Sternberg Astronomical Institute
Gas content and kinematics of disks of galaxies.
1
2
Smirnova A.A., Zasov A.V.
1.Special Astrophysical Observatory of the Russian Academy of Sciences (SAO), Karachaevo-Cerkessia, Russia
2 .Sternberg Astronomical Institute, Moscow, Russia
There are many factors which determine the gas content in galaxies (a star formation history is the most essential of them). Their
efficiency strongly differs from one galaxy to another. Nevertheless the total mass of gas in late-type galaxies is closely tied with such
properties of galaxies as their optical diameter D or their velocity of rotation.
Here we develop the idea proposed earlier by Zasov (see [1],[2]) that the maximum mass of gas contained in a disk of galaxy is
determined by the threshold condition for gravitational stability of rotating gas layer, which corresponds to the critical value of gas surface
density scrit = Ck/QpG (where C is the velocity dispersion of gas clouds, k – epicyclic frequency, Q – Toomre’ s parameter). It is easy to show
that if C is approximately constant, then the total mass of gas Mgas (which is assumed to be proportional to MH for late-type galaxies)
is proportional to specific angular momentum of a galaxy VC RHI, where VC is the rotational velocity and RHI is the radius of HI disk (the latter
is well correlated with its optical diameter D).
C
HI
M
3/2
= 2 h (k/G)V C R HI
To check the universality of this rule for various galaxies, and also to compare quantitatively MHI and the “critical” mass MCHI expected
when the surface gas density s Is close to scrit allover a disk, we have examined several samples of late type galaxies possessing very
different properties - from clumpy irregular galaxies to galaxies with extremely low surface brightness, such as Malin1. For comparison of
galaxies with different surface brightness we used the radial optical disk scale (R0) instead of their optical diameters. All the data for
MHI,VC and R0 were taken from literature (see references in [3]).
Figure 1
Figure 2
Figure 3
asterisks – dwarf galaxies with quiet star forming
open asterisks -- galaxies with high contents of HI
open triangles – clumpy irregular galaxies
circles – edge-on late type spiral galaxies
diamonds – low surface brightness galaxies
crosses – galaxies from cluster Uma
squares – Malin1-type objects
This figure 1 shows the existence of a tight correlation between MHI and VCR0. If this relationship is just the reflection of the well known
correlation between MHI and R0, one would expect the worsening of correlation after the velocity VC. Actually, though the MHI correlates both
with R0 (Fig. 2) and VC (Fig. 3), the relationship “mass of gas” - specific angular momentum” has the highest coefficient of correlation. It enables
to conclude that this interdependence is not reduced to a combination of two simple relations, and reflects a real connection between the
gas mass and the angular moment of a disk. It is worth noting that the different types of galaxies follow the same regularity. Dashed line and
C
two parallel lines at the diagram illustrate the theoretically expected relationship between the M HI (a critical value of hydrogen mass MHI ) and
VCR0, and the uncertainly of the estimation of MC.
Figure 4
C
This diagram (Fig.4) illustrates the relationship between the expected values of M HI theoretically
expected for marginally stability of a disk and the observed mass MHI. With some exceptions, late-type
galaxies tend to keep their gas content at the level close to the critical one (within a factor of two) or a
little lower. A good agreement between these quantities allows to conclude that the condition of gravitational stability of a gas layer is the important factor determining the quantity of mass of a gas in galaxies
with different star formation rate in the presence epoch , and, hence it may be considered as the factor
regulating the efficiency of star formation and the rate of depletion of gas.
Conclusions
1 It is confirmed that the gas content in late-type galaxies in most cases is restricted (within the uncertainly of about factor 2) by the
critical value MHI determined by the condition of gravitational stability of gaseous disk.
2 The paucity of galaxies where the observed mass of HI significantly exceeds the critical value MCHI gives evidence that most
galaxies have already experienced the efficient star formation earlier in their history, after which they have reduced the rate of gas
depletion.
3 Even the unusual Malin1-type galaxies follow the same way of gas depletion: their extremely high mass of HI is close to those
expected for galaxies with such a huge specific angular momentum.
4 A proportionality between MHI and VCR0, answers the question why does the relative mass of hydrogen MHI /M tot (where Mtot is the
total mass of a galaxy) increase along the sequence from the fast to slow rotating late-type galaxies.
References
1. Zasov A.V. 1974 Soviet Astr. 51 1225
2. Zasov, Z. V.; Rubtsova, T. V. 1989 Soviet Astr. Lett 15 118 390 829
3. Zasov A.V., Smirnova A.A. 2004 Astr. Lett in press