The Fate of Gas in Merging Galaxies

Gas is collisional, compressible, and complicated!
A variety of outcomes are possible, and gas is redistributed on all scales.

Inflow into nuclear regions (r < few hundred pc). (Negroponte & White 1983; Noguchi 1987, Barnes & Hernquist 1991, 1996;Mihos etal 1992; Mihos & Hernquist 1994, 1996; Bekki 1998; Gerritsen & Icke 1999; Springel 2000) Fueling AGN Triggering central starbursts Discrete cores (Hernquist & Barnes 1991; Mihos & Hernquist 1994; Springel 2000) (Hernquist & Barnes 1991)

Redevelopment of gas disks on kiloparsec scales. Earlier models showed most gas driven into nuclear regions, with little gas left behind to rebuild disks. More recent models show progenitors with extended gas disks can result in more substantial (~ few x 10⁹ M_sun ) disks in the remnant (Mihos 2002; Naab etal 2002).

Ejection of gas into tidal debris (r ~ 10 - 100 kpc). Late infall/accretion of gas (Hibbard & Mihos 1995) Gas/star offsets in tidal tails (Mihos 2001) Formation of tidal dwarf galaxies? (Barnes & Hernquist 1992; Elmegreen etal 1993; Hibbard etal ongoing) Formation of large scale (10s of kpc) HI "rings".

Complications:

Early inflow is coupled to the self-gravitating response of the disks (pre-merger evolution):

Gas Angular Momentum	Physical Torques	Gravitational Torques

	(from Mihos & Hernquist 1996)

Survival and return of diffuse gas will be very sensitive to environmental effects.

Cluster tides can liberate a significant amount of diffuse material, and ram pressure stripping can sweep away diffuse gas.
Reformation of disks may be inhibited in cluster ellipticals

an admittedly cartoonish example

The Fate of Gas in Merging Galaxies

Inflow into nuclear regions (r < few hundred pc).

Redevelopment of gas disks on kiloparsec scales.

Ejection of gas into tidal debris (r ~ 10 - 100 kpc).

Complications: