Merger Remnants and Elliptical Galaxies

So we have a qualitative picture of how mergers may lead to elliptical 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 gas
  • fuels X-ray halo
environment
  • E's more common in high density environments

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)








Surface Brightness Profiles:

Over a large range in radius (100 pc - many kpc) the remnant shows an r1/4 law profile.
But the central starburst may leave behind a prominant (dominant?) poststarburst population in the center of the remnant. Surface brightness cusps? Kinematically distinct cores? Metallicity gradients?
Model Observation
Mihos & Hernquist (1994) Hibbard & Yun (1999)

 


A good candidate: Centaurus A
Optical

Malin/AAT
Deep Optical + HI

Malin etal 1983 and Schiminovich etal 1994