Mergers, Progenitors, and Remnants -- Prevolution and Evolution

Salient Questions:

What is a normal elliptical?
What physical processes shape the properties of the merger remnant?
How do these processes depend on the merging progenitors?

The Role of Processed Gas

A: The ISM of spiral galaxies consists largely of cold neutral and molecular gas (HI and H₂).
B: The ISM of ellipticals consists largely of hot X-ray gas.
How do we go from point A to point B?

make stars (good! fixes central density problem)

make star clusters (good! explains differences in S_N)

heat gas via shocks or starburst winds (good! explains X-ray halos)

The Basic Picture

Stellar Dynamics: tranformation from a rotationally supported disk to a kinematically hot spheroid.
Gas Dynamics: rapid merger-driven inflow of gas, triggering a central starburst (and AGN?)
Star Formation: makes central starburst population and young GCs
Feedback: generates wind and hot halo

A good qualitative picture, and one with much observational and theoretical support. But...

Some Potential Problems

X-ray observations of mergers and "young ellipticals" show that they are deficient in hot gas compared to ellipticals (Schweizer & Fabbiano 1995; Mackie & Fabbiano 1997; Read & Ponman 1998; Sansom etal 2000). Where is the hot halo?

Observations of mergers (Schweizer etal 1996) and "young ellipticals" (Brown etal 2000) reveal a relatively low current GC specific frequency (S_N=0.7-1.4, compared to ~ 5 for "typical" ellipticals) -- could mergers produce ellipticals which have too few globular clusters?

Is there a problem? Evolution and "Prevolution"

Evolution:

Just wait -- the remnant will evolve with time...

X-ray halos: Stellar evolution and gas return will feed a hot halo (eg Sansom etal 2000).

Globular Clusters: The remnant fades with time (but only raises S_N ~ 2-3; Schweizer etal 1996; Brown etal 2000)

Prevolution:
Our understanding of the details of mergers relies primarily on z~0 studies (both observational and theoretical). The things which merged to form ellipticals at higher redshift may well be different than the things we observe at z~0:

different gas content

different gas distributions

different local/global stability properties

So far we have focused largely on starburst triggering and gas inflow. But how would these things affect merger properties?
These mergers may have

a larger fraction of baryons processed through a starburst: higher S_N?

more gas to fuel starburst and drive superwinds or more gas to be collisionally heated: more luminous X-ray halo?

a higher ratio of global:central star formation: KDCs less common in old E's?

Question: why should ellipticals which form from different progenitors look the same?
Question: why should ellipticals formed at z=0 look like ones formed at higher redshift?

Where do we go from here?

Better initial conditions (high redshift observations)
Better gas physics and starburst modeling