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1.
A. V. Tutukov 《Astronomy Reports》2006,50(6):439-450
We consider the evolution of galaxies in dense galactic clusters. Observations and theoretical estimates indicate that this evolution may be specified to a large extent by collisions between galaxies, as well as interactions between the gaseous components of disk galaxies and intergalactic gas. We analyze collisions between disk galaxies with gaseous components using a simple model based on a comparison of the duration of a collision and the characteristic cooling time for the gas heated by the collision, and also of the relative masses of stars and gas in the colliding disk galaxies. This model is used to analyze scenarios for collisions between disk galaxies with various masses as a function of their relative velocities. Our analysis indicates that galaxies can merge, lose one or both of their gaseous components, or totally disintegrate as a result of a collision; ultimately, a new galaxy may form from the gas lost by the colliding galaxies. Disk galaxies with mass M G and velocities exceeding ~300 (M G/1010 M ⊙)1/2 km/s in intergalactic gas in clusters with densities ~10?27 g/cm3 can lose their gas due to the pressure of inflowing intergalactic gas, thereby developing into E(SO) galaxies. 相似文献
2.
The dynamics of the dust and gas in the tidal region of the M81 galaxy group have been analyzed, and the drift of the dust relative to the gas has been estimated, including the drift due to the action of radiation pressure from stars in M81. It is concluded that a large fraction of the gas in the tidal region is in the form of ionized hydrogen HII that shields the observedHI gas from the extragalactic Lyman continuum: the observed atomic gas could be only 10% of the total mass of gas. Only then it is possible to satisfactorily explain the excess dust abundance, which exceeds the Galactic value by a factor of six. By analogy, extended HI disks in galaxies with sizes appreciably larger than the stellar disks could be surrounded by HII envelopes with a comparable or greater mass. Such disks could play an important role in supporting prolonged star formation in galaxies with extended HI disks. Associated observational manifestations are discussed. Such HII envelopes outside HI disks could be detectable in absorption in Ly α and lines of ions of heavy elements. 相似文献
3.
We have calculated evolutionary tracks for stars with high abundances of heavy elements. An abundance increase from the solar level (~0.02) to 0.1 (for ΔY/ΔZ=0–2.4), which corresponds to the central regions of the disk components of giant galaxies, shifts the main sequence towards lower effective temperatures; however, it does not appreciably affect either the luminosity of the stars or their lifetime on the main sequence. Increasing the heavy-element abundance to 0.2 for ΔY/ΔZ=2.4 shifts the main sequence towards higher effective temperatures, appreciably increases the luminosity of the stars, and substantially accelerates their evolution. 相似文献
4.
Results of numerical simulations of a collision of the gaseous components of two identical disk galaxies during a head-on
collision of the galaxies in the polar direction are presented. When the relative velocity of the galaxy collision is small,
their gaseous components merge. At high relative velocities (100–500 km/s), the massive stellar components of the galaxies
(M
g
= 109
M
⊙) pass through each other nearly freely, leaving behind the gaseous components, which are decelerated and heated by the collision.
If the overall gaseous component of the colliding galaxies is able to cool to the virial temperature during the collision,
a new galaxy forms. At velocities V ≥ 500 km/s, the gaseous component does not have time to cool, and the gas is scattered into intergalactic space, supplying
it with heavy elements produced in supernovae in the colliding galaxies. High-velocity (V ≥ 100 km/s) collisions of identical low-mass galaxies (M
g
≤ 109
M
⊙) whose mass is dominated by the mass of gas lead to the disruption of their stellar components. The overall gaseous component
forms a new galaxy when V ≤ 500 km/s, and is scattered into intergalactic space if the velocity becomes higher than this. A galaxy collision increases
the star-formation rates in the disk galaxies by nearly a factor of 100. Rotation of the colliding galaxies in the same direction
increases the changes of the disruption of both the stellar and gaseous components of the galaxies. The merger of galaxies
during their collision can explain the presence of gaseous disks rotating opposite to the rotation of the stellar component
in some ordinary elliptical galaxies. Moreover, galaxy mergers can help explain the origin of a comparatively young stellar
population in some elliptical galaxies. 相似文献
5.
Published data on rotation curves and the radial distribution of the surface density of neutral hydrogen (HI) in galaxies
with a low gas content are used to calculate radial profiles of the volume density of HI in the planes of the galactic disks.
