65. Distribution of stars in the Galaxy. Clusters. General structure of the Galaxy.
end of form beginning of form Knowing the distances to the stars allows us to approach the study of their distribution in space, and hence the structure of the Galaxy. In order to characterize the number of stars in different parts of the Galaxy, the concept of stellar density is introduced, which is analogous to the concept of the concentration of molecules. Stellar density is the number of stars in a unit volume of space. The unit of volume is usually taken to be 1 cubic parsec. In the vicinity of the Sun, the stellar density is about 0.12 stars per cubic parsec, in other words, each star has an average volume of over 8 ps3; the average distance between the stars is about 2 ps. To find out how the stellar density changes in different directions, the number of stars per unit area (for example, 1 square degree) in different parts of the sky is counted.
The first thing that catches the eye in such calculations is an unusually strong increase in the concentration of stars as one approaches the band of the Milky Way, the middle line of which forms a large circle in the sky. On the contrary, as we approach the pole of this circle, the concentration of stars decreases rapidly. This fact is already at the end of the 18th century. allowed V. Herschel to draw the correct conclusion that our star system has an oblate shape, and the Sun should be close to the plane of symmetry of this formation. spherical sector, the radius of which is determined by the formula
lg r m =1 + 0.2 (m * M)
end of form beginning of form To characterize how many stars of different luminosities are contained in a given region of space, the luminosity function j (M) is introduced, which shows what proportion of the total number of stars has a given value of absolute stellar magnitude, say, from M to M + 1.
end of form beginning of form Clusters of galaxies are gravitationally bound systems galaxies, one of the largest structures in universe. The sizes of clusters of galaxies can reach 10 8 light years.
Accumulations are conditionally divided into two types:
regular - clusters of a regular spherical shape, in which elliptical and lenticular galaxies, with a clearly defined central part. At the centers of such clusters are giant elliptical galaxies. An example of a regular cluster - Cluster of Veronica's Hair.
irregular - clusters without a definite shape, inferior to regular ones in the number of galaxies. Clusters of this species are dominated by spiral galaxies. Example - Virgo Cluster.
Cluster masses vary from 10 13 to 10 15 solar masses.
The structure of the galaxy
The distribution of stars in the Galaxy has two pronounced features: firstly, a very high concentration of stars in the galactic plane, and secondly, a large concentration in the center of the Galaxy. So, if in the vicinity of the Sun, in the disk, one star falls on 16 cubic parsecs, then in the center of the Galaxy there are 10,000 stars in one cubic parsec. In the plane of the Galaxy, in addition to an increased concentration of stars, there is also an increased concentration of dust and gas.
Dimensions of the Galaxy: – the diameter of the disk of the Galaxy is about 30 kpc (100,000 light years), is about 1000 light years thick.
The Sun is located very far from the nucleus of the Galaxy - at a distance of 8 kpc (about 26,000 light years).
The center of the Galaxy is located in the constellation Sagittarius in the direction of? = 17h46.1m, ? = –28°51′.
The galaxy consists of a disk, a halo and a corona. The central, most compact region of the Galaxy is called the nucleus. There is a high concentration of stars in the core: there are thousands of stars in every cubic parsec. If we lived on a planet near a star located near the core of the Galaxy, then dozens of stars would be visible in the sky, comparable in brightness to the Moon. A massive black hole is assumed to exist at the center of the Galaxy. Almost all molecular matter of the interstellar medium is concentrated in the annular region of the galactic disk (3–7 kpc); there is the largest number of pulsars, supernova remnants and sources of infrared radiation. The visible radiation of the central regions of the Galaxy is completely hidden from us by powerful layers of absorbing matter.
The galaxy contains two main subsystems (two components), nested one into the other and gravitationally bound to each other. The first is called spherical - a halo, its stars are concentrated towards the center of the galaxy, and the density of matter, which is high in the center of the galaxy, decreases rather quickly with distance from it. The central, densest part of the halo, within a few thousand light-years from the center of the Galaxy, is called the bulge. The second subsystem is a massive stellar disk. It looks like two plates folded at the edges. The concentration of stars in the disk is much greater than in the halo. The stars inside the disk move in circular paths around the center of the Galaxy. The Sun is located in the stellar disk between the spiral arms.
The stars of the galactic disk were called population type I, the stars of the halo - population type II. The disk, the flat component of the Galaxy, includes stars of the early spectral classes O and B, stars in open clusters, and dark dusty nebulae. Halos, on the contrary, are made up of objects that arose in the early stages of the evolution of the Galaxy: stars of globular clusters, stars of the RR Lyrae type. The stars of the flat component, compared with the stars of the spherical component, are distinguished by a high abundance of heavy elements. The age of the population of the spherical component exceeds 12 billion years. It is usually taken as the age of the Galaxy itself.
Compared to the halo, the disk rotates noticeably faster. The disk rotation speed is not the same at different distances from the center. The mass of the disk is estimated at 150 billion M. There are spiral branches (sleeves) in the disk. Young stars and star formation centers are located mainly along the arms.
The disk and the halo surrounding it are immersed in the corona. At present, it is believed that the size of the corona of the Galaxy is 10 times larger than the size of the disk.
- Specialty HAC RF01.03.02
- Number of pages 144
1 Methods for determining distances to galaxies.
1.1 Introductory remarks.
12 Photometric methods.
1.2.1 Supernovae and new stars.
1.2.2 Blue and red supergiants.
1.2.3 Cepheids.
1.2.4 Red giants.
1.2.5 KE Lyra.
1.2.6 Using the object luminosity function.
1.2.7 Surface brightness fluctuation method (SVR).
1.3 Spectral methods.
1.3.1 Using the Hubble relation.
1.3.2 Using the Tully-Fisher (TR) relationship.
1.3.3 Using the Faber-Jackson dependency.
1.4 Other methods.
1.5 Comparison of methods for determining distances.
2 The brightest stars in galaxies and their photometry.
2.1 The brightest stars in galaxies.
2.2 Blue and red supergiants.
2.2.1 Method calibration.
2.2.2 Accuracy of the brightest star method.
2.2.3 Budush; her method of the brightest stars.
2.3 Red giants and the TSCR method.
2.3.1 Influence of metallicity and age.
2.3.2 Influence of bright SG and AGB stars and the density of stellar fields on the accuracy of the TRGB method.
2.4 Photometry of stars in galaxies.
2.4.1 Photographic methods.
2.4.2 Aperture photometry with PCVISTA.
2.4.3 Photometry with DAOPHOT.
2.4.4 Features of photometry of HST images.
2.5 Comparison of photometry accuracy of different methods.
2.5.1 Comparison of photographic and CCD photometry.
2.5.2 Comparison of Zeiss-1000 - BTA results.
3 Local complex of galaxies and its spatial structure.
3.1 Introduction.
3.2 Local complex of galaxies.
3.3 Local group of galaxies.
3.3.1 Galaxy ICIO.
3.3.2 Galaxy LGS3.
3.3.3 Galaxy DDO210.
3.3.4 New galaxies of the Local Group.
3.4 Group M81 + NGC2403.
3.5 Group IC342/Maffei.
3.6 Group M101.
3.7 Cloud of galaxies CVn.
3.8 Distribution of galaxies in the Local Complex, velocity anisotropy.
4 The structure of galaxies in the direction of the cluster in
Virgo. Determination of the Hubble constant.
