Changing Environments

At higher redshift much of the action took place in cluster (or proto-cluster) environment. How do these types of interactions differ from those in the field environment?

Cluster tides: Tidal field strips cluster galaxies (eg Malamuth & Richstone, Merritt, early 1980s), drives disk instabilities (Byrd & Valtonen 1990).
Mergers and close encounters in high density environment may drive Butcher-Oemler effect (Lavery & Henry 1988)
High speed encounters ("harassment"; Moore etal 1998): repeated impulsive heating
Hot ICM interactions (eg Gunn & Gott 1972; Quilis etal 2000): ram pressure induced starburst and gaseous stripping of cluster galaxies.

Focus on slow collisions and mergers: Given the high velocity dispersion of rich clusters, do we expect these types of encounters to occur?

Theoretical modeling:

Ghigna etal (1998) argue no: high relative velocities between cluster galaxies, coupled with the cluster tidal field, shuts off merging inside the virial radius of the cluster after z=0.5.

but Dubinski (1998) says yes: mergers of infalling subclumps do happen; between z=0.4 and z=0, 1/4 of massive galaxies have merged other than with the central cD.

animation courtesy John Dubinski

Observationally, we see evidence for slow interactions and mergers in clusters.

Abell 103, 114, 118 z=0.3 Couch etal (1998)

3C324 cluster z=1.2
(Dickinson 1994)
the objects are too close if it was all high speed harassment...
MS1054-03 z=0.83
(van Dokkum etal 1999)

How will cluster interactions differ from those in the field?

Cluster tides:

can add energy to binary orbit, increasing merger time;

tidally liberate a significant amount of stellar material

Cluster infall/merger model

r_s 0.3 Mpc

r₂₀₀ 2 Mpc

M₂₀₀ 10¹⁵ M_sun

r_peri 0.5 Mpc

r_apo 2.0 Mpc

What is the role of cluster hot gas?

Strips away low density gas:

outer HI disk, tidal debris stripped: feed hot halo?
shuts down reaccretion (no reforming disks in cluster E's)

But it is unclear how it affects the inner gas

Gunn & Gott (1972) argued it would strip gas into ~ few kpc, but not the inner molecular gas.

Quilis etal (2000): everything stripped w/in 100 Myr (H₂ -- > HI --> stripping). They argue cluster S0's form through stripping and rapid starburst truncation. But:

Molecular gas not observed to be stripped

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

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

Starbursts have duration: many "E+A" galaxies may even have appreciable ongoing star formation (Poggianti etal 1999; Smail etal 2000)

So cluster interactions and mergers happen and cluster galaxies have starburst fuel: premature to abandon collisional formation mechanisms for cluster E+A and S0 galaxies...

Theoretical modeling: Ghigna etal (1998) argue no: high relative velocities between cluster galaxies, coupled with the cluster tidal field, shuts off merging inside the virial radius of the cluster after z=0.5. but Dubinski (1998) says yes: mergers of infalling subclumps do happen; between z=0.4 and z=0, 1/4 of massive galaxies have merged other than with the central cD.
	animation courtesy John Dubinski

	Observationally, we *see* evidence for slow interactions and mergers in clusters. Abell 103, 114, 118 z=0.3 Couch etal (1998)
3C324 cluster z=1.2 (Dickinson 1994)	the objects are too close if it was all high speed harassment...	MS1054-03 z=0.83 (van Dokkum etal 1999)