Galaxy Evolution: the formation of S0's

Mergers are a prime candidate for driving evolution in field galaxy morphology.

(Malin etal 1983; Schiminovich etal 1994)

E's:

Major mergers may transform spirals into ellipticals (Toomre 1977; Schweizer 1978, 1982).

S0's:

reaccretion of gas onto (merger-spawned) spheroid (eg, Schweizer 1990; Steinmetz & Navarro 2000),

disk "survival" in unequal mass mergers (Bekki 1998; Naab 2000; Bendo & Barnes 2001).

disk heating from minor mergers (eg, Toth and Ostriker 1992; Mihos & Hernquist 1994; Walker etal 1996).

Cluster E's:

are old. If mergers built E's in clusters, they did so at high redshift during the assembly of the protoclusters.
Very little contribution from late, gas rich, starbursting mergers.
The few mergers which might fall in from the outskirts at later times will be "swept clean". No CenA-like "smoking guns"...

Cluster S0's:

show strong evolution. Can field processes work in clusters?

Mechanism	Pros	Cons
Major mergers and reaccretion of disks	mergers possible in infalling groups, spheroids built...	...but not enough merging at late times, and reaccretion shuts off quickly.
Unequal mass mergers and disk survival	mergers possible, spheroids built...	...but mergers destroy stellar disks, so gas survival is crucial, and tides/RPS may strip the gas before it resettles.
Satellite mergers	bound satellite populations exist...	...but cluster tides can remove them, and why are S0's enhanced in clusters?

So field mechanisms are likely not the explanation for cluster S0's, unless they are made entirely in the group environment before accreting into clusters.

Cluster mechanisms for S0 formation:

Collisional heating + RPS (Moore etal 1999; Quilis etal 1999)

Possible, but efficacy of RPS on molecular gas is still uncertain:

Molecular gas not observed to be stripped

HI deficient spirals in Virgo contain significant quantities of molecular gas (Kenney & Young 1989)

no CO deficiency observed in cluster spirals (Casoli etal 1998).

Some poststarburst galaxies may even have appreciable ongoing star formation. If RPS is fast (Quilis etal 1999), these signatures shouldn't coexist.

And how do we make a luminous spheroid?

Strangulation (Larson etal 1980; Bekki etal 2002)

Easier to strip hot halo (if it exists!), possibly resulting in slow evolution to anemic or red disks, but again, how to make spheroid?

Different paths to different types of S0's: a heterogeneous class.