4.1 Introduction.
4.2 The structure of the cluster of galaxies in Virgo.
4.3. Preliminary selection of galaxies by parameters.
4.4 Observations and photometry of stars.
4.5 Accuracy of photometry and distance measurements.
4.6 Spatial distribution galaxies.
4.7 Determination of the Hubble constant.
4.8 Comparison of results.
5 Group NGC1023.
5.1 Introduction.
5.2 The NGC1023 group and its composition.
5.3 Observations of galaxies in the NGC1023 group.
5.4 Photometry of stars in BTA and HST images.
5.5 Determination of distances to the galaxies of the group.
5.5.1 Definition by the brightest supergiants.
5.5.2. Determination of distances based on the TRGB method.
5.6 The problem of the galaxy NGC1023a.
5.7 Distribution of distances of galaxies of the group.
5.8 Determination of the Hubble constant in the direction of NGC1023.
6 Spatial structure of irregular galaxies
6.1 Introductory remarks.
6.2 Spiral and irregular galaxies.
6.2.4 Stellar composition of galaxies.
6.3 Periphery of galaxies.
6.3.1 Galaxies seen "face-on" and "edge-on".
6.3.4 The boundaries of galaxies.
6.4. Red giant disks and the hidden mass of irregular galaxies.
Introduction to the thesis (part of the abstract) on the topic "Spatial distribution and structure of galaxies based on the study of the brightest stars"
Formulation of the problem
Historically, at the beginning of the 20th century, literally an explosion in the study of stars and star clusters, both in our Galaxy and in other star systems, created the basis on which extragalactic astronomy proper appeared. The emergence of a new direction in astronomy took place thanks to the work of Hertzsprung and Ressel, Duncan and Abbe, Leavitt and Bailey, Shapley and Hubble, Lundmark and Curtis, which established an almost modern understanding of the scale of the universe.
In its further development, extragalactic astronomy went to such distances where individual stars were no longer visible, but as before, astronomers involved in extragalactic research published a large number of works that were somehow related to stellar topics: with the elucidation of luminosities stars, building distance scales, studying the evolutionary stages of certain types of stars.
Studies of stars in other galaxies allow astronomers to solve several problems at once. First, clarify the scale of distances. It is clear that without knowing the exact distances, we do not know the main parameters of galaxies - sizes, masses, luminosities. Opening in 1929 Hubble's relationship between the radial velocities of galaxies and their distances makes it possible to quickly determine the distance to any galaxy based on a simple measurement of its radial velocity. However, we cannot use this method if we are studying non-Hubble motions of galaxies, i.e. motions of galaxies associated not with the expansion of the Universe, but with the usual laws of gravity. In this case, we need a distance estimate based not on the measurement of speed, but on the basis of the measurement of other parameters. It is known that galaxies at distances up to 10 Mpc have their own velocities, which are comparable to their speed in the Hubble expansion of the Universe. The summation of two almost identical velocity vectors, one of which has a random direction, leads to strange and completely unrealistic results if we use the Hubble dependence when studying the spatial distribution of galaxies. Those. and in this case we cannot measure distances based on the radial velocities of galaxies.
Secondly, since all galaxies are composed of stars, by studying the distribution and evolution of stars in a galaxy, we somehow answer the question of the morphology and evolution of the galaxy itself. Those. The obtained information on the stellar composition of the galaxy limits the variety of models used for the origin and evolution of the entire stellar system. Thus, if we want to know the origin and evolution of galaxies, we absolutely need to study the stellar population of different types of galaxies to the deepest possible photometric limit.
In the era of photographic astronomy, studies of the stellar population of galaxies were carried out with the largest telescopes in the world. But still, even in this nearby galaxy, like M31, P-type stellar population, i.e. red giants, was at the limit of photometric measurements. Such a technical limitation of possibilities has led to the fact that the stellar population has been studied in detail and in depth only in the galaxies of the Local Group, where, fortunately, galaxies of almost all types are present. In the 1940s, Baade divided the entire population of galaxies into two types: bright young supergiants (type I), located in a thin disk, and old red giants (type II), occupying a more voluminous halo. Later, Baade and Sandage pointed to the presence of Type II Local Populations in all galaxies; old stars that were clearly visible on the periphery of galaxies. In the images of more distant galaxies, only bright supergiants were visible, which Hubble used in its time to determine the distances to galaxies when calculating the expansion parameter of the Universe.
The technical progress of the 1990s in the development of observational facilities led to the fact that rather faint stars became available in galaxies outside the Local Group, and it became possible to actually compare the parameters of the stellar population of many galaxies. At the same time, the transition to a CCD matrix was also marked by a regression in the study of the global parameters of the distribution of the stellar population of galaxies. It has become simply impossible to study a galaxy 30 arc minutes in size with a light receiver 3 arc minutes in size. And only now are CCD matrices appearing, comparable in size to previous photographic plates.
General characteristics of the work RELEVANCE.
The relevance of the work has several manifestations:
The theory of star formation and the evolution of galaxies, the determination of the initial mass function under various physical conditions, as well as the stages of evolution of single massive stars require direct images of galaxies. Only a comparison of observations and theory can give further progress in astrophysics. We have obtained a large amount of observational material, which is already yielding secondary astrophysical results in the form of LBV star candidates, which are then confirmed spectrally. It is known that a program of direct images of galaxies "for the future" is currently being carried out on the HST; these images will be required only after the outbreak of a P-type supernova (supergiant) in such a galaxy. The archive we have is slightly inferior to what is currently being created on HST.
At present, the problem of determining the exact distances to galaxies, both distant and close, has become the main one in the work of large telescopes. If for large distances the goal of such work is to determine the Hubble constant with maximum accuracy, then at small distances the goal is to search for local inhomogeneities in the distribution of galaxies. And for this, exact values of the distances to the galaxies of the Local Complex are needed. In the first approximation, we have already obtained data on the spatial distribution of galaxies. In addition, the calibration of distance methods requires accurate values for those few key galaxies that are basic.
Only now, after the advent of modern matrices, it has become possible to deeply study the stellar composition of galaxies. This immediately opened the way for reconstructing the history of star formation in galaxies. And the only source material for this is direct images of galaxies resolvable to stars, made in different filters.
The history of the study of weak structures of galaxies has more than a dozen years. This became especially important after obtaining extended rotation curves of spiral and irregular galaxies from radio observations. The results obtained indicated the existence of significant invisible masses, and the search for the optical manifestation of these masses is being intensively pursued in many observatories. Our results show the existence around galaxies of late types of extended disks, consisting of the old stellar population - red giants. Taking into account the mass of these disks can weaken the problem of invisible masses.
GOAL OF THE WORK.
The objectives of this dissertation work are:
1. Obtaining the largest possible homogeneous array of images of galaxies in the northern sky with velocities less than 500 km/s and determining the distances to galaxies based on the photometry of their brightest stars.
Fig. 2. Resolution for stars of galaxies observed in two opposite directions - in the Virgo cluster and in the N001023 group. Determination of distances to named groups and calculation, on the basis of the obtained results, of the Hubble constant in two opposite directions.
3. Study of the stellar composition of the periphery of irregular and spiral galaxies. Determination of the spatial forms of galaxies on long distances from the center.
SCIENTIFIC NOVELTY.
For a large number galaxies on the b-m telescope obtained deep images in two) A colors, which made it possible to resolve the galaxies into stars. The photometry of the stars in the images was carried out and color-magnitude diagrams were constructed. Based on these data, distances were determined for 92 galaxies, including those in such distant systems as the Virgo cluster or the N001023 group. For most galaxies, distance measurements have been made for the first time.
The measured distances were used to determine the Hubble constant in two opposite directions, which made it possible to estimate the velocity gradient between the Local Group and the N001023 group, which turned out to be small and does not exceed the measurement errors.