A self-consistent model for the disks is used, taking into account the self-gravitation of the gas and the presence of a pseudo-isothermal,
massive halo. Eleven low-surface-brightness (LSB) galaxies and three S0 galaxies in which HI is detected are considered. The
gaseous and stellar disks are taken to be in equilibrium and axially symmetric, and the velocity dispersion in the stellar
disk to be equal to the marginal value for gravitational perturbations; in general, this gives an upper limit for the gas
density. It is shown that, on average, the gas volume densities are two orders of magnitude lower in LSB galaxies than in
galaxies with normal brightnesses at the same R values, while the three S0 galaxies occupy an intermediate position. The volume density of gas observed at the galaxy peripheries
are less than 10−27 g/cm3, even in the plane of the disk. The role of the UV background in ionizing outer regions is discussed. The obtained gas densities
can be used to estimate the star-forming efficiency in regions of low density. 相似文献
6.
The paper analyzes possible origins of stars located in intergalactic space that are not bound to specific galaxies, which
comprise 15–50% of all stars in galaxy clusters. Some such stars can form in streams of intergalactic gas flowing around gas-rich
disk galaxies moving in the cluster. Others may be the products of the decay of young, low-mass, spheroidal galaxies after
the loss of their gaseous components during an initial burst of star formation. The decay of low-mass disk galaxies moving
at high speeds after they have lost their gaseous components due to the pressure of the incident flow of dense intergalactic
gas is possible in the cluster core. The largest fraction of intergalactic stars are probably produced by the partial disruption
of galaxies as a result of close passages, collisions, or mergers. Collisions of low-mass, gas-rich galaxies are especially
good suppliers of intergalactic stars. Both stars from decaying stellar components of galaxies and stars arising in the gaseous
components of colliding galaxies can be supplied to the intergalactic medium. The merger of galaxies harboring supermassive
black holes in their nuclei could lead to the partial or total disruption of these galaxies during the deceleration of the
binary black hole that is formed during the merger. An enhanced density of intergalactic stars is observed in the cores of
galaxy clusters, underscoring the role of galaxy collisions in the formation of the intergalactic stellar population, since
the frequency of galaxy collisions grows with their density. 相似文献
7.
8.
Arguments indicating that galaxies and galaxy clusters should be considered open, forming systems are presented. Galaxies interact with the intergalactic medium, and are not in virial equilibrium (determined by gravitation and rotation). The usual interpretation of the rotation curves of the outer regions of galaxies beyond the visible stellar disk—that they imply the presence of a massive dark-matter halo— could be erroneous in this case: if the intergalactic medium is being accreted in these regions, the orbital speeds of clouds of neutral hydrogen will not be determined purely by the gravitation of the mass inside their orbits. Galaxy clusters accrete matter (intergalactic gas and galaxies) from the filaments of the large-scale structure at whose intersections they are located. Only their inner regions can approach virial equilibrium. Therefore, the high speeds of galaxies and the high temperature of the intergalactic gas in clusters does not necessarily imply the presence of a high mass of dark matter in galaxy clusters. 相似文献
9.
We explore the hypothesis that the outer boundaries (“cutoffs”) of the stellar disks observed in many galaxies are determined by the condition of local gravitational (Jeans) stability for the gaseous protodisks at large galactocentric distances. The ratio of the surface density of the disk Σdisk to the critical value for Jeans instability Σcrit is computed for a number of galaxies, assuming that the gas velocity dispersion in the forming disk corresponded to its current thickness and that the disk itself is in a quasi-equilibrium state. The mean estimated stellar velocity dispersion in the vicinity of the cutoff (12 km/s) is close to the typical velocity dispersions of gaseous clouds in disk galaxies. At greater distances, such velocity dispersions should ensure gravitational stability of the disk both at the present epoch and in the past. The cutoff radius of the disk R cut is correlated with other disk parameters, and the ratio Σdisk/Σcrit at R cut is close to unity in most cases. We conclude that the available observational data agree well with the hypothesis that stellar disk cutoffs are due to a rapid decrease in the star-formation rate beyond R cut, where the gaseous disk has always been stable. 相似文献
10.
We consider the formation of massive stars under the assumption that a young star accretes material from the protostellar cloud through its accretion disk while losing gas in the polar directions via its stellar wind. The mass of the star reaches its maximum when the intensity of the gradually strengthening stellar wind of the young star becomes equal to the accretion rate. We show that the maximum mass of the forming stars increases with the temperature of gas in the protostellar cloud T 0, since the rate at which the protostellar matter is accreted increases with T 0. Numerical modeling indicates that the maximum mass of the forming stars increases to ~900 M ⊙ for T 0 ~ 300 K. Such high temperatures of the protostellar gas can be reached either in dense star-formation regions or in the vicinity of bright active galactic nuclei. It is also shown that, the lower the abundance of heavy elements in the initial stellar material Z, the larger the maximum mass of the star, since the mass-loss rate due to the stellar wind decreases with decreasing Z. This suggests that supermassive stars with masses up to 106 M ⊙ could be formed at early stages in the evolution of the Universe, in young galaxies that are almost devoid of heavy elements. Under the current conditions, for T 0 = (30–100) K, the maximum mass of a star can reach ~100M ⊙, as is confirmed by observations. Another opportunity for the most massive stars to increase their masses emerges in connection with the formation and early stages of evolution of the most massive close binary systems: the most massive stars can be produced either by coalescence of the binary components or via mass transfer in such systems. 相似文献
11.
