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.

The low surface brightness disc galaxy Malin 2 challenges the "standard" theory of galaxy evolution by its enormous total mass ∼ 2 · 10 12 M which must have been formed without recent major merger events. The aim of our work is to create a coherent picture of this exotic object by using the new optical multicolor photometric and spectroscopic observations at Apache Point Observatory as well as archival datasets from Gemini and wide-field surveys. We performed the Malin 2 mass modelling, estimated the contribution of the host dark halo and found that it had acquired its low central density ρ 0 0.003 M /pc 3 and the huge isothermal sphere core radius r c = 27.3 kpc before the disc subsystem was formed. Our spectroscopic data analysis reveals complex kinematics of stars and gas in the very inner region (r = 5 − 7 kpc). We measured the oxygen abundance in several clumps and concluded that the gas metallicity decreases from the solar value in the centre to a half of that at 20 − 30 kpc. We found a small satellite projected onto the galaxy disc at 14 kpc from the centre and measured its mass (1/500 of the host galaxy) and gas metallicity (similar to that of Malin 2 disc at the same distance). One of the unique properties of Malin 2 turned to be the apparent imbalance of the interstellar media: the molecular gas is in excess with respect to the atomic gas for given values of the gas equilibrium turbulent pressure. We explain this imbalance by the presence of a significant portion of the dark gas not observable in CO and the Hi 21-cm lines. We also show that the depletion time of the observed molecular gas traced by CO is nearly the same as in normal galaxies. Our modelling of the UV-to-optical spectral energy distribution favours the exponentially declined star formation history over a single-burst scenario. We argue that the massive and rarefied dark halo which had formed before the disc component well describes all the observed properties of Malin 2 and there is no need to assume additional "catastrophic" scenarios (such as a major merging) proposed previously to explain the origin of giant LSB galaxies.

In this article, we test the conclusion of Donato et al. (2009) concerning the universality of the DM halo surface density µ 0D = ρ 0 r 0 . According to our study, the dispersion of values of µ 0D is twice higher than that found by Donato et al. (2009). We conclude, in contrast with Donato et al. (2009), that the DM surface density and its Newtonian acceleration are not constant but correlate with the luminosity, morphological type, (B − V ) 0 colour index, and the content of neutral hydrogen. These DM parameters are higher for more luminous systems of early types with red colour and low gas content. We also found that the correlation of DM parameters with colour index appears to be the manifestation of a stronger relation between DM halo mass and the colour of a galaxy. This finding is in agreement with cosmological simulations (Guo et al, 2011). These results leave little room for the recently claimed universality of DM column density. We also found that isolated galaxies in our sample (contained in the Analysis of the interstellar Medium of Isolated GAlaxies (AMIGA) catalogue) do not differ significantly in their value of µ 0D from the entire sample. Thus, since the AMIGA catalogue gives a sample of galaxies that have not interacted with a significant mass neighbour in the past 3 Gyr, the difference between the systems with low and high values of µ 0D is not related to the merging events during this period of time.

Dark matter in galaxies, its abundance, and its distribution remain a subject of long-standing discussion, especially in view of the fact that neither dark matter particles nor dark matter bodies have yet been found. Experts' opinions range from a very large number of completely dark galaxies exist to nonbaryonic dark matter does not exist at all in any significant amounts. We discuss astronomical evidence for the existence of dark matter and its connection with visible matter and examine attempts to estimate its mass and distribution in galaxies from photometry, dynamics, gravitational lensing, and other observations (the cosmological aspects of the existence of dark matter are not considered in this review). In our view, the presence of dark matter in and around galaxies is a well-established fact. We conclude with an overview of mechanisms by which a dark halo can influence intragalactic processes.Comment: 82 pages, 35 figure

Modern galaxy formation theory suggests that the misalignment between stellar and gaseous components usually results from an external gas accretion and/or interaction with other galaxies. The extreme case of the kinematic misalignment is demonstrated by so-called galaxies with counterrotation that possess two distinct components rotating in opposite directions with respect to each other. We provide an in-deep analysis of galaxies with counterrotation from IllustrisTNG100 cosmological simulations. We have found 25 galaxies with substantial stellar counterrotation in the stellar mass range of 2 × 109 − 3 × 1010 M⊙ . In our sample the stellar counterrotation is a result of an external gas infall happened ≈2 − 8 Gyr ago. The infall leads to the initial removal of pre-existing gas, which is captured and mixed together with the infalling component. The gas mixture ends up in the counterrotating gaseous disc. We show that $\approx 90\%$ of the stellar counterrotation formed in-situ, in the counterrotating gas. During the early phases of the infall, gas can be found in inclined extended and rather thin disc-like structures, and in some galaxies they are similar to (nearly-) polar disc or ring-like structures. We discuss a possible link between the gas infall, AGN activity and the formation of misaligned components. In particular, we suggest that the AGN activity does not cause the counterrotation, although it is efficiently triggered by the retrograde gas infall, and it correlates well with the misaligned component appearance. We also find evidence of the stellar disc heating visible as an increase of the vertical-to-radial velocity dispersion ratio above unity in both co- and counterrotating components, which implies the importance of the kinematical misalignment in shaping the velocity ellipsoids in disc galaxies.