9. GAS ACCRETION IN GALAXIES

There are two modes of gas accretion, cold flow accretion and hot gas accretion via major mergers. In the first scenario, cold gas is provided along filaments, while in the major merger case, a source of hot gas, which will eventually feed star formation by cooling, is supplied to the galaxy.

The cold flows occur in filamentary streams following the cosmic web of large-scale structure, fig. 26. However, cold flows are rarely observed, but this can be due to the small covering factor of the filaments: the best indirect evidence in favor of cold flows is perhaps the study of star formation in dwarf galaxies, where several distinct episodes of star formation are detected.

Figure 26. Flux of cold streams through a hot halo at z = 2.5. The circle marks the halo virial radius. The hydrodynamical simulated galaxy has M = 1012 M. Figure from [131].

Major mergers are subdominant in star-forming galaxies at z ~ 2 where the star formation rate density peaks, as measured by Sersic profile fitting. A sample of submillimeter galaxies at z = 1-3, considered to be the most extreme star-forming galaxies, shows that their morphologies with WFC3/IR imaging are predominantly disk-like [126]. However the most luminous starbursts almost invariably show signs of major mergers [127]. Moreover, AGN host galaxies are mostly disks as measured by morphology [128].

At high redshift, major mergers especially between the most massive galaxies are more common. Usually, the mechanism invoked in merger-induced starbursts is a global inflow of gas towards the central kpc, resulting in a nuclear starburst. Major galaxy mergers lead to cloud agglomeration, angular momentum loss and cooling flows that feed star formation [129, 130]. Galaxy interactions and mergers drive star formation, and a variety of stellar structures can then be formed: for example, it has been proposed that the pile-up mechanism forms massive tidal dwarf galaxies which survive as dwarf satellites around the merger remnant, fig. 27.

Figure 27. AMR grid hydrodynamic simulation of a major merger from Chapon et al. Pile-up of material in a massive tidal dwarf galaxy at the tip of a tidal tail is visible. Figure from [129].