A BRIEF OVERVIEW OF DWARF GALAXIES

N.R.Begisheva; I.U.Tajibaev

Central Asian Research Journal For Interdisciplinary Studies (CARJIS)

www.carjis.org DOI: 10.24412/2181-2454-2022-6-563-569

A BRIEF OVERVIEW OF DWARF GALAXIES

N.R.Begisheva I.U.Tajibaev

Chirchik State Pedagogical Institute of Tashkent region

ABSTRACT

The article gives a brief overview of dwarf galaxies (dE-galaxies). They do not represent any group that stands out sharply in their features, and their properties continue the properties of ordinary elliptical galaxies, extrapolating into the region of low luminosities and sizes.

Keywords: dwarf galaxies - dwarf, elliptical, spheroidal, irregular, blue, spiral, ultra-compact galaxies.

Discovering new amazing pages of astronomy is the study of galaxies. In Zasov's brochure "Dwarf Galaxies" [1] it is told about galaxies of a special type about which very little has been said and written so far, about the so-called dwarf galaxies.

The world of galaxies is surprisingly diverse. When trying to describe a typical galaxy, one may encounter considerable difficulties, such as linear size, luminosity, mass, relative gas content, chemical composition and, finally, their appearance vary greatly from one object to another. Especially impressive is the difference in mass, which, as in the case of stars, is one of the most important characteristics of galaxies. But if the masses of stars differ at least a few hundred times, then galaxies - more than a million times. The largest and smallest galaxies differ in mass, about the same as stars from planets. First of all, it is appropriate to note that most of the observed galaxies are many thousands of parsecs in size and consist of billions of stars continuously moving in a rarefied gas-dust medium.

Dwarf galaxies are called low-luminosity galaxies. And if we do not take into account the two elliptical satellites of the spiral galaxy closest to us - the Andromeda Nebula - which, by luminosity, lie somewhere on the border between dwarf and normal galaxies, the first indisputably dwarf systems were discovered only in the late 30s of our century [1]. The discovery was made at the Harvard University Observatory in South Africa.

In Shapley's article [1] " Distribution of 89 thousand galaxies in the South Polar

Central Asian Research Journal For Interdisciplinary Studies (CARJIS)

www.carjis.org DOI: 10.24412/2181-2454-2022-6-563-569

region", a table with the results of counting galaxies on the studied photographic plates was given. In the line reserved for the description of the photo plate with this cluster, the full number of registered galaxies of 1772 objects was noted, and a note was made to it. And it indicated that 1 sq. degree of the sky was excluded from consideration, since a previously unknown star cluster was noticed there, interfering with the counting of galaxies (it turned out to be a hindrance when performing the main task). The following discoveries came with the appearance at the disposal of astronomers of the Palomar Sky Survey photographs taken in the 50s using the wide-angle 120-centimeter Schmidt camera of the Mount Palomar Observatory [1].

Subsequently, it turned out that there are various types of dwarf galaxies, including quite unlike the systems found in the constellations of the Sculptor and the Furnace.

The situation changed dramatically in the 70s, which was due to two reasons. Firstly, the new large astronomical instruments that appeared made it possible to study a large number of previously inaccessible dwarf galaxies, and this in many cases led to the identification of their unusual properties that needed to be explained. Secondly, the study of the processes occurring in galaxies of various types has shown that observations of dwarf galaxies can serve as a touchstone to verify the correctness of existing ideas about how galaxies were formed, how they evolve and how they interact with their surroundings.

Most often, dwarf galaxies include such galaxies whose absolute stellar magnitude is at least - 16m, the corresponding luminosity value does not exceed 4 * 108 Lc (Lc is the luminosity of the Sun), which is about 100 times less than the luminosity of such galaxies as ours or the Andromeda Nebula. That is why dwarf galaxies are very difficult to observe from long distances [1,2]. There are several main types of dwarf galaxies:

-dwarf elliptical galaxy (dE), similar to elliptical galaxies;

- dwarf spheroidal galaxy (dSph), a subtype of dE, characterized by particularly low surface brightness;

- a dwarf irregular galaxy (dIr) appears to be the equivalent of ordinary irregular galaxies in the region of dwarf galaxies. Clouds of ionized hydrogen have been detected in a number of nearby dIr galaxies;

- dwarf blue compact galaxy (dBCG or BCD) has signs of active star formation;

- ultra-compact dwarf galaxies (UCD) are a class of very compact galaxies

Central Asian Research Journal For Interdisciplinary Studies (CARJIS)

www.carjis.org DOI: 10.24412/2181-2454-2022-6-563-569

containing about 108 stars with a characteristic transverse size of about 50 pcs;

- dwarf spiral galaxy, an analogue of spiral galaxies, but unlike normal galaxies, it is extremely rare.

The first lists of dwarf galaxies were published by the famous astrophysicist F. Zwicky in the second half of the 1960s based on direct images of the Palomar Sky Survey. Currently there are many catalogues of dwarf galaxies. But the information in such catalogs is insufficient for a complete physical study of dwarf galaxies [3].

