Evolution and Prevolution

The merger scenario explains many properties of ellipticals, but some nagging suspicions remain:

1. X-ray halos:

Studies of nearby merger remnants/young ellipticals show underluminous X-ray halos (Schweizer & Fabbiano 1993; Mackie & Fabbiano 1997; Read & Ponman 1998; Sansom etal 2001).

(Schweizer & Fabbiano 1993)

2. Globular Cluster Specific Frequency:

The number of globular clusters per unit spheroid luminosity is much greater for E's than for spirals.
Globular clusters may be made in mergers, but perhaps not enough. The specific frequency of merger remnants appears low (~2-3) compared to ellipticals (~5) (Schweizer etal 1996; Brown etal 2000).

(Harris 1991)

3. Central Parameter Relationships:

Young mergers sometimes show large nuclear density spikes, unlike "typical" ellipticals.

Do we have a problem? Not necessarily.

Prevolution

Progenitor Evolution: What merged "then" may have been very different from what's merging now!

Gas content:
Over the course of cosmic history, the universe has been processing gas into stars. Galaxies in the young universe might be much more gas-rich.

Disk stability:
The higher gas fractions may have the disks of young galaxies closer to gravitational instabilities which drive star formation.

Models of gas distributions in galaxies

present-day galaxy
high redshift galaxy

Kaufmann 1996

Implications and Speculations:

With higher gas fractions and less stable disks, high redshift mergers may:

process a higher fraction of baryons through a starburst: heightened GC specific frequency?

process more gas through starburst winds and supernovae: more luminous X-ray halos?

process fractionally less gas through a central starburst: inhibit central density spikes?

So why should merger remnants today look like those from the high-redshift universe?