The study of the stellar composition of the periphery of galaxies led to the discovery of extended thick disks in irregular galaxies, consisting of old stars, red giants. The sizes of such disks are 2-3 times larger than the apparent sizes of galaxies at the level of 25 "A/P". Based on the spatial distribution of red giants, galaxies are found to have clearly defined boundaries.
SCIENTIFIC AND PRACTICAL VALUE.
The 6-m telescope obtained multicolor images of about 100 resolvable galaxies. In these galaxies, the colors and brightness of all visible stars. Hypergiants and supergiants with the highest luminosity have been identified.
Based on the works in which the author was directly involved, for the first time a large and homogeneous array of data was obtained on measuring distances for all galaxies in the northern sky with velocities less than 500 km/s. The data obtained make it possible to analyze the non-Hubble motions of the galaxies of the Local Complex, which limits the choice of a model for the formation of the Local "pancake" of galaxies.
The composition and spatial structure of the nearest groups of galaxies in the northern sky have been determined. The results of the work make it possible to carry out statistical comparisons of the parameters of groups of galaxies.
A study was made of the structure of space in the direction of the cluster of galaxies in Virgo. Several relatively close galaxies have been found located between the cluster and the Local Group. Distances were determined and galaxies belonging to the cluster itself and located in different parts of the periphery and center of the cluster were identified.
The distance to the Virgo and Coma Veronica clusters was determined and the Hubble constant was calculated. The brightness of the brightest stars of 10 galaxies of the N001023 group, which lies at a distance of 10 Me, was measured. The distances to galaxies are determined and the Hubble constant in this direction is calculated. A conclusion is made about a small velocity gradient between the Local Group and the N001023 group, which can be explained by the non-dominant mass of the Virgo galaxy cluster.
FOR DEFENSE ARE SUPPLIED:
Fig. 1. Results of work on the development and implementation of a technique for photometry of stars on automatic microdensitometers AMD1 and AMD2 of the OJSC RAS.
2. Derivation of the calibration dependence of the method for determining distances from blue and red supergiants.
3. Results of photometry of stars in 50 galaxies of the Local Complex and determination of distances to these galaxies.
4. Results of determining the distances of up to 24 galaxies in the direction of the Virgo cluster. Determination of the Hubble constant.
Fig. 5. Results of determining distances to galaxies of the NOC1023 group and determining the Hubble constant in the opposite direction from the cluster in Virgo. Conclusion of a small velocity gradient between the Local group and the NGO1023 group.
6. Results of a study of the spatial distribution of late-type stars in irregular galaxies. Discovery of extended disks of red giants around irregular galaxies.
WORK APPROVAL.
The main results obtained in the dissertation were reported at the seminars of JSC RAS, SAI, AI OPbSU, as well as at conferences:
France, 1993, In ESO/OHP Workshop "Dwarf Galaxies" eds. Meylan G., Prugniel P., Observatoire de Haute-Provence, France, 109.
South Africa, 1998, in lAU Symp. 192, The Stellar Content of Local Group Galaxies, ed. Whitelock P., and Gannon R., 15.
Finland, 2000 "Galaxies in the M81 Group and IC342/Maffei Complex: The Structure and Stellar Populations", ASP Conference Series, 209, 345.
Russia, 2001, All-Russian Astronomical Conference, August 6-12, St. Petersburg. Report: "Spatial distribution of late-type stars in irregular galaxies".
Mexico, 2002, Cozumel, April 8-12, "Stars as a Tracer of the Shape of Irregular Galaxies Haloes".
1. Tikhonov N.A., Results of hypersensitization in hydrogen of astrofilms of the Kaz-NII of the technical project, 1984, Soobshch.SAO, 40, 81-85.
2. N. A. Tikhonov, Photometry of stars and galaxies on direct BTA images. Errors in photometry AMD-1, 1989, Soobshch.SAO, 58, 80-86.
3. Tikhonov N.A., Bilkina B.I., Karachentsev ID., Georgiev Ts.B., Distance of nearby galaxies N00 2366.1С 2574, and NOG 4236 from photographic photometry of their brightest stars, 1991, A&AS, 89, 1-3.
4. Georgiev Ts. V., Tikhonov N.A., Karachentsev ID., Bilkina B.I. The brightest stars and the distance to the dwarf galaxy HoIX, 1991, A&AS, 89, 529-536.
5. Georgiev Ts.B., Tikhonov N.A., Karachentsev I.D., The brightest candidates for globular clusters galaxies M81, 1991, Letters to AJ, 17, 387.
6. Ts. B. Georgiev, N. A. Tikhonov, and I. D. Karachentsev, Estimates of B and V quantities for candidates for globular clusters of the galaxy M 81, 1991, Letters to Azh, 17, nil, 994-998.
7. Tikhonov N.A., Georgiev T.E., Bilkina B.I. Stellar photometry on the 6-m telescope plates, 1991, OAO, 67, 114-118.
8. Karachentsev I.D., Tikhonov N.A., Georgiev Ts.B., Bilkina B.I., Sharina M.E., Distances of nearby galaxies N0 0 1560, NGO 2976 and DDO 165 from their brightest stars, 1991, A&AS, 91, 503-512.
9. Georgiev Ts.B., Tikhonov N.A., Bilkina B.I., The brightest blue and red stars in the galaxy M81, 1992, A&AS, 95, 581-588.
10. Georgiev Ts.B., Tikhonov N.A., Bilkina B.I., The distribution of blue and stars around the M81, A&AS, 96, 569-581.
11. Tikhonov N.A., Karachentsev I.D., Bilkina B.I., Sharina M.E., Distances to three nearby dwarf galaxies from photometry of their brightest stars, 1992, A&A Trans, 1, 269-282.
12. Georgiev Ts.B., Bilkina B.I., Tikhonov N.A., Getov R., Nedialkov P., The precise coordinates of the supergiants and globular cluster candidates of the galaxy M 81, 1993, Bull SAO, 36, 43.
13. Karachentsev I.D., Tikhonov N.A., Photometric distances to the nearby galaxies 10 10, 10 342 and UA 86, visible throught the Milky Way, 1993, A&A, 100, 227-235.
14. Tikhonov N.A., Karachentsev I.D., Photometric distances to five dwarf galaxies in the vicinity of M 81, 1993, A&A, 275, 39.
15. Karachentsev I., Tikhonov N., Sazonova L., The brightest stars in three irregular dwarfs around M 81, 1994, A&AS, 106, 555.
16. Karachentsev I., Tikhonov N., Sazonova L., NGC 1569 and UGCA 92 - a nearby pair of galaxies in the Milky Way zone, 1994, Letters to Soviet AJ, 20, 90.
17. Karachentsev L, Tikhonov N., New photometric distances for dwarf galaxies in the Local Volume, 1994, A&A, 286, 718.
18. Tikhonov N., Karachentsev L, Maffei 2, a nearby galaxy shielded by the Milky Way, 1994, Bull. SAO, 38, 3.
19. Georgiev Ts., Vilkina V., Karachentsev I., Tikhonov N. Stellar photometry and distances to close galaxies: Two differences in estimates for the parameter "ra per X" bl. 1994, Obornik with the report of VAN, Sofia, p.49.
20. Tikhonov N., Irregular galaxy Casl - a new member of the Local Group, Astron.Nachr., 1996, 317, 175-178.
21. Tikhonov N., Sazonova L., A color - magnitude diagram for Pisces dwarf galaxy, AN, 1996, 317, 179-186.
22. M. E. Sharina, I. D. Karachentsev, and N. A. Tikhonov, Photometric distance to the galaxy N0 0 6946 and its companion, 1996, Letters to Azh, 23, 430-434.
