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 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.

A2218 – a cluster at redshift 0.23

Gravitationally lensed background galaxies

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Color-magnitude-density relation

Cluster galaxies are mostly red sequence.

Fewer blue galaxies in clusters; a continuous at low-L.

trend at high-L, more abrupt Luminosity is more important than environment Even isolated regions have passive galaxies What makes galaxies blue  red in clusters?

Balogh et al. 2004

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Ram-pressure stripping

Observed in HI + optical/H a .

P= d f r ICM /dz v s 2 . When this pressure exceeds restoring gravitational force, gas is stripped: < r ICM v 2 Solve  stripping radius .

Virgo: 52% truncated (vs. 12% in field).

Starvation? 6% “anemic”.

Harrassment? 6% enhanced.

Simulations work… but cluster center).

r ICM needs to be high (like near 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 1 b M m r

Galaxy Collisions, Tides and Harassment

Perturbation to velocity of star in galaxy 1 size x force gradient Time of encounter V Perturber, galaxy 2 

v

*  2

GM r b

3

b V



E

 4

G

2

M

2

m r

2 3

b

4

V

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

Red Peak Blue Peak

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

t <0.5 Gyr  if transformations are more common in the past, longer timescales permitted  Also need to occur not exclusively in clusters.

Mechanisms



Ram-pressure

 Needs dense ICM and high velocities - clusters 

Collisions / harassment

Density too low  Most effective in groups: Groups are preferred place!

 "Strangulation"  Removal of the gas halo: no more fuel supply  Similar to ram-pressure stripping but much easier!

Transformation too rapid

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Clusters in X-rays

Every photon in sacred!

Spectra fit with plasma model for (T,n pixel.

,Z) in each 2-D Cooling flow core relaxed .

or clusters: cool Center has lower T, peaked X-ray SB, higher metallicity.

70-90% of clusters have cool cores:

Surface Brightness

 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

   Suppose halos of all sizes are self-similar. Then:    E s thermal = E T x kinetic Free-free  2  L x  T x 2 .

Combining  L .

x L x  kT = MT ½  s 4 x .

= ½ (+VT) Observations show And s T x 0.64

.

L x  T x relation at group scales. 3 , What assumption is wrong?

Xue & Wu 2000

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Entropy

A useful quantity to examine is “entropy”, S(R)  T/n e 2/3 .

Self-similar case: S  T. Observed: S  T 2/3 .

Smaller systems have more diffuse hot gas.

  L x lowered relative to   Radial profiles suggest cores, i.e. some process has set an “entropy floor” in the ICM.

Cooling? Feedback? Ponman, Sanderson, Finoguenov 2003

De Grandi + Molendi 2001

Metallicity

    Clusters all have Z~0.3Z

 .

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

  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?

   Why does intracluster gas show excess entropy?

What is responsible for enriching the ICM?

Are any/all of these answers related to our understanding of field galaxy formation?