Mergers and the Central Parameter Relationships for Ellipticals

The Central Parameter Relationships

HST studies of the nuclei of elliptical galaxies have revealed the existence of a "central parameter relationship" (eg Lauer etal 1995, Byun etal 1996; Gebhardt 1996; Faber etal 1997).

Ellipticals have a range of nuclear surface brightness profiles, which can be characterized by the "Nuker law". Ellipticals generally fall along a sequence from power law cusp galaxies to flat core galaxies.

Correlations exist between

inner slope (gamma),
break radius (r_b), and
total magnitude (M_V)

These are known as the central parameter relationships:

data from Faber etal (1997)

Look at the discrepant points:

NGC 4486B: companion to M87, possible stripped object?
Fornax A: lots of tidal debris, merger remnant?

Begs the question: Are mergers consistent with the central parameter relationship for ellipticals?

NICMOS imaging of the nuclei of young merger remnants

Sample

NGC 34	tidal tail; AGN?	NGC 3921	merger remnant
NGC 455	loops & plumes	NGC 7252	merger remnant
NGC 474	shells	NGC 7585	shells
NGC 2418	plumes	NGC 7727	tails
NGC 3656	disturbed isophotes	UGC 5814	plume + dust
IC 51	shells	UGC 9829	distorted

Observations

NIC1: F110W
NIC2: F110W, F160W, F187W

Preliminary Results:

van der Marel, Mihos, Zurek, Hernquist, & Heckman 2001

Comments:

Dust is still a problem (ie NGC 3656's artificially large break radius).
Most nuclei are consistent with normal ellipticals.
Four interesting objects:

NGC 34	NGC 3921	NGC 7252	NGC 7727

The three most discrepant points all show long tidal features, indicative of a major merger.
But the other young merger, NGC 7727, sits on the canonical central parameter relationship.
Evolution could drive these objects back towards "normal" ellipticals, but very strong evolution may be needed.