23. Sharina M.E., Karachentsev I.D., Tikhonov N.A., Photometric distances to NGC 628 and its four companions, 1996, A&AS, 119, n3. 499-507.
24. Georgiev Ts. V., Tikhonov N.A., Karachentsev I.D., Ivanov V.D. Globular cluster candidates in the galaxies NGC 2366.1C 2574 and NGC 4236, 1996, A&A Trans, 11, 39-46.
25. Tikhonov N.A., Georgiev Ts. V., Karachentsev I.D., Brightest star cluster candidates in eight late-type galaxies of the local complex, 1996, A&A Trans, 11, 47-58.
26. Ts. B. Georgiev, I. D. Karachentsev, and N. A. Tikhonov, Distance modules up to 13 nearby isolated dwarf galaxies, Letters to Azh., 1997, 23, 586-594.
27. Tikhonov N. A., The deep stellar photometry of the ICIO, 1998, in lAU Symposium 192, ed. P. Whitelock and R. Cannon, 15.
28. Tikhonov N.A., Karachentsev I.D., CCD photometry and distances of six resolved irregular galaxies in Canes Venatici, 1998, A&AS, 128, 325-330.
29. Sharina M. E., Karachentsev I. D., Tikhonov N. A., Distances to Eight Nearby Isolated Low-Luminosity Galaxies, 1999, AstL, 25, 322S.
30. Tikhonov N.A., Karachentsev I.D., Distances to the Two New Companions of M 31, 1999, AstL, 25, 332.
31. Drozdovskii 1.0., Tikhonov N.A., The stellar content and distance to the nearby blue compact dwarf galaxy NGC 6789, 2000, A&AS, 142, 347D.
32. Aparicio A., Tikhonov N.A., Karachentsev I.D., DDO 187: do dwarf galaxies have extended, old halos? 2000, AJ, 119, 177A.
33. Aparicio A., Tikhonov N.A., The spatial and age distribution of stellar population in DDO 190, 2000, AJ, 119, 2183A.
34. Lee M., Aparicio A., Tikhonov N, Byin Y.-I, Kim E., Stellar populations and the Local Group membership of the dwarf galaxy DDO 210, 1999, AJ, 118, 853-861.
35. Tikhonov N.A., Galazutdinova O.A., Drozdovskii I.O., Distances to 24 Galaxies in the Direction of the Virgo Cluster and a Determination of the Hubble Constant, 2000, Afz, 43, 367.
STRUCTURE OF THE THEsis
The dissertation consists of an Introduction, six chapters, a Conclusion, a list of cited literature and an Appendix.
Dissertation conclusion on the topic "Astrophysics, radio astronomy", Tikhonov, Nikolai Alexandrovich
The main conclusions of this chapter concern irregular and, to a lesser extent, spiral galaxies. Therefore, these types of galaxies should be considered in more detail, focusing on the differences and similarities between them. We touch, to a minimum extent, those parameters of galaxies that do not manifest themselves in any way in our studies.
6.2.1 Issues of classification of galaxies.
Historically, the entire classification of galaxies has been created on the basis of images obtained in the blue rays of the spectrum. Naturally, in these images, those objects that have a blue color stand out especially brightly, i.e. star forming regions with bright young stars. In spiral galaxies such regions form spectacularly prominent branches, and in irregular galaxies they form bright areas scattered almost randomly over the bodies of galaxies.
The apparent difference in the distribution of star-forming regions became the initial boundary that separated spiral and irregular galaxies, regardless of whether the classification is carried out according to Hubble, Vaucouleur, or van den Bergh 192,193,194]. In some classification systems, the authors tried to take into account other parameters of galaxies, in addition to their appearance, but the simplest Hubble classification remained the most common.
Naturally, there are physical reasons for the difference in the distribution of star-forming regions in spiral and irregular galaxies. First of all, this is the difference in masses and rotation speeds; however, the initial classification proceeded only from the type of galaxies. At the same time, the boundary between these two types of galaxies is very relative, since many bright irregular galaxies have signs of spiral arms or a bar-like structure in the center of the galaxy. The Large Magellanic Cloud, which serves as an example of a typical irregular galaxy, has a bar and faint signs spiral structure, which is characteristic of Sc-type galaxies. Signs of the spiral structure of irregular galaxies are especially noticeable in the radio range when studying the distribution of neutral hydrogen. As a rule, around an irregular galaxy there is an extended gas cloud, in which signs of spiral branches are often visible (for example, in ICIO 196], Holl, IC2574).
The consequence of such a smooth transition of their general properties from spiral galaxies to irregular galaxies is subjectivity in the morphological definitions of the types of galaxies by different authors. Moreover, if the first photographic plates were sensitive to infrared rays, and not to blue, then the classification of galaxies would be different, since star-forming regions would not be distinguished in galaxies in the most noticeable way. In such infrared images, those regions of galaxies that contain the old stellar population - red giants - are best seen.
Any galaxy in the IR range has a smoothed appearance, without contrasting spiral arms or star formation regions, and the disk and bulge of the galaxy are most pronounced. In the images in the IR range of Irr, the galaxies are visible as disk dwarf galaxies, oriented towards us at different angles. This is clearly seen in the IR atlas of galaxies. Thus, if the classification of galaxies were initially carried out on the basis of images in the infrared range, then both spiral and irregular galaxies would fall into the same group of disk galaxies.
6.2.2 Comparison of general parameters of spiral and irregular galaxies.
The continuity of the transition from spiral galaxies to irregular ones is visible when considering the global parameters of a sequence of galaxies, i.e. from spiral ones: Sa Sb Sc to irregular ones: Sd Sm Im . All parameters: masses, sizes, hydrogen content indicate a single class of galaxies. The photometric parameters of galaxies have a similar continuity: luminosity and color. ticks, we did not try to meticulously find out the exact type of galaxy. As further experience showed, the distribution parameters of the stellar population in dwarf spiral and irregular galaxies are approximately the same. This once again emphasizes that both types of galaxies should be united under one name - disk.
6.2.3 Spatial forms of galaxies.
Let us turn to the spatial structure of galaxies. The flattened shapes of spiral galaxies need no explanation. When describing this type of galaxies, the bulge and disk of the galaxy are usually distinguished on the basis of photometry. Since the extended and flat curves of the radial velocities of spiral galaxies require their explanation in the form of the presence of significant masses of invisible matter, an extended halo is often added to the morphology of galaxies. Attempts to find a visible manifestation of such a halo have been made repeatedly. Moreover, in many cases, the absence of a central cluster or bulge in irregular galaxies leads to the fact that only the exponential disk component of the galaxy is visible on photometric sections without signs of other components.
To elucidate the shapes of irregular galaxies along the Z axis, observations of edge-on galaxies are required. A search for such galaxies in the LEDA catalog, with selection by rotation speed, axis ratio, and size, led us to compile a list of several dozen galaxies, most of which are located at large distances. With deep surface photometry, it is possible to reveal the existence of low surface brightness subsystems and measure their photometric characteristics. The low brightness of the subsystem does not at all mean its small influence on the life of the galaxy, since the mass of such a subsystem can be quite large due to of great importance M/L.
UGCB760, VTA. 1800s
20 40 60 in RADIUS (arcsec)
Position (PRCSEC)
Rice. 29: Color distribution (Y - Z) along the major axis of the N008760 galaxy and its isophote up to HE - 27A5
On fig. Figure 29 shows the results of surface photometry of the irregular galaxy 11008760 obtained by us at the VTA. The isophotes of this galaxy show that, at deep photometric limits, the shape of the outer parts of the galaxy is close to an oval. Second, the galaxy's faint isophotes extend along the major axis much farther than the main body of the galaxy, where bright stars and star-forming regions are visible.
