Testing the Merger Hypothesis

 
Dynamical modeling of galaxy mergers:
 
  • Collisional perturbation
  • Self gravitating response
  • Gas inflow + starburst activity
  • Merger and relaxation


(from Mihos & Hernquist 1996)

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

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)

Do we see these signatures in nearby ellipticals?
 
The nearest elliptical: Centaurus A 
  • distance: 3.5 Mpc
  • diffuse stellar shells (Malin etal 1983)
  • gaseous shells (Schiminovich etal 1994)
  • rotating gas disk (van Gorkom etal 1990)
  • rotating PN system (Hui etal 1995)

(Malin etal 1983; Schiminovich etal 1994)

 
 
Approximately 1/2 of all field ellipticals have shell-like structures (Schweizer 1996; Colbert etal 2001):
 

(from Schweizer 1996)