The article [4] presents a list of 75 isolated dwarf galaxies of late types that do not have neighboring galaxies with a relative radial velocity difference of less than 500 km/s and projection distances of less than 500 kpc. They were isolated among ~2000 dwarf galaxies located in the volume of a Local supercluster limited by the radial velocity VLC<3500 km/s. Isolated dwarf galaxies of late types do not differ in their sizes, luminosities and amplitude of internal movements. However, the median value of the mass of neutral hydrogen per unit of luminosity is twice as high for them as for dwarf galaxies of late types in the groups [4]. It is also described about the identification of 10 supposed isolated spheroidal dwarf galaxies. The discovery of isolated dwarf galaxies inhabited exclusively by old stars is of great interest for modern cosmological scenarios of galaxy formation.

To capture individual stars in photographs of these dwarf galaxies, i.e., as astronomers say, to resolve galaxies into stars, it was possible at great cost in the 40s by V. Baade, who worked on the largest telescope in the world at that time - the 2.5-meter Mount Palomar reflector. I must say that even at the present time, even with the help of the best telescopes, it is not an easy task to resolve the satellites of the Andromeda Nebula to the stars. For a long time, the stellar composition of these small galaxies, as well as the central region of the Andromeda Nebula itself, remained mysterious. The photos did not show the presence of the brightest stars -blue supergiants, although these stars are confidently observed in the spiral branches of the nearby Andromeda Nebula. Having set himself the task of resolving the central part of the Andromeda Nebula and its elliptical satellites to the stars, V. Baade began to seriously prepare for its implementation. These objects were known to have a reddish color, and he assumed (and was not mistaken) that this is the color of the brightest stars that they contain. Therefore , V. Baade rejected plates that react to blue rays, commonly used in astronomical photography, and chose the most sensitive of the available photographic plates at that time, perceiving orange and red colors. However, these plates had significantly lower sensitivity than the "blue" ones, and in

Central Asian Research Journal For Interdisciplinary Studies (CARJIS)

www.carjis.org DOI: 10.24412/2181-2454-2022-6-563-569

order to increase it, it was necessary to specially treat them with ammonia before using the plates. But even after that, the sensitivity was not too high, and in order to have at least some hope of capturing stars on them that were inaccessible to the "blue" plates, it was necessary to count on many hours of exposure. The fact is that many hours of exposure cannot be done on highly sensitive "blue" plates: after 1.5 hours, the faint glow of the night sky tightened them with a dense veil. According to Baade's calculations, such an approach was supposed to make it possible to obtain stars on the "red" plates 0.5 t (1.6 times) weaker than on the "blue" ones. How else can the penetrating power of the telescope be raised, i.e. his ability to register faint stars? People familiar with the specifics of astronomical observations are well aware that the capabilities of a telescope as an optical instrument vary greatly from night to night, even if they are equally clear, and sometimes during one night. This is due to the different state of the atmosphere, and for large telescopes - also with the state of the mirror lens, the reflecting surface of which is subject to temperature deformations due to temperature differences, both between different parts of the mirror and between the mirror and the air environment. And only recently have we learned how to make large mirrors out of a substance that is practically not subjected to thermal expansion. Subsequently, Baade wrote about this: "It was impossible to hope to achieve success if you simply insert a "red" plate into the cassette of a 2.5-meter telescope, make an exposure, develop and try to see something. It was quite clear that the stars would be very faint and in all likelihood extremely closely spaced. This is at the limit of the resolution of a 2.5-meter telescope, and, obviously, one should be very careful and not neglect the slightest chance. In order to keep the resolution as high as possible, it was necessary, firstly, to make observations only when obtaining the best images, when the turbulent disk of stars is very small. Secondly, it was worth observing only on those nights when the shape of the mirror was close to ideal, without the "blockage" of the edges, which always leads to an increase in the disk of the star. Thirdly (and this was the main problem), something had to be done when the focus changes occurred due to the fact that the mirror of the 2.5-meter telescope is made of old-brand glass. Even when the nights in this sense were satisfactory, there were changes in the focal length from 1.5 to 2 mm, and there were also nights when these changes reached 5-6 mm." As a result, V. Baade had to invent his own way of continuously checking the correctness of the image focusing, which allowed not to interrupt the many-hour exposure. The preparation for the decisive observations lasted more than a year. Finally, in the autumn of 1943 over the course of several

Central Asian Research Journal For Interdisciplinary Studies (CARJIS)

www.carjis.org DOI: 10.24412/2181-2454-2022-6-563-569

nights, the long-awaited negatives were obtained with exceptionally good image quality, in which the satellites of the Andromeda Nebula (as well as its central part, consisting of similar stars) were strewn with the smallest points of stars. So from a distance of almost 700 thousand, the brightest stars of dwarf elliptical galaxies looked like pcs. It should be said that one important circumstance contributed to the success of their detection. There were really dark nights over the observatory, because the war-related blackout of the giant city of Los Angeles with its busy suburbs located nearby had not yet been canceled.

