Transcript Slide 1

Optical observations of clusters of
galaxies
Coma Cluster
A larger image of the Coma Cluster
The Virgo Cluster
The Virgo Supercluster
Optical Catalogs
• The Abell Catalog (1958)
The Abell Cluster Catalog
• Use the Palomar Sky Survey, complete photographic
all sky survey made at Mount Palomar observatory in
the 1950's.
• clusters have to satisfy four criteria:
– Richness: A cluster must have a minimum population of 50
members within a magnitude range of m3 to m3+2 (where m3 is
the magnitude of the third brightest member of the cluster)
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Group 0: 30-49 galaxies
Group 1: 50-79 galaxies
Group 2: 80-129 galaxies
Group 3: 130-199 galaxies
Group 4: 200-299 galaxies
Group 5: more than 299 galaxies
– Compactness: the “Abell radius,” is defined as 1.72/z
arcminutes, (or 1.5H-1 Mpc)
– Distance: A cluster should have a nominal redshift of between
0.02 and 0.2
– Galactic-Latitude: Areas ouside the Milky Way (even if some
clusters where incleded)
The Abell Cluster Catalog
• The Abell catalog of rich clusters of galaxies
contains 4,073 rich galaxy clusters: 2,712 from
the" Northern Survey” and 1,361 in the Southern
Survey
• Clusters are identified as: Abell X, where X = 1
to 4076
– A426, the Perseus Cluster
– A1656, the Coma Cluster
– A3526, the Centaurus Cluster
• The selection criteria are rather arbitrary
• The catalogue is highly contaminated
Optical Catalogs
• The Abell Catalog (1958)
• The Zwicky Catalog (1861-1968)
The Zwicky Catalog
• Uses a subset of the Palomar Sky Survey: Zwicky identified clusters
in 560 of the POSS fields.
• To determine cluster diameters, Zwicky drew isopleths at the level
where the cluster density was twice that of the background density
of galaxies.
• The number of cluster members was determined by:
– counting all galaxies within the isopleth
– within three magnitudes of the brightest member,
– subtracting the background count.
• All Zwicky clusters are rich clusters, having at least 50 members
within 3 magnitudes of the brightest member.
• Same clusters are classified larger and more populous by Zwicky
than it is by Abell.
• Zwicky clusters may contain two or three Abell clusters. E.g.
Zwicky Hercules contains Abell clusters 2147, 2151 and 2152.
Recent Cluster catalogues
• To avoid projection problems we should
know the galaxy cluster member redshift
• This is expensive in terms of time
The 2dF survey
APM and 2dF sky coverage
2dF
Show movie
Recent Cluster catalogues
• To avoid projection problems we should
know the galaxy cluster member redshift
• This is expensive in terms of time
• Photometric survey
Red Sequence
The Red Sequence Cluster Survey
The Red-Sequence Cluster Survey is the largest area,
moderately deep imaging survey ever undertaken on
4m class telescopes. The survey comprises 100 square
degrees of imaging in 2 filters (R and z), to a depth
sufficient to find galaxy clusters to z~1.4 (2 mags past
M* at z=1)
Property
Symmetry
Regular Clusters
Irregular Clusters
Marked spherical symmetry
Little or no symmetry
Concentration
High concentration of members
toward cluster center
No marked concentration to a
unique cluster center; often two or
more nuclei of concentration are
present
Collisions
Numerous collisions and close
encounters
Collisions and close encounters are
relatively rare
All or nearly all galaxies in the first 3
or 4 magnitude intervals are
elliptical and/or S0 galaxies
All types of galaxies are usually
present except in the poor groups,
which may not contain giant
ellipticals. Late-type spirals and/or
irregular galaxies present
Order of 103 or more
Order of 10 to 103
Order of 1 - 10
Order of 1 - 10
Probably absent or unimportant
Often present. Double and multiple
systems of galaxies common
Radial velocities dispersion
Order of 103 km/sec
Order of 102 - 103 km/sec
Mass (from Virial Theorem)
Order of 1015 Msun
Order of 1012 - 1014 Msun
Types of galaxies
Number of galaxies
Diameter (Mpc)
Subclustering
Other characteristics
Cluster often centered about one or
two giant elliptical galaxies
Examples
Coma cluster (A1656); Corona
Borealis cluster (A2065)
Virgo cluster, Hercules cluster
(A2151)
Optical Classification of clusters of
galaxies
•cD - single dominant cD (elliptical) galaxy (A2029, A2199)
•B - dominant binary, like Coma
•L - linear array of galaxies (Perseus)
•C - single core of galaxies
•F - flattened (IRAS 09104+4109)
•I - irregular distribution (Hercules)
Emission Processes of Clusters of
Galaxies in the X-ray Band
Cluster Gas Density
Observables Relations
