Interactions of Cluster Galaxies

Lots of galaxies, so the probability of collision is likely:       But they are high-speed encounters (venc » sv,gal), so energy deposition and dynamical friction are much weaker than field encounters (energy input, dyn friction both ~ v-2). Strong interactions rare; mergers rarer (Ostriker 1980). Effects from the cluster potential dominate (eg., Byrd & Valtonen 1990; Henriksen & Byrd 1996).  inside ~ 0.5 Mpc for Coma-like potential Other factors also thought important:  Ram pressure stripping (Gunn and Gott 1977) Strangulation (Larson, Tinsley, & Caldwell 1980)

But while individual high speed encounters may not heat much, repeated encounters inside the tidal field of the cluster can drive evolution -- so-called "galaxy harassment"  (Moore etal 1996, 1998; Lake etal 1998). flat rotation curve: dE/E ~ b-2. ® divergent heating (except that halos truncate...). Add resonances and the cluster tidal field, and strong evolution results. low L spirals mostly affected, due to slowly rising rotation curves and low density cores. heating transforms into faint spheroidals, fueling of low luminosity AGN hosts, tidal destruction of LSBs. high L spirals spared most of the effects, except for modest heating (Moore etal 1999).

(courtesy Paul Bode) The revival of slow collisions: hierarchical structure. Galaxies accrete small groups (sgroup < scluster) from the surrounding environment ("cosmic web"). slow encounters and mergers possible in infalling groups evolutionary "preprocessing" of galaxies competition between the binding energy of the group and the tidal field of the cluster. The Mice,  projected ~ 5 Mpc from Coma (courtesy John Hibbard)

Interactions in Cluster Collapse Simulations

• followed subhalos in W_m=1, logM=14.7 cluster
• merger rate ~ 5-10% at z<0.5 in cluster outskirts
• no mergers within virial radius (1.6 Mpc).
  

Dubinski (1998): dark matter only, Wm = 1 small cluster, logM=14.0, s=550 km/s halos replaced w/ disk galaxy models at z=2 between z=1 and z=0.4, intense activity

from Dubinski (1998)

Gnedin (2003): Traced halo trajectories in simulations with varying cosmologies. Characterized close encounters...
...and encounter velocities. Merger probability ~ 20%-30%. Substructure (scales ~ 100 kpc) is a major heating source.

MS 1054-03 van Dokkum etal 2000

Active dynamical environment: cluster potential substructure heating high speed encounters (harassment) slow encounters  mergers? ram pressure stripping Obvious, but worthwhile points: Not every cluster is Coma!  And now, we're observing clusters out to higher and higher redshifts!