By this time, astronomers were well acquainted with the most diverse types of stars, but the stars photographed by V. Baade puzzled the scientist. For ordinary red stars, they had too high luminosity. It seemed strange that there are almost no such stars in the observed stellar neighborhood of the Sun, and in dwarf elliptical galaxies they make the main contribution to the radiation of the galaxy. It was only after some time that a guess dawned on V. Baade: the globular clusters of our Galaxy consist of exactly the same stars. These clusters are rather distant associations of hundreds of thousands of stars (the nearest of them is several thousand light-years away from us). Their age exceeds 10 billion years, i.e. they are real relics of the stellar world. Further research confirmed V. Baade's guess. The brightest stars of dwarf elliptical galaxies, as well as globular clusters, turned out to be red giants of high luminosity - stars that have greatly inflated and changed their internal structure, since during their long life the main nuclear fuel (hydrogen) has largely been exhausted in the stellar bowels. A characteristic feature of dwarf galaxy stars is the low content of heavy chemical elements in the stellar atmosphere (although not as low as in globular clusters). Looking ahead, we note that this so-called deficiency of heavy elements is characteristic of dwarf galaxies of all types. "Normal" elliptical galaxies, which are not dwarf in their luminosity, also consist of old stars, although not as much depleted of heavy elements as in dwarf galaxies. Apparently, star formation in "normal" E-galaxies almost ended many billions of years ago, the history of dE-galaxies, as it turned out, may be different. This is clearly seen by the example of all the same satellites of the Andromeda Nebula [1].

In recent years, the study of isolated galaxies has noticeably intensified. Galaxies located in a region with a low local density of matter serve as a reference sample for studying the influence of the environment on the processes of star formation and evolution of galaxies, as well as for testing various models of the origin of galaxies. Isolated dwarf galaxies, including those of low surface brightness,

Central Asian Research Journal For Interdisciplinary Studies (CARJIS)

www.carjis.org DOI: 10.24412/2181-2454-2022-6-563-569

are of particular interest for these tasks.

Typical characteristics of dwarf galaxies (linear dimensions of several kpc, absolute B magnitudes weaker than - 16m, average surface brightness <SB> weaker than 25B mag/arcsec2) [5] explain the difficulty of detecting such objects at large distances. Therefore, dwarf galaxies are poorly represented in catalogs limited by the apparent magnitude CGCG [6], m <15.7) or angular diameter, a>1\ (UGC [7], ASO [8]). Most of the objects in the UGC catalog, which were classified by their low surface brightness as "dwarfs", turned out to be ordinary spiral galaxies after measurements of radial velocities. The relative number of true dwarf galaxies in the catalog [7] is only 2%. In the catalog of isolated galaxies [6], created on the basis of the CGCG catalog, dwarf galaxies are also few in number. The detection of dwarf galaxies, most of which have low surface brightness, is a separate, very time-consuming task.

The situation with the representation of dwarf systems changes dramatically when a sample of galaxies is considered, limited not by the flow, but by distance. Thus, in the catalog of neighboring CNG galaxies [5], which includes 451 objects of Local volume with distances up to 10 Mpc, about 85% of the galaxies are dwarf, mainly of low and very low surface brightness.

The theory of the origin of galaxies is based on the principle of gravitational instability. The principle states that the particles of matter cannot always be in a uniformly distributed state in space.

REFERENCES

1. A.V.Zasov Dwarf Galaxies, Znanie, Moscow, 1984

2. I.D.Karachentsev, E.I.Kaisina Dwarf galaxies in Local volume // Astrophysical Bulletin, 2019, volume 74, No. 2, pp. 119-137

3. S.Kutlimuradov, I.Tajibaev, S.Nuritdinov Special consolidated catalog of dwarf galaxies in the universe up to distances 121 Mpk // Uzbek Physical Journal, 2020, 22(4), 203-217

4. V.E.Karachentseva, I.D.Karachentsev, M.E.Sharina Isolated dwarf galaxies in the local supercluster and its surroundings // Astrophysics, 2010, 54, 4, 513-527

5. I.D.Karachentsev, V.E.Karachentseva, W.K.Huchtmeier, D.I.Makarov, Astron.J., 2004, 127, 2031 (CNG)

6. F.Zwicky, E.Herzog, M.Karpowich, C.T.Kowal, P. Wild Catalogue of Galaxies and Clusters of Galaxies, California Institute of Technology, Pasadena, 1961-1968, I-

Central Asian Research Journal For Interdisciplinary Studies (CARJIS)

www.carjis.org DOI: 10.24412/2181-2454-2022-6-563-569

VI (CGCG).

7. P.Nilson Uppsala General Catalogue of Galaxies, Uppsala Astron. Observ., 1973, 6, 1 (UGC)

8. E. Lauberts The ESO/Uppsala Survey of the ESO (B) Atlas, Munich: ESO, 1982

9. V.E.Karachentseva Catalog of isolated Galaxies // Post. CAO, 1973, 8, 3

A BRIEF OVERVIEW OF DWARF GALAXIES Текст научной статьи по специальности «Науки о Земле и смежные экологические науки»

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