The formation and dynamical state of the brightest cluster galaxies Pei-Li Ho 2009, 6, 17

Main Reference The origin of the brightest cluster galaxies Dubinski, J. 1998, ApJ, 502, 141 The hierarchical formation of the brightest cluster galaxies De Lucia, G., & Blaizot, J. 2007, MNRAS, 375, 2 The dynamical state of brightest cluster galaxies and the formation of clusters Coziol et al. 2009, AJ, 137, 6

Content What is the brightest cluster galaxies (BCGs)? How special they are? The dynamical state of BCGs The formation of BCGs i. Cooling flows ii. Galactic cannibalism iii. Hierarchical formation

What is the brightest cluster galaxies? How special they are? Brightest cluster galaxies are the very bright galaxies that inhabit the cores of galaxy clusters. They are among the most luminous and most massive galaxies in the universe at the present epoch. (2-4 mag brighter than normal galaxies) BCGs lie close to the peaks of the X-ray emission in concentrated X-ray bright clusters (Jones & Forman 1984).

Their rest-frame velocities have small offsets from those of their host clusters (Quintana & Lawrie 1982). They exhibit different luminosity profiles than typical cluster elliptical galaxies (with an extended envelope of excess light proportion to cluster’s rechness)(Oemler 1976). They host higher fraction of radio-loud AGN than other galaxies with the same mass. (Best et al. 2006)

Indicate the interaction with ICM, and cooling flow.

The dynamical state of BCGs --Are they relaxed? The location they lie The peculiar velocity of BCGs The alignment of BCG * * * The growth of BCGs are along with cluster growth.

The formation of BCGs — Cooling flow Early theoretical studies suggested that BCGs are formed as a result of star formation in cooling flows expected in the high-density, rapid cooling centers of cluster X-ray halo (Fabian 1994). Recent X-ray observations performed with Chandra and XMM show that cooling flow rates in nearby clusters are too low to explain the masses of BCGs. In the perspective of hierarchical merging, cooling flow are probably the main fuel for galaxy mass-growth at high redshifts, in dense and lower mass halo. This source is removed possibly due to AGN feedback.

Dynamical friction The more they eat, the larger the envelope grow, and then they eat more! dvM/dt ∞ - lnΛ G2ρM/vM3 vM

The formation of BCGs — Galactic catabolism ‘Galactic catabolism’ means BCGs grow in situ by accretion of the smaller mass companion galaxies through dynamical friction and tidal stripping (e.g. Ostriker & Tremaine 1975) This mechanism turns out to be too slow due to the high velocity dispersion of companion galaxies. The estimation in ‘Galactic dynamics’ (Binney & Tremaine 1987) suggests that a typical giant galaxy has eaten one or two satellites of luminosity ~0.1L* in a Hubble time. (L*=10^10L⊙) Nevertheless, cannibalism may be responsible for the formation of diffuse stellar envelopes extending up to hundreds of kiloparsecs (the ‘cD’ galaxies).

The formation of BCGs — Hierarchical formation In a hierarchical structure formation theory, groups of galaxies will form before the formation and virialization of a cluster. In small groups, galaxy-galaxy mergers are most efficient with low velocity dispersions. They are indeed driven by dynamical friction, but it is the accretion rate of galaxies into the proto-cluster, along with cluster growth itself, that regulates and sets the conditions for galaxy merging (De Lucia & Malizot, 2007)

N-body Simulation Dubinski 1998 http://www.cita.utoronto.ca/~dubinski/bigcluster.html

Millennium Run Simulation De Lucia & Blaizot 2007, http://www.mpa-garching.mpg.de/galform/virgo/millennium/

Summary The dynamical states of BCGs including non-zero peculiar velocity, offset from cluster global center, and alignment to the large scale structure all indicate an hierarchical formation history. ‘Dynamical friction’ play an important role in the growth of BCGs, both in cannibalism and galaxy-galaxy mergers. The N-body simulations clearly show the origin of BCGs is related to the collapse and merging of cluster.

From Beers & Gellar, 1983, ApJ, 274, 491

From Coziol et al., 2009, AJ,