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41.
Stellar velocity dispersion data at galactocentric distance of two disk radial scale lengths (R = 2h), available in the literature allowed us to determine the upper limits of disk local surface densities at a given R and (by extrapolation) total masses of disks proceeding from the marginal gravitational stability condition. A comparison
of the obtained disk masses with the photometric estimates based on the stellar population models indicates the absence of
strong dynamical overheating inmost spiral galaxies and hence the absence of significant major merging events, which were
able to heat dynamically the inner parts of disks. The same conclusion is valid for some of S0 galaxies. However, a significant
part of the latter possesses stellar velocity dispersion, which exceeds the threshold value needed for gravitational stability.
Dynamically overheated disks occur both among paired and isolated galaxies. Disk to total mass ratios within R = 4h found for marginally stable disks in most cases lie in the range 0.5–0.8 with the absence of the clearly defined correlation
of this ratio with color index or morphological type. 相似文献
42.
We have performed photometric B, V, and R observations of nine disk galaxies that presumably have abnormally low total mass-to-light (M/L) ratios for given color indices. Our data on surface photometry are used to analyze the possible causes of anomalous M/L estimates. In many cases, these can be the result of errors in photometry or rotational velocity determination but can also
reflect the real peculiarities of the stellar composition of galaxies. Comparison of the photometric and dynamical disk mass
estimates obtained by analyzing the rotational velocities shows that low M/L values for a given color index are probably real for some of the galaxies. This is primarily true of NGC 4826 (Sab), NGC
5347 (Sab), and NGC 6814 (Sb). The small number of such galaxies suggests that the stellar initial mass function is universal.
However, a small fraction of galaxies probably may have a non-typical mass function “depleted” in low-mass stars. Such galaxies
require a more careful study. 相似文献
43.
Special Astrophysical Observatory, USSR Academy of Sciences: P. K. Shternberg State Astronomical Institute. Translated from Astrofizika, Vol. 28, No. 2, pp. 245–254, March–April, 1988. 相似文献
44.
The results of a search for galaxies with straight structural elements, usually spiral-arm rows (“rows” in the terminology of Vorontsov-Vel'yaminov), are reported. The list of galaxies that possess (or probably possess) such rows includes about 200 objects, of which about 70% are brighter than 14m. On the whole, galaxies with rows make up 6–8% of all spiral galaxies with well-developed spiral patterns. Most galaxies with rows are gas-rich Sbc-Scd spirals. The fraction of interacting galaxies among them is appreciably higher than among galaxies without rows. Earlier conclusions that, as a rule, the lengths of rows are similar to their galactocentric distances and that the angles between adjacent rows are concentrated near 120° are confirmed. It is concluded that the rows must be transient hydrodynamic structures that develop in normal galaxies. 相似文献
45.
A series of numerical N-body simulations is performed in order to dynamically model the properties of four galaxies (NGC 5603, NGC 3198, NGC 891, and NGC 1566) with known rotation curves, radial disk scales L, and velocity dispersions of old disk stars at various galactocentric distances r. Each model includes a three-dimensional collisionless disk and rigid spherical components, whose relative mass μ was treated as a free parameter that differed from simulation to simulation. The observed disk stellar velocity dispersions were assumed to be equal to or (in the general case) greater than the corresponding line-of-sight projections of the simulated values for the adopted μ after the initially unstable disk is heated and arrives at a steady state. A comparison of the simulated and observed rotational velocities and velocity dispersions provides evidence for “light” disks with μ≥2 in the disk (r<4L). 相似文献
46.
Total mass-to-light ratio M/L
B
for S0 — Irr galaxies, whereM is the dynamical mass within the optical radius R
25 (corresponding to 25
m
/sq. arcsec in the B band), increases systematically with (B-V)0 color, but slower than that is predicted by stellar population evolution models without dark halo. It shows that the mean
ratio between dark halo and stellar masses is higher for more “blue“ galaxies. However some galaxies don’t follow this general
trend. The properties of galaxies with extremely high and extremely low values of M/L
B
ratios are compared, and different factors, accounting for the extremes, are analyzed. The conclusion is that in some cases
too high or too low M/L
B
ratios are associated with observational errors, in other cases—with non-typical dark halo mass fraction, and in some cases
— with peculiarities of disc stellar population. Particularly, discs of some galaxies with low M/L
B
ratios turn out to be unusually “light” for their luminosity and colors, which indicates a substantial deficit of low mass
stars as the most probable cause of low M/L
B
. 相似文献
47.
Surface BVRI photometry is presented for two spiral galaxies with a complex photometric structure: NGC 834 and NGC 1134. We propose to introduce the combined color indices Q BVI and Q VRI to investigate the photometric structure of the galaxies. These color indices depend only slightly on selective absorption, which allows them to be used to study the photometric structure of “dusty” galaxies. Evolutionary stellar-population models show that Q BVI is most sensitive to the presence of blue stars, while Q VRI depends on local Hα equivalent width. A ring with active star formation manifests itself on the Q BVI map for NGC 834 at a distance of ～15 from its center, and a spiral structure shows up on the Q VRI map for NGC 1134 in its inner region. The Q BVI –Q VRI diagram can provide information about the current stage of a star's formation in various galactic regions. A comparison of the color indices for the galaxies with their model values allows us to estimate the color excesses and extinction in various galactic regions. 相似文献
48.
Estimates of the masses of supermassive black holes (M bh ) in the nuclei of disk galaxies with known rotation curves are compared with estimates of the rotational velocities V m and the “indicative” masses of the galaxies M i . Although there is a correlation between M bh and V m or M i , it is appreciably weaker than the correlation with the central velocity dispersion. The values of M bh for early-type galaxies (S0-Sab), which have more massive bulges, are, on average, higher than the values for late-type galaxies with the same rotational velocities. We conclude that the black-hole masses are determined primarily by the properties of the bulge and not the rotational velocity or the mass of the galaxy. 相似文献
49.
50.
A sample of 66 galaxies from the catalog of Bettoni et al. (CISM) with anomalously high molecular-to-atomic hydrogen mass ratios (M mol/M HI > 2) is analyzed. The sample galaxies do not differ systematically from the other galaxies in the catalog with the same morphological types, in terms of their photometric parameters, rotational velocities, dust contents, or the integrated masses of gas (for galaxies with the same linear sizes and disk angular momenta). This suggests that the overabundances of H2 are due to the molecularization of HI. Galaxies with bars and active nuclei are found more frequently among galaxies with M mol estimates in CISM. In a small fraction of cases, high M mol/M HI ratios are due to overestimation of M mol due to overstimating of the conversion factor for the translation of CO-line intensities into the number of H2 molecules along the line of sight. It is argued that the molecularization of the bulk of the gas mass could be due to the concentration of gas in the inner regions of the galactic disks and the resulting high gas pressures and relative low star-formation efficiencies, as is indeed observed in galaxies with high M mol/M HI ratios. 相似文献