One can see the continuation of the disk component beyond the limits of the main body of the galaxy. Nearby is the color change from the center of the galaxy to the faintest isophotes.
Photometric measurements have shown that the main body of the galaxy has a color of (Y-th) = 0.25, which is quite typical for irregular galaxies. Measurements of the color of regions far from the main body of the galaxy give the value (V - K) = 1.2. This result means that the faint = 27.5""/P") and extended (3 times larger than the size of the main body) outer parts of this galaxy should consist of red stars. It was not possible to find out the type of these stars, since the galaxy is located further photometric limits of VTA.
After such a result, it became clear that studies of nearby irregular galaxies were needed in order to be able to say more definitely about the stellar composition and spatial forms of the faint outer parts of galaxies.
Rice. 30: Comparison of the metallicity of red supergiant giant (M81) and dwarf galaxies (Holl). The position of the supergiant branch is very sensitive to the metallicity of the galaxy
6.2-4 Stellar composition of galaxies.
The stellar composition of spiral and irregular galaxies is exactly the same. Based on the G-R diagram alone, it is almost impossible to determine the type of galaxy. Some influence is introduced by the statistical effect, brighter blue and red supergiants are born in giant galaxies. However, the mass of the galaxy still manifests itself in the parameters of the born stars. in massive galaxies heavy elements, formed during the evolution of stars, remain within the galaxy, enriching the interstellar medium with metals. As a result, all subsequent generations of stars in massive galaxies have an increased metallicity. On fig. 30 shows a comparison of the G - R diagrams of a massive (M81) and dwarf (Holl) galaxies. The different positions of the branches of the red supergiants are clearly visible, which is an indicator of their metal personality. For the old stellar population - red giants - in massive galaxies, the existence of stars in a large range of metallicity 210] is observed, which affects the width of the giant branch. In dwarf galaxies, narrow giant branches (Fig. 3$) and low metallicity values are observed. The surface density of giants varies exponentially, which corresponds to the disk component (Fig. 32). A similar behavior of red giants was also found by us in the IC1613 galaxy.
Rice. 32: Variation in the surface density of red giants in the F5 field of the ICIO galaxy. A jump in the density of giants is visible at the disk boundary, which does not drop to zero beyond the disk boundary. A similar effect is observed in spiral galaxy MZZ. Graph scale in minutes of arc from the center.
Considering these results and everything said earlier about irregular galaxies, it could be assumed that it is the old red giant stars that form the extended periphery of galaxies, especially since the existence of red giants on the outskirts of the Local Group galaxies has been known since the time of W. Vaade. A few years ago, Minity and his colleagues announced that they had found red giant halos around two galaxies: WLM and NGC3109, but the publications did not investigate the issue of changes in the density of giants with distance from the center and the size of such halos.
To determine the law of change in the surface density of stars of various types, including giants, deep observations of nearby galaxies were needed.
Rice. 33: Change in the density of stars in the galaxies BB0 187 and BB0190 from the center to the edge. It is noticeable that the red giants have not reached their boundary and continue beyond our image. Graph scale in arcseconds. laid flat, as seen in ICIO.
Our observations with the 2.5-m Nordic telescope of the galaxies DD0187 and DDO 190 confirmed that these irregular galaxies seen face-on also exhibit an exponential drop in the surface density of red giants from the center to the edge of the galaxy. Moreover, the extent of the structure of red giants far exceeds the size of the main body of each galaxy (Fig. 33). The edge of this halo/disk is outside of the applied CCD array. An exponential change in the density of giants has also been found in other irregular galaxies. Since all the studied galaxies behave in the same way, we can speak, as an established fact, about the exponential law of change in the density of the old stellar population - red giants, which corresponds to the disk component. However, this still does not prove the existence of discs.
The reality of disks can only be confirmed from edge-on observations of galaxies. Observations of such galaxies to search for the visible manifestation of a massive halo have been carried out repeatedly using a variety of equipment and in different regions of the spectrum. The discovery of such a halo has been repeatedly announced. illustrative example The complexity of this task can be traced in publications. Several independent researchers have announced the discovery of such a halo around N005007. Subsequent observations with a fast telescope with a total exposure of 24 hours (!) closed the question of the existence of a visible halo of this galaxy.
Among nearby irregular galaxies seen edge-on, attention is drawn to the dwarf in Pegasus, which has been repeatedly studied. BTA observations of several fields allowed us to fully trace the change in the density of stars of different types in it, both along the major and minor axes. The results are presented in fig. 34, 35. They prove that, firstly, the structure of red giants is three times larger than the main body of the galaxy. Second, the shape of the distribution along the b axis is close to an oval or ellipse. Thirdly, no halo consisting of red giants is noticeable.
Rice. 34: The boundaries of the Pegasus Dwarf galaxy based on the study of red giants. Locations of BTA images are marked.
AGB blue stars Q O O
PegDw W « «(W Joko* 0 0 oooooooooo
200 400 600 majoraxis
Rice. 35: Surface density distribution of different types of stars along the major axis of the Pegasus Dwarf galaxy. The boundary of the disk is visible, where there is a sharp drop in the density of red giants. about 1
Our further results are based on the photometry of NCT images obtained by us from the archive of free access. The search for NCT galaxies, resolvable as red giants and visible face-on and edge-on, gave us about two dozen candidates for study. Unfortunately, the field of view of the NCT, insufficient for us, sometimes hindered the goals of our work - to trace the parameters of the distribution of stars.
After standard photometric processing, G-R diagrams were constructed for these galaxies and stars of various types were identified. Their research showed:
1) For face-on galaxies, the decrease in the surface density of red giants follows an exponential law (Fig. 36).
-|-1-1-1-E-1-1-1-1-1-1-1-1--<тГ
PGC39032 / w "".
15 red giants Z sh
Rice. Fig. 36: Exponential change in the density of red giants in the dwarf galaxy RCC39032 from center to edge based on NCT observations
2) Not a single edge-on galaxy has an extended, along axis 2, halo of red giants (Fig. 37).
3) The shape of the distribution of red giants along the b-axis has the form of an oval or ellipse (Fig. 38).
Taking into account the randomness of the sampling and the uniformity of the results obtained in terms of the shape of the distribution of giants in all the studied galaxies, it can be argued that most of the galaxies have such a red giant distribution law. Deviations from the general rule are possible, for example, in interacting galaxies.
It should be noted that among the studied galaxies there were both irregular and non-giant spiral galaxies. We have not found significant differences between them in the laws of distribution of red giants along axis 2, with the exception of the gradient of the fall in the density of giants.
6.3.2 Spatial distribution of stars.
By selecting stars of different types on the G - R diagram, we can see their distribution in the galaxy image or calculate the parameters of their spatial distribution over the body of the galaxy.
It is well known that the young stellar population of irregular galaxies is concentrated in star-forming regions that are randomly scattered throughout the body of the galaxy. However, the apparent randomness immediately disappears if we trace the change in the surface density of young stars along the radius of the galaxy. On the graphs of Fig. 33 shows that the general, close to exponential, distribution is superimposed by local fluctuations associated with individual star formation regions.
