Galaxy Clusters Perseus Cluster in Xrays Why study

Galaxy Clusters Perseus Cluster in X-rays

Why study clusters? Clusters are the largest virialized objects in the Universe. n n n Cosmology: tail of density peak distribution Impact of extreme environments Physics of galaxy formation + feedback Magnifying lenses on the universe Practical: Many galaxies in single field Negative: Only ~5% of galaxies are in clusters!

Cluster galaxies, mostly red. A 2218 – a cluster at redshift 0. 23 Gravitationally lensed background galaxies

Color-magnitude-density relation n n Cluster galaxies are mostly red sequence. Fewer blue galaxies in clusters; a continuous trend at high-L, more abrupt at low-L. Luminosity is more important than environment Even isolated regions have passive galaxies What makes galaxies blue red in clusters? Balogh et al. 2004

Ram-pressure stripping n n n n Observed in HI + optical/Ha. P=r. ICMv 2. When this pressure exceeds restoring gravitational force, gas is stripped: df/dz s < r. ICMv 2 Solve stripping radius. Virgo: 52% truncated (vs. 12% in field). Starvation? 6% “anemic”. Harrassment? 6% enhanced. Simulations work… but r. ICM needs to be high (like near cluster center). Virgo galaxy normal galaxy

Galaxy Collisions, Tides and Harassment • Tidal truncation • Slow encounter • Depends on gradient of potential • Big impact on the dark halo, but not significant for stellar component • Impulsive heating • Fast encounter • Importance increases as relative velocity decreases • Harassment • The cumulative effect of repeated encounters

Galaxy Collisions, Tides and Harassment Perturbation to velocity of star in galaxy 1 Galaxy 1 size x force gradient Time of encounter m r b V M Perturber, galaxy 2 Change of internal energy of galaxy 1 Binney & Tremaine “Galactic Dynamics”

Strangulation - removal of the gas halo Quite slow because gas reservoir needs to be depleted, which happens on several Gyr timescales. First suggested by Larson, Tinsley & Caldwell, 1984

Timescales for Galaxy Transformation ØHow rapid must the blue®red Red Peak Blue Peak transition be? ØTwo gaussian model always fits the data well – there is no room for an intermediate population. Øcolour evolves rapidly if timescale for star formation to stop is short Øif transformations occur uniformly in time: need t<0. 5 Gyr Øif transformations are more common in the past, longer timescales permitted ØAlso need to occur not exclusively in clusters.

Mechanisms n Ram-pressure n n Collisions / harassment n n Needs dense ICM and high velocities - clusters Density too low Most effective in groups: Groups are preferred place! "Strangulation" Removal of the gas halo: no more fuel supply n Similar to ram-pressure stripping but much easier! n Transformation too rapid

Clusters in X-rays n n Every photon in sacred! Spectra fit with plasma model for (T, ne, Z) in each 2 -D pixel. Cooling flow or cool core clusters: Center has lower T, peaked X-ray SB, higher metallicity. 70 -90% of clusters have cool cores: relaxed.

Surface Brightness n n n King model + isothermal hot gas produces a cored SB distribution (Cavaliere +Fusco -Femiano 1976) called a beta model: Chandra data shows additional cavities: Hot, low pressure bouyant. Possibly associated with intermittent AGN?

X-ray Scaling Relations n Suppose halos of all sizes are self-similar. Then: n n n Ethermal = Ekinetic k. Tx = ½ s 2 T x s 2. Free-free Lx = MT½ (+VT) L x T x 2. Combining Lx s 4. Observations show Lx Tx 3, with an even steeper relation at group scales. And s Tx 0. 64. What assumption is wrong? Xue & Wu 2000

Ponman, Sanderson, Finoguenov 2003 Entropy n n n A useful quantity to examine is “entropy”, S(R) T/ne 2/3. Self-similar case: S T. Observed: S T 2/3. Smaller systems have more diffuse hot gas. n n n Lx lowered relative to self-similar expectations. Radial profiles suggest cores, i. e. some process has set an “entropy floor” in the ICM. Cooling? Feedback?

De Grandi + Molendi 2001 Metallicity n n Clusters all have Z~0. 3 Z. Cool core clusters show elevated central metallicity. Central region shows more enrichment form Type I’s; outskirts from Type II’s. Could feedback that injected metals also inject energy? Probably not, but debated still. Finoguenov et al 2000

Clusters: Not so simple n n Decades ago, clusters were thought to be the simplest possible systems: Giant balls of gas in hydrostatic equilibrium sprinkled with old, passively evolving galaxies. Now, more questions than answers: Why are clusters galaxies so red and dead? n Why does intracluster gas show excess entropy? n What is responsible for enriching the ICM? n Are any/all of these answers related to our understanding of field galaxy formation? n