Galaxy Clusters Perseus Cluster in X-rays

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

A2218 – a cluster at redshift 0.23 Gravitationally lensed background galaxies Cluster galaxies, mostly red.

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

Ram-pressure stripping Observed in HI + optical/H . Observed in HI + optical/H . P=  ICM v 2. When this pressure exceeds restoring gravitational force, gas is stripped: d  /dz  <  ICM v 2 P=  ICM v 2. When this pressure exceeds restoring gravitational force, gas is stripped: d  /dz  <  ICM v 2 Solve  stripping radius. Solve  stripping radius. Virgo: 52% truncated (vs. 12% in field). Virgo: 52% truncated (vs. 12% in field). Starvation? 6% “anemic”. Starvation? 6% “anemic”. Harrassment? 6% enhanced. Harrassment? 6% enhanced. Simulations work… but  ICM needs to be high (like near cluster center). Simulations work… but  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 V M m b Perturber, galaxy 2 r Perturbation to velocity of star in galaxy 1 Galaxy 1 size x force gradient Change of internal energy of galaxy 1 Time of encounter Binney & Tremaine “Galactic Dynamics”

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

Timescales for Galaxy Transformation  How rapid must the blue  red transition be?  Two gaussian model always fits the data well – there is no room for an intermediate population.  colour evolves rapidly if timescale for star formation to stop is short  if transformations occur uni- formly in time: need  <0.5 Gyr  if transformations are more common in the past, longer timescales permitted  Also need to occur not exclusively in clusters. Blue Peak Red Peak

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

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

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

X-ray Scaling Relations Suppose halos of all sizes are self-similar. Then: Suppose halos of all sizes are self-similar. Then: E thermal = E kinetic  kT x = ½  2  T x   2. E thermal = E kinetic  kT x = ½  2  T x   2. Free-free  L x = MT ½ (+VT)  L x  T x 2. Free-free  L x = MT ½ (+VT)  L x  T x 2. Combining  L x   4. Combining  L x   4. Observations show L x  T x 3, with an even steeper relation at group scales. And  T x Observations show L x  T x 3, with an even steeper relation at group scales. And  T x What assumption is wrong? What assumption is wrong? Xue & Wu 2000

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

Metallicity Clusters all have Z~0.3Z . Clusters all have Z~0.3Z . Cool core clusters show elevated central metallicity. Cool core clusters show elevated central metallicity. Central region shows more enrichment form Type I’s; outskirts from Type II’s. 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. Could feedback that injected metals also inject energy? Probably not, but debated still. Finoguenov et al 2000 De Grandi + Molendi 2001

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