For the older population - stars of the extended asymptotic giant branch, the distribution has a smaller density drop gradient. And the smallest gradient has the ancient population - the red giants. It would be interesting to check this dependence for obviously the most ancient population - horizontal branch stars, however, in those galaxies where these stars are reachable, we see their insufficient number for statistical studies. The clearly visible dependence of the age of stars and parameters of spatial density can have a completely logical explanation: although star formation occurs most intensively near the center of the galaxy, the orbits of stars become larger and larger with time, and over a period of several billion years, stars can move to the periphery of galaxies . Difficult to
Rice. Fig. 37: Decline in the density of red giants along axis 2 in several edge-on galaxies
Rice. 38: An image of a nearly edge-on dwarf galaxy shows the positions of the red giants found. The general form of the distribution is to set an oval or an ellipse, how such an effect can be verified in observations. Probably, only modeling the evolution of the galaxy disk can help with the regeneration of such hypotheses.
6.3.3 Structure of irregular galaxies.
Summarizing what has been said in other sections, one can imagine the structure of an irregular galaxy as follows: the most extended star system in all coordinates is formed by red giants. The shape of their distribution is a thick disk with an exponential drop in the surface density of giants from the center to the edge. The thickness of the disk is almost the same throughout its entire length. Younger star systems have their subsystems nested in this disk. The younger the stellar population, the thinner the disk it forms. And although the youngest stellar population, blue supergiants, is distributed over individual chaotic star-forming regions, in general, it also obeys a general pattern. All nested subsystems do not avoid each other, i.e. Star-forming regions may contain old red giants. For the most dwarf galaxies, where one star-forming region occupies the entire galaxy, this scheme is very arbitrary, but the relative sizes of the disks of the young population and the old population are also valid for such galaxies.
If, however, to complete the survey of the structure of irregular galaxies, radio data are also involved, then it turns out that the entire stellar system is immersed in a disk or cloud of neutral hydrogen. The size of the disk from HI, as it follows from the statistics of 171 galaxies, is approximately 5-6 times larger than the visible body of the galaxy at the level of Hv = 25" *. For a direct comparison of the sizes of hydrogen disks and disks from red giants, we have too little data.
In the ICIO galaxy, the sizes of both disks are approximately equal. For a galaxy in Pegasus, the hydrogen disk is almost half the size of a disk of red giants. And the galaxy NGC4449, which has one of the most extended hydrogen disks, is unlikely to have an equally extended disk of red giants. kah is confirmed not only by our observations. We have already mentioned reports by Minity and his colleagues about the discovery of a halo. Having obtained an image of only a part of the galaxy, they took the size of the thick disk along the b axis as a manifestation of a halo, which they reported without attempting to investigate the distribution of stars in these galaxies along the major axis.
In our studies, we did not touch on giant galaxies, but if we consider the structure of our Galaxy, then for it there already exists the concept of a "thick disk" for a low-metal old population. As for the term "halo", it seems to us that it is applicable to spherical, but not to flattened systems, although this is only a matter of terminology.
6.3.4 The boundaries of galaxies.
The question of the boundaries of galaxies has probably not been fully explored yet. Nevertheless, our results can make a certain contribution to its solution. It is usually believed that the stellar density at the edges of galaxies gradually decreases to zero, and the boundaries of galaxies, as such, simply do not exist. We measured the behavior of the most extended subsystem, consisting of red giants, along the Z axis. In those edge-on galaxies, data on which we obtained from photometry of images, the behavior of the density of red giants was uniform: there was an exponential decrease in density to zero (Fig. 37) . Those. the galaxy along the Z axis has a sharply defined edge, and its stellar population has a well-defined boundary, and does not gradually disappear.
It is more difficult to study the behavior of stellar density along the radius of the galaxy at the point where the stars disappear. For edge-on galaxies, the disk size is more convenient to determine. A galaxy in Pegasus along the major axis shows a sharp drop in the number of red giants to zero (Fig. 36). Those. the galaxy has a quite sharp disk boundary, beyond which there are practically no red giants. Galaxy J10, in the first approximation, behaves in a similar way. The density of stars decreases, and at some distance from the center of the galaxy, a sharp decrease in their number is observed (Fig. 33). However, in this case, the decrease does not occur to zero. It is noticeable that red giants also exist outside the radius of their density jump, but beyond this limit they have a different spatial distribution than that which they had closer to the center. It is interesting to note that red giants are distributed similarly in the MZZ spiral galaxy. Those. exponential density drop, jump and continuation beyond the radius of this jump. There was an assumption that this behavior is related to the mass of the galaxy (ICIO is the most massive irregular galaxy, after the Magellanic clouds, in the Local Group), but a small galaxy was found with the same behavior of red giants (Fig. 37). The parameters of red giants beyond the jump radius are unknown, do they differ in age and metallicity? What is the type of spatial distribution for these distant stars? Unfortunately, today we cannot answer these questions. Research is needed on large telescopes with a wide field.
How large is the statistics of our research to speak of the existence of thick disks in late-type galaxies as a widespread or general phenomenon? In all galaxies that had sufficiently deep images, we revealed extended structures of giant giants
Having studied the NCT archive, we found images of 16 galaxies seen edge-on or face-on, and resolvable as red giants. These galaxies are located at distances of 2-5 Me. Their list: N002976, VB053, 000165, K52, K73, 000190, 000187, iOSA438, P00481 1 1, P0C39032, ROS9962, N002366, i0C8320, iOCA442, N00625, N001560.
The exponential drop in density for face-on galaxies and the appearance of the distribution of red giants around edge-on galaxies proves that in all these cases we see manifestations of thick disks.
6.4 Disks of red giants and the hidden mass of irregular galaxies.
Radio observations in H1 of spiral and dwarf galaxies showed little difference in the behavior of the rotation curves of galaxies. For both types of galaxies, for an explanation
To change the shape of the rotation curves, the presence of significant masses of invisible matter is required. Could the extended disks we have found in all irregular galaxies be the invisible matter we are looking for? The masses of the red giants themselves, which we observe in the disks, are of course completely insufficient. Using our observations of the 1C1613 galaxy, we determined the parameters of the fall in the density of giants towards the edge and calculated their total number and mass in the entire galaxy. It turned out that Mred/Lgal = 0.16. Those. taking into account the mass of giant branch stars slightly increases the mass of the entire galaxy. However, it should be remembered that the stage of a red giant is a relatively short stage in the life of a star. Therefore, significant corrections should be made to the mass of the disk, taking into account the number of less massive stars and those stars that have already passed the red giant stage. It would be interesting, based on very deep observations of nearby galaxies, to check the population of subgiant branches and to calculate their contribution to the total mass of the galaxy, but this is a matter for the future.
Conclusion
Summing up the results of the work, let us dwell once again on the main results.
The 6-m telescope obtained deep multicolor images of about 100 stellar-resolvable galaxies. Data archive created. These galaxies can be referred to when studying the stellar population, primarily high-luminosity variable stars of the LBV type. In the studied galaxies, the colors and brightness of all visible stars were measured. Hypergiants and supergiants of the highest luminosity are distinguished.
A large and homogeneous array of distance measurement data was obtained for all galaxies in the northern sky with velocities less than 500 km/s. The results obtained personally by the dissertator are very significant among the total amount of data. The obtained distance measurements make it possible to analyze the non-Hubble motions of the galaxies of the Local Complex, which limits the choice of a model for the formation of the Local "pancake" of galaxies.
Based on distance measurements, the composition and spatial structure of the nearest groups of galaxies in the northern sky is determined. The results of the work make it possible to carry out statistical comparisons of the parameters of groups of galaxies.
A study was made of the distribution of galaxies in the direction of the cluster of galaxies in Virgo. Several relatively close galaxies have been found located between the cluster and the Local Group. Distances were determined and galaxies belonging to the cluster itself and located in different parts of the periphery and center of the cluster were identified.