T-M
Virial Equilibrium
Kinetic Energy for the gas
Thermodynamic
T-M relation
Status of The IGM
Age of Clusters ~ few Gyr; R ~ 1-2 Mpc
T ~ 1-10 keV; Gas highly ionized
Electrons free mean path
Gas may be treated as a fluid
Timescale for Coulomb Collisions
Electrons are in kinetic equilibrium
Maxwellian velocity distribution
Timescale for soundwave propagation
Gas is in hydrostatic equilibrium
Intracluster Medium
Hydrostatic equilibrium (spherical symmetry)
We can measure the Cluster mass
Dynamical Properties of the Galaxies
Isothermal Cluster
King profile
Beta Profile
Emission Processes of Clusters of
Galaxies in the X-ray Band
•The IGM is a Plasma
•Electrons are accelerated by the ions
•They emit for Bremsstrahlung
•Electrons are in kinetic equilibrium (Maxwellian V distr. )
•Cluster emission is mainly thermal Bremsstrahlung
Emission Processes of Clusters of
Galaxies in the X-ray Band
Beside IGM contains
some metals (0.3 Solar)
They produce line emission
X-ray Observations
•Gas density
•Gas Temperature
•Gas chemical composition
•If assume hydrostatic equilibrium
Cluster Mass
Clusters –Cosmology
connection
Clusters are useful cosmological tools
Evolution of N(M,z) to constrain
cosmological parameters
Rosati, Borgani & Norman 03
But: matter is dark & we need light to
see/count/measure galaxy clusters…
Instead of M we can either use
LX  ngas2 (T) Volume
or
Tgas
Observables Relations
T-M
Virial Equilibrium
Kinetic Energy for the gas
Thermodynamic
T-M relation
X-ray scaling laws: M  T3/2
Evrard, Metzler & Navarro (1996) use gasdynamic simulations to assess the
accuracy of X-ray mass estimations & conclude that within an overdensity
between 500 and 2500, the masses from -model are good. The scatter can
be reduced if M is estimated from the tight M-T relation observed in
simulations:
M500 = 2.22e15 (T/10 keV)3/2 h50-1 Msun
-model
law
X-ray scaling laws: M  T3/2
Nevalainen et al. (2000) using a ASCA (clusters: 6) & ROSAT
(groups: 3) T profiles: (i) in the 1-10 keV range, M1000  T 1.8
[preheating due to SN?], but (ii) at T>4 keV, M1000  T 3/2 [they
claim, but measure 1.80.5 at 90%…] & norm 50% [!!!] lower than
EMN :
EMN96
X-ray scaling laws: M  T3/2
Finoguenov et al. (2001) use a flux-limited sample of 63 RASS
clusters (T mainly from ASCA) & 39 systems btw 0.7-10 keV with
ASCA T profile.
(i) Steeper profile than 3/2,
high scatter in groups
(ii) deviations from simulations
due to pre-heating [makes flat
ngas] & z_formation
(iii) M from -model:  depends
on T
EMN96
X-ray scaling laws: M  T3/2
Allen et al. (2001): 7 massive clusters observed with
Chandra, M2500-T2500 relation.
slope of 1.52  0.36 &
normalization lower
than 40%.
ME01
Observables Relations
L-M
X-ray Luminosity
Observables Relations
L-T
Theoretically
However from an observation point of view
X-ray scaling laws: self-similar?
We have a consistent picture at T>3 keV, but also
evidence that cool clusters/groups may be not just
a scaled version of high-T clusters [review in Mulchaey
2000]
T3
T5
X-ray scaling laws: evolution
Luminosity Function
Local (left) & high-z (right) XLF: no evolution evident
below 3e44 erg/s, but present at 3 level above it (i.e.
more massive systems are rare at z>0.5)
Rosati et al. 03
Temperature Function & cosmological
constraints
Markevitch 98
Henry 00
Cosmology in the WMAP era
1-st year results of the temperature
anisotropies in the CMB from MAP
(Bennett et al., Spergel et al 03) put alone
constraints on tot, bh2, mh2.
Cosmology in the WMAP era
However, the final answer to the cosmology quest is not
given:
• the cosmological parameters in CMB are degenerate…
complementary
• the equation of state of Dark Energy & its evolution
with redshift is not known
• given that, we can play the reverse game: fix the
cosmology & see what your cosmology-dependent data
require
Cosmology in the WMAP era
In non-flat cosmologies, there is degeneracy in m- space (e.g. =0 is
consistent with MAP results, but requires H0=32 and tot=1.28…).
To get tighter & non-degenerated constraints, one needs to add
something else, like, P(k) from 2dF & Lyman- forest, Hubble KP, SN Ia,
clusters survey…: complementarity
Allen etal 02
Cosmology in the WMAP era
The equation of state of the Dark Energy & its evolution with
time: only post-MAP CMB surveys (i.e. Planck in 2007), SN Ia, Xray/SZ clusters can answer in the next future
Cosmology in the WMAP era
The equation of state of the Dark Energy & its evolution with
time: only post-MAP CMB surveys (i.e. Planck in 2007), SN Ia, Xray/SZ clusters can answer in the next future
Mohr et al.