The equilibrium thickness of the isothermal layers of interstellar gas and volume gas densities ρ gas in the plane of the disk as a function of galactocentric distance R are computed for seven spiral galaxies (including the Milky Way) using an axisymmetrical model. In this model, the thickness of the stellar disk varies with R and remains approximately equal to the minimum thickness of a stable equilibrium disk. We found the disk thickness to increase toward the periphery in at least five of the seven galaxies. The density of the stellar disk decreases with R faster than ρ gas , so that gas dominates at the disk peripheries in terms of density. A comparison of the azimuthally averaged star formation rate SFR and the gas density shows the absence of a universal Schmidt law SFR ~ρ gas n for galaxies. However, the SFRs in various galaxies are better correlated with the volume than the gas surface density. The parameter n in the Schmidt law formally calculated using the least-squares method lies in the interval 0.8–2.4, being, on average, close to 1.5. The values of n calculated separately for the molecular gas display substantial scatter, but are, on average, close to unity. The value of n appears to increase with decreasing ρ gas , so that the fraction of gas that actively participates in star formation decreases with n. 相似文献
12.
Spectroscopic observations of three lenticular (S0) galaxies (NGC 1167, NGC 4150, and NGC 6340) and one SBa galaxy (NGC 2273) have been taken with the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences aimed to study the structure and kinematic properties of early-type disk galaxies. The radial profiles of the stellar radial velocities and the velocity dispersion are measured. N-body simulations are used to construct dynamical models of galaxies containing a stellar disk, bulge, and halo. The masses of individual components are estimated formaximum-mass disk models. A comparison of models with estimated rotational velocities and the stellar velocity dispersion suggests that the stellar disks in lenticular galaxies are “overheated”; i.e., there is a significant excess velocity dispersion over the minimum level required to maintain the stability of the disk. This supports the hypothesis that the stellar disks of S0 galaxies were subject to strong gravitational perturbations. The relative thickness of the stellar disks in the S0 galaxies considered substantially exceed the typical disk thickness of spiral galaxies. 相似文献
13.
The influence of close passages of galaxies on the shapes of disk galaxies and the distribution of stars in them is studied for several types of interactions in the framework of the restricted N-body problem. Depending on the conditions adopted, either two spiral density waves or ring structures are formed in the stellar disk of the galaxy. These structures can generate star formation fronts with the corresponding shape, as are observed in disk galaxies. Our calculations can also be applied to study the influence of the passage of a nearby star on a protoplanetary disk. The formation of ring structures there could specify the type of planet formation in the outer regions of the planetary system and the distribution of semimajor axes for the planetary orbits. We use the same model to study the generation and evolution of spiral density waves in the stellar disks of galaxies as a result of the recently found asymmetry of the gravitational potential in the massive dark haloes in disk galaxies. The dipole component of the gravitational field of the halo can continuously permanently generate the spiral structure in disk galaxies. 相似文献
14.
Numerical simulations of the chemical evolution of disk galaxies taking into account the influence of Population III stars
are considered. The probability that stars with peculiar chemical compositions are present in the solar neigborhood is analyzed,
and possibilities for their detection considered. For various assumptions about the slope of the initial mass function for
Population III stars and the critical metallicity, the radius surrounding the Sun containing at least one such star is 10–12
pc. Such objects could be studied using modern large telescopes. The influence of Population III stars on the chemical evolution
of disk galaxies is investigated. Taking into account the first stars in early stages leads to an earlier onset of chemical
enrichment of the ISM and a characteristic chemical composition of the gas, but all traces of this enrichment have disappeared
by the current epoch. 相似文献
15.
E. S. Safonova 《Astronomy Reports》2011,55(11):1016-1024
We propose a new method for estimating the HI deficiency in galaxies. The method is based on a semi-empirical relationship
between the total mass of HI and specific angular momentum of isolated galaxies. The atomic-hydrogen deficiency is estimated
for nearby spiral galaxies and for spiral galaxies in the Virgo and Coma clusters. The mean HI deficiencies determined for
these samples using our method are similar to those obtained with conventional methods, although there are considerable differences
in some cases. The HI deficiency in nearby galaxies does not depend on their degree of isolation, and there is no systematic
discrepancy between their HI and “normal” masses. Significant HI deficiencies are observed in the Virgo and Coma clusters,
out to distances of 1.5 and 3–4 Mpc from the cluster centers, respectively. At such distances, the ram pressure is too small
to sweep a considerable amount of gas from the galactic disks. Either these galaxies have passed through the dense cluster
center, or their gas deficiency is due to the fact that the halo had stopped accreting onto the disk when the galaxy entered
the cluster. 相似文献
16.