The distance to the clusters in Virgo was determined, which turned out to be 17.0 Mpc and Coma Veronica, equal to 90 Mpc. On this basis, the Hubble constant was calculated equal to R0 = 77 ± 7 km/s/Mpc.
Based on the photometry of the BTA and HST images, the brightness of the brightest stars in 10 galaxies of the N001023 group, located at a distance of 10 Mpc, was measured. The distances to galaxies are determined and the Hubble constant in this direction is calculated. It is concluded that there is a small velocity gradient between the Local Group and the NGC1023 group, which can be
121 to be explained by the relatively small mass of the cluster of galaxies in Virgo in comparison with all the surrounding galaxies.
Based on studies of the spatial distributions of red giants in late-type galaxies, thick and extended disks of old stars have been discovered. The size of such disks is 2-3 times larger than the size of the visible body of the galaxy. It is found that the boundaries of these disks have rather sharp edges, beyond which there are very few stars.
Despite the large-scale studies of the distances to the galaxies of the northern sky, there are no fewer questions for the future than there were before the start of work. But these questions are of a different quality, since now, especially in connection with the work of space telescopes, it has become possible to make accurate measurements that can change our understanding of near space. This concerns the composition, structure, and kinematics of nearby groups of galaxies, the distances to which are intensively determined by the TCM method.
The periphery of galaxies is attracting more and more attention, especially because of the search for dark matter and the history of the formation and evolution of galactic disks. It is remarkable that in autumn 2002 the first meeting on the periphery of galaxies will be held at the Lovell Observatory.
Thanks
Over the long years that the work on the topic of the dissertation presented by me was carried out, many people, in one way or another, assisted me in my work. I am grateful to them for this support.
But it is especially pleasant for me to express my gratitude to those whose help I felt constantly. Without the highest qualifications of Galina Korotkova, the work on the dissertation would have dragged on for an incredibly long time. Olga Galazutdinova's passion and tenacity in doing the work allowed me to get results on a large number of objects in Virgo and N001023 in a fairly short period of time. Igor Drozdovsky, with his small service programs, gave us great help in the photometry of tens of thousands of stars.
I am grateful to the Russian Foundation for Basic Research, whose grants I received (95-02-05781, 97-02-17163,00-02-16584), for financial support for eight years, which allowed me to conduct research more effectively.
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Usually galaxies are found in small groups containing ten members, often combined into vast clusters of hundreds and thousands of galaxies. Our Galaxy is part of the so-called Local Group, which includes three giant spiral galaxies (our Galaxy, the Andromeda nebula and the nebula in the constellation Triangulum), as well as more than 15 dwarf elliptical and irregular galaxies, the largest of which are the Magellanic Clouds. The average size of galaxy clusters is about 3 Mpc. In some cases, their diameter can exceed 10–20 Mps. They are divided into scattered (irregular) and spherical (regular) clusters. Open clusters do not have a regular shape and have blurred outlines. The galaxies in them are very weakly concentrated towards the center. An example of a giant open cluster is the nearest cluster of galaxies in the constellation Virgo. In the sky, it occupies about 120 square meters. degrees and contains several thousand predominantly spiral galaxies. The distance to the center of this cluster is about 11 Mpc. Spherical clusters of galaxies are more compact than open ones and have spherical symmetry. Their members are noticeably concentrated towards the center. An example of a spherical cluster is the cluster of galaxies in the constellation Coma Berenices, which contains a lot of elliptical and lenticular galaxies (Fig. 242). Its diameter is almost 12 degrees. It contains about 30,000 galaxies brighter than 19 photographic magnitude. The distance to the center of the cluster is about 70 Mpc. Many rich clusters of galaxies are associated with powerful extended X-ray sources, the nature of which is most likely associated with the presence of hot intergalactic gas, similar to the coronas of individual galaxies.
There is reason to believe that clusters of galaxies, in turn, are also unevenly distributed. According to some studies, the clusters and groups of galaxies surrounding us form a grandiose system - the Supergalaxy. In this case, individual galaxies apparently concentrate towards a certain plane, which can be called the equatorial plane of the Supergalaxy. The cluster of galaxies just discussed in the constellation Virgo is at the center of such a gigantic system. The mass of our Supergalaxy should be about 1015 solar masses, and its diameter should be about 50 Mpc. However, the reality of the existence of such clusters of second-order galaxies currently remains controversial. If they exist, then only as a weakly expressed inhomogeneity of the distribution of galaxies in the Universe, since the distances between them can slightly exceed their sizes. On the evolution of galaxies The ratio of the total amount of stellar and interstellar matter in the Galaxy changes with time, since stars form from interstellar diffuse matter, and at the end of their evolutionary path they return only part of the matter to interstellar space; some of it remains in white dwarfs. Thus, the amount of interstellar matter in our Galaxy should decrease with time. The same should happen in other galaxies. Being processed in the stellar depths, the matter of the Galaxy gradually changes its chemical composition, being enriched with helium and heavy elements. It is assumed that the Galaxy was formed from a gas cloud, which consisted mainly of hydrogen. It is even possible that, apart from hydrogen, it did not contain any other elements. Helium and heavy elements were formed in this case as a result of thermonuclear reactions inside stars. The formation of heavy elements begins with the triple helium reaction 3He4 ® C 12, then C 12 combines with a-particles, protons and neutrons, the products of these reactions undergo further transformations, and thus more and more complex nuclei appear. However, the formation of the heaviest nuclei, such as uranium and thorium, cannot be explained by gradual growth. In this case, one would inevitably have to go through the stage of unstable radioactive isotopes, which would decay faster than they could capture the next nucleon. Therefore, it is assumed that the heaviest elements at the end of the periodic table are formed during supernova explosions. A supernova explosion is the result of the rapid contraction of a star. At the same time, the temperature rises catastrophically, chain thermonuclear reactions take place in the contracting atmosphere, and powerful neutron fluxes arise. The intensity of neutron fluxes can be so high that intermediate unstable nuclei do not have time to collapse. Before that happens, they capture new neutrons and become stable. As already mentioned, the abundance of heavy elements in the stars of the spherical component is much less than in the stars of the flat subsystem. This is apparently due to the fact that the stars of the spherical component were formed in the very initial stage of the evolution of the Galaxy, when the interstellar gas was still poor in heavy elements. At that time, the interstellar gas was an almost spherical cloud, the concentration of which increased towards the center. The stars of the spherical component that formed in this epoch also retained the same distribution. As a result of collisions of clouds of interstellar gas, their speed gradually decreased, kinetic energy turned into thermal energy, and the general shape and size of the gas cloud changed. Calculations show that in the case of rapid rotation, such a cloud should have taken the form of an oblate disk, which is what we observe in our Galaxy. Stars formed at a later time therefore form a flat subsystem. By the time the interstellar gas formed into a flat disk, it had been processed in the stellar interior, the abundance of heavy elements had increased significantly and the stars of the flat component were therefore also rich in heavy elements. Often the stars of the flat component are called second generation stars, and the stars of the spherical component are called first generation stars, to emphasize the fact that the stars of the flat component were formed from matter that had already been in the stellar interior. The evolution of other spiral galaxies probably proceeds in a similar way. The shape of the spiral arms, in which the interstellar gas is concentrated, is apparently determined by the direction of the lines of force of the general galactic magnetic field. The elasticity of the magnetic field, to which the interstellar gas is "glued", limits the flattening of the gaseous disk. If only gravity acted on the interstellar gas, its compression would continue indefinitely. In this case, due to its high density, it would quickly condense into stars and practically disappear. There is reason to believe that the rate of star formation is approximately proportional to the square of the density of the interstellar gas.
