Gravitational stability and dynamical overheating of galactic stellar disks

Abstract

We use the marginal stability condition for galactic disks and the stellar velocity dispersion data published by different authors to place upper limits on the disk local surface density at two radial scalelengths R = 2h. Extrapolating these estimates, we constrain the total mass of the disks and compare these estimates to those based on the photometry and color of stellar populations.The comparison reveals that the stellar disks of most of spiral galaxies in our sample cannot be substantially overheated and are therefore unlikely to have experienced a significant merging event in their history. The same conclusion applies to some, but not all of the S0 galaxies we consider. However, a substantial part of the early type galaxies do show the stellar velocity dispersion well in excess of the gravitational stability threshold suggesting a major merger event in the past. We find dynamically overheated disks among both seemingly isolated galaxies and those forming pairs. The ratio of the marginal stability disk mass estimate to the total galaxy mass within four radial scalelengths remains within a range of 0.4-0.8. We see no evidence for a noticeable running of this ratio with either the morphological type or color index.

Numerical modelling of the vertical structure and dark halo parameters in disc galaxies

“…12 corresponds to a model galaxy with different parameters of the disc and spherical subsystem. The figure includes our modelling of the objects studied by Mikhailova et al (2001) and Zasov et al (1991) in addition to results of the present paper. The upper curve corresponds to mostly bulgeless models.…”

“…Relation between the vertical scale height, local disc surface density and spheroidal subsystem mass M s enables us to include the disc thickness into the modelling. At the same time, if the velocity dispersion is close to that required for the marginal disc stability, the thickness of the stellar disc is tightly connected with the spherical-to-disc components mass ratio (Zasov et al 1991;Mikhailova et al 2001;Sotnikova & Rodionov 2006).…”

Does the stellar disc flattening depend on the galaxy type?

“…Thus, since the relative thickness of the disc depends on the contribution of the spherical component ('bulge+dark halo') (h/z0 ∝ M dark /M total ; Zasov, Makarov & Mikhailova 1991;Zasov et al 2002, see also the discussion in MSR10 and , this means that the relative contribution of the spherical component does not vary systematically along the morphological Hubble sequence of bright spiral galaxies. In turn, since bulges contribute more to the mass of the spherical component in the early-type disc galaxies, the dark halos themselves should reveal higher mass fraction in the case of the late-type (mostly bulgeless) galaxies.…”

Gravitational stability and mass estimation of stellar disks

“…To compare the obtained disk masses with the photometric estimates we calculated disk mass-to-light ratios in B band (M/L B ) d . For Sa-S0 galaxies the contribution of bulge was taken into account (for more details see Zasov et al 2011).…”