We study the evolution of the [O/Fe]-[Fe/H] relation and the dependence of the iron abundance on distance from the galactic plane z in a one-zone model for a disk galaxy, starting from the beginning of star formation. We obtain good agreement with the observational data, including, for the first time, agreement for the [Fe/H]-z relation out to heights of 16 kpc. We also study the influence of the presence of dark matter in the galaxies on the star-formation rate. Comparison of the observed luminosity of the Galaxy with the model prediction places constraints on the fractional mass of dark matter, which cannot be much larger than the fractional mass of visible matter, at least within the assumed radius of the Galaxy, ~20 kpc. We studied the evolution of disk galaxies with various masses, which should obey the Tully-Fisher relation, M ? R2. The Tully-Fisher relation can be explained as a combination of a selection effect related to the observed surface brightnesses of galaxies with large radii and the conditions for the formation for elliptical galaxies. 相似文献
17.
N. A. Tikhonov 《Astronomy Reports》2006,50(7):517-525
Stellar photometry obtained using the Hubble Space Telescope is used to study the distributions of the number densities of
stars of various ages in 12 irregular and dwarf spiral galaxies viewed edge-on. Two subsystems can be distinguished in all
the galaxies: a thin disk comprised of young stars and a thick disk containing a large fraction of old stars (primarily red
giants) in the system. Variations of the stellar number density in the thin and thick disks in the Z direction perpendicular to the plane of the galaxy follow an exponential law. The size of the thin disk corresponds to the
visible size of the galaxy at the μ = 25 mag/arcsec2 isophote, while the thick disk is a factor of two to three larger. In addition to a thick disk, the massive irregular galaxy
M82 also has a more extended stellar halo that is flattened at the galactic poles. The results of our previous study of 12
face-on galaxies are used together with the new results presented here to construct an empirical model for the stellar structure
of irregular galaxies.
Original Russian Text ? N.A. Tikhonov, 2006, published in Astronomicheskiĭ Zhurnal, 2006, Vol. 83, No. 7, pp. 579–588. 相似文献
18.
Seven early-type galaxies that are members of the massive X-ray group containing NGC 80 have been studied using two-dimensional spectroscopy with the 6-m telescope of the Special Astrophysical Observatory. We searched for evidence for the synchronous secular evolution of the galaxies in the group. The bulges of five of the seven galaxies appear to be old, with the average age of the bulge stars being 10–15 billion years. Signs of a relatively recent star-formation burst are observed in the small S0 galaxy IC 1548, whose average bulge age is 3 billion years and average core age is 1.5 billion years. A circumnuclear polar gas ring was also detected in this galaxy; in its outer regions, it makes a smooth transition to a gas disk that counter-rotates relative to the stars. IC 1548 probably underwent a close interaction, which resulted in its transformation from a spiral to a lenticular galaxy; the same interaction may also have induced the central burst of star formation. In the giant E0 galaxy NGC 83, a compact massive stellar-gas disk with a radius of about 2 kpc and very rapid rotation is observed, with ongoing star formation; the so-called “minor merger” is likely to have occurred there. We conclude that the NGC 80 group is in a state of formation, with the small NGC 83 subgroup “falling into” the large, old NGC 80 subgroup. 相似文献
19.
A. S. Saburova 《Astronomy Reports》2011,55(5):409-415
The disk masses of four low-surface-brightness (LSB) galaxies are estimated using the criterion of marginal gravitational
stability. The constructed mass models are close to those for a maximum disk. The results show that the disks of LSB galaxies
may be significantly more massive than is usually believed based on their brightnesses. In this case, their surface densities
and masses are fairly typical for higher-surface-brightness spirals. Alternatively, it may be that LSB disks are dynamically
overheated. 相似文献
20.
A. A. Kabanov 《Astronomy Reports》2010,54(6):489-495
The impact of uncertainties in the relative efficiency of nucleosynthesis of various elements in stars on models of the chemical
evolution of disk galaxies is studied using a single-zone model for the galactic evolution. The dependences of the abundances
of 12C, 14N, 16O, and 56Fe on nucleosynthesis models are compared. The influence of the uncertainty in iron production by Type Ia supernovae on its
abundance in a galaxy is also considered. It is concluded that differences in nucleosynthesis models can appreciably affect
the results of modeling the early stages of galactic evolution, but this influence becomes insignificant at ages t> 109 yr. Uncertainties in the amount of iron ejected by Type Ia supernovae do not significantly influence the total galactic abundance
of iron. 相似文献