If the galaxy rotates slowly, then the interstellar gas is collected by gravity in the center. Apparently, in such galaxies the magnetic field is weaker and hinders the compression of the interstellar gas less than in rapidly rotating ones. The high density of interstellar gas in the central region leads to the fact that it is quickly consumed, turning into stars. As a result, slowly rotating galaxies should have an approximately spherical shape with a sharp increase in stellar density in the center. We know that elliptical galaxies have just such characteristics. Apparently, the reason for their difference from the spiral ones lies in the slower rotation. From what has been said above, it is also clear why there are few stars of early classes and little interstellar gas in elliptical galaxies.
Thus, the evolution of galaxies can be traced from the stage of a gaseous cloud of approximately spherical shape. The cloud consists of hydrogen, it is not uniform. Separate clumps of gas, moving, collide with each other - the loss of kinetic energy leads to cloud compression. If it rotates quickly, a spiral galaxy is obtained, if it rotates slowly, an elliptical one. It is natural to ask why the matter in the Universe broke up into separate gas clouds, which later became galaxies, why we observe the expansion of these galaxies, in what form was the matter in the Universe before the formation of galaxies.
Usually galaxies are found in small groups containing ten members, often combined into vast clusters of hundreds and thousands of galaxies. Our Galaxy is part of the so-called Local Group, which includes three giant spiral galaxies (our Galaxy, the Andromeda nebula and the nebula in the constellation Triangulum), as well as more than 15 dwarf elliptical and irregular galaxies, the largest of which are the Magellanic Clouds. The average size of galaxy clusters is about 3 Mpc. In some cases, their diameter can exceed 10–20 Mpc. They are divided into scattered (irregular) and spherical (regular) clusters. Open clusters do not have a regular shape and have blurred outlines. The galaxies in them are very weakly concentrated towards the center. An example of a giant open cluster is the closest cluster of galaxies to us in the constellation Virgo (241). In the sky, it occupies about 120 square meters. degrees and contains several thousand predominantly spiral galaxies. The distance to the center of this cluster is about 11 Mpc. Spherical clusters of galaxies are more compact than open ones and have spherical symmetry. Their members are noticeably concentrated towards the center. An example of a spherical cluster is the cluster of galaxies in the constellation Coma Berenices, which contains a large number of elliptical and lenticular galaxies (242). Its diameter is almost 12 degrees. It contains about 30,000 galaxies brighter than 19 photographic magnitude. The distance to the center of the cluster is about 70 Mpc. Many rich clusters of galaxies are associated with powerful extended X-ray sources, the nature of which is most likely associated with the presence of hot intergalactic gas, similar to the coronas of individual galaxies. There is reason to believe that clusters of galaxies, in turn, are also unevenly distributed. According to some studies, the clusters and groups of galaxies surrounding us form a grandiose system - the Supergalaxy. In this case, individual galaxies apparently concentrate towards a certain plane, which can be called the equatorial plane of the Supergalaxy. The cluster of galaxies just discussed in the constellation Virgo is at the center of such a gigantic system. The mass of our Supergalaxy should be about 1015 solar masses, and its diameter should be about 50 Mpc. However, the reality of the existence of such clusters of second-order galaxies currently remains controversial. If they exist, then only as a weakly expressed inhomogeneity in the distribution of galaxies in the Universe, since the distances between them can slightly exceed their sizes.
The most striking feature of the spatial distribution of globular clusters in the Galaxy is a strong concentration towards its center. On fig. 8-8 shows the distribution of globular clusters over the entire celestial sphere, here the center of the Galaxy is in the center of the figure, the north pole of the Galaxy is at the top. There is no visible zone of avoidance along the plane of the Galaxy, so interstellar extinction in the disk does not hide a significant number of clusters from us.
On fig. 8-9 shows the distribution of globular clusters along the distance from the center of the Galaxy. There is a strong concentration towards the center - most globular clusters are located in a sphere with a radius of ≈ 10 kpc. It is within this radius that almost all globular clusters formed from matter are located. single protogalactic cloud and formed subsystems of the thick disk (clusters with > -1.0) and halo proper (less metallic clusters with extreme blue horizontal branches). Metal-poor clusters with horizontal branches anomalously red for their metallicity form a spheroidal subsystem accreted halo radius ≈ 20 kpc. About a dozen more distant clusters belong to the same subsystem (see Fig. 8-9), among which there are several objects with anomalously high metal contents.
Clusters of the accreted halo are believed to be selected by the gravitational field of the Galaxy from satellite galaxies. On fig. 8-10 schematically shows this structure according to Borkova and Marsakov from the Southern Federal University. Here, the letter C denotes the center of the Galaxy, S is the approximate position of the Sun. At the same time, accumulations with a high content of metals belong to the oblate subsystem. We will dwell on a more detailed substantiation of the division of globular clusters into subsystems in § 11.3 and § 14.3.
Globular clusters are also common in other galaxies, and their spatial distribution in spiral galaxies resembles the distribution in our Galaxy. Noticeably different from the Galactic clusters of the Magellanic Clouds. The main difference is that along with old objects, the same as in our Galaxy, young clusters are also observed in the Magellanic Clouds - the so-called blue globular clusters. Probably, in the Magellanic Clouds, the epoch of the formation of globular clusters either continues or ended relatively recently. It seems that there are no young globular clusters in our Galaxy similar to the blue clusters of the Magellanic Clouds, so the era of the formation of globular clusters in our Galaxy ended a very long time ago.
Globular clusters are evolving objects that gradually lose stars in the process. dynamic evolution . Thus, all clusters for which it was possible to obtain a high-quality optical image showed traces of tidal interaction with the Galaxy in the form of extended deformations (tidal tails). Currently, such lost stars are also observed in the form of increases in stellar density along the galactic orbits of clusters. Some clusters that orbit near the galactic center are destroyed by its tidal action. At the same time, the galactic orbits of clusters also evolve due to dynamic friction.
On fig. 8-11 is a dependency diagram masses of globular clusters
from their galactocentric positions. Dashed lines mark the region of slow evolution of globular clusters. The upper line corresponds to the critical value of the mass that is stable for effects of dynamic friction
, leading to a slowdown of a massive star cluster and its fall to the center of the Galaxy, and the lower one - for dissipation effects
taking into account tidal clusters during the flight through the galactic plane. The reason for dynamic friction is external: a massive globular cluster moving through the stars of the field attracts the stars it meets on its way and forces them to fly around itself behind along a hyperbolic trajectory, due to which an increased density of stars is formed behind it, creating a decelerating acceleration. As a result, the cluster slows down and begins to approach the galactic center along a spiral trajectory until it falls on it in a finite time. The greater the mass of the cluster, the shorter this time. Dissipation (evaporation) of globular clusters occurs due to the internal mechanism of stellar-stellar relaxation that is constantly operating in the cluster, distributing stars according to velocities according to Maxwell's law. As a result, the stars that have received the largest increments of speed leave the system. This process is significantly accelerated by the passage of the cluster near the galactic core and through the galactic disk. Thus, with a high probability we can say that the clusters lying on the diagram outside the area bounded by these two lines are already ending their life path.
It's interesting that accreted globular clusters discover the dependence of their masses on their position in the Galaxy. The solid lines in the figure represent direct regressions for genetically related (black dots) and accreted (open circles) globular clusters. It can be seen that genetically related clusters show no change in the average mass with increasing distance from the galactic center. On the other hand, there is a clear anticorrelation for accreted clusters. Thus, the question that needs to be answered arises, why is there an increasing deficit of massive globular clusters in the outer halo with increasing galactocentric distance (almost empty upper right corner in the diagram)?
