Merger-Driven Galaxy Evolution

So we have a qualitative picture of how mergers may change galaxies:

violent relaxation	destroys disks produces spheriods changes rotational motion to random
gas inflow	"sweeps" cold gas into center shuts off future star formation
starburst	depletes cold gas (neutral/molecular hydrogen) fuels X-ray halo (10⁶ K ionized gas)
environment	collisions more common in high density environments

Merger remnants resemble Elliptical galaxies in many ways. Can we be more quantitative, and can we look for signatures for a merger origin in ellipticals?

Tidal Morphology:
Violent relaxation is efficient, setting up an smooth density profile w/in a few dynamical times at most -- the inner regions quickly relax.
In the outskirts, the mixing time is much longer, and extended tidal debris has a long survival time.

Gas content:
Not all the cold gas is driven to the center. Much is thrown out in the tidal tails, then rains down onto the remnant over a Hubble time.
Some gas settles into a warped disk, or into diffuse loops.

Kinematics:

Much of the orbital angular momentum is transferred to the dark halos, and to the loosely bound outer portions of the remnant. Rotation rates (i.e., v/sigma) rise as a function of radius.

Simulated "slit spectra" of merger remnants

From Hernquist (1992)

A good candidate: Centaurus A

Deep Optical + HI

Malin etal 1983 and Schiminovich etal 1994

What about "minor mergers"?

Look at NGC 3628, in the Leo Group:

Tidal Morphology: Violent relaxation is efficient, setting up an smooth density profile w/in a few dynamical times at most -- the inner regions quickly relax. In the outskirts, the mixing time is much longer, and extended tidal debris has a long survival time.

Gas content: Not all the cold gas is driven to the center. Much is thrown out in the tidal tails, then rains down onto the remnant over a Hubble time. Some gas settles into a warped disk, or into diffuse loops.