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Kupy galaxií – lekce II
Pavel Jáchym
Clusters – overview
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Classification
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concentration (compact – open)
distribution of brightest members
presence or absence of a cD galaxy
sub-clustering
morphology of dominant galaxies
single
dominant
cD
◦ Rood & Sastry classification:
Cluster type
Content
E:S0:Sp
Symmetry
Example
Regular
E/S0 rich
3:4:2
Spherical
Coma
linear
array of
galaxies
Intermediate
Sp-poor
1:4:2
intermediate
A194
flattened
dominant
binary
(Coma)
Irregular
Sp-rich
1:2:3
Irregular
Virgo
single
core of
galaxies
irregular
Compact groups

Hickson (1982)
◦ consist of 4-7 galaxies within an area of only
few 100 kpc diameter
◦ typical spacing 20-40 kpc
◦ more Sp galaxies than expected
◦ very short lifetimes against merging
◦ Stephan’s Quintet, Seyfert’s Sextet (see Figs.)
◦ M/L ~150 – 500
 large DM halos around individual galaxies
 or a common halo encompassing the whole
group
Cosmological simulations
can serve as a powerful cosmological probe of the
nature of mysterious dark matter and dark energy
 Cosmological simulations

◦ 85% of DM, 10% of hot gas and 2-5% of stars
◦ complicated astrophysics problem involving
 nonlinear collapse
 merging of dark matter
 radiative cooling of gas
 star formation
 chemical enrichment of the intergalactic medium by
supernovae and energy feedback.
Cosmological simulations
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Rich and complex structure of the gas density and
temperature distributions
◦ such as strong and highly aspherical accretion shocks surrounding the
cluster and
◦ turbulent gas motions within the cluster
The cluster gas is also
enriched with heavy
elements ("metals"), as the
metal-enriched gas is
stripped off from galaxies
when they orbit within the
cluster.
Local structures

Supergalactic plane
◦ sheet-like structure that contains the Local supercluster, the Coma
supercluster, the Pisces-Cetus supercluster, and the Shapley concentration
◦ it separates two giant voids – the Northern and the Southern Local supervoids
◦ is the reference plane for the system of supergalactic coordinates

Supergalactic coordinates – MW in the center, plane (x,y) coincides with
the supergal. plane, axis y points to the Virgo cluster
Local group
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Cumulative luminosity
function of local group
galaxies is consistent with a
Schechter function:
Virial radius

the characteristic or virial radius (Rv) of a cluster
◦ defined from the theory of structure collapse in an expanding universe as
radius within which the mean density of the cluster is 200 times the critical
density of the Universe
3H 2
 crit 
8G
with Hubble constant H=H(z)
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radius of a sphere, centered on the cluster, within which virial equil. Holds
the gas is heated by the gravitational infall to temperatures close to the
virial temperature
kT ~
GMm p
Rv
which ranges in clusters from 1 to 15 keV. The total X-ray luminosities range
from about 1043 erg s-1 to 1046 erg s-1
ICM – temperature profile

XMM-Newton
◦ profiles show a clear decline beyond
≈ 0.2 R200
◦ there is no evidence of profile
evolution with redshift out to
z ≈ 0.3

In the center the temperature
falls to typically a third or a half
of the temperature in the
outskirts of the cluster
ICM – metallicity
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The mean metallicity profile
shows a peak in the center, and
gently declines out to 0.2 R180
Beyond 0.2 R180 the metallicity
is ≈ 0.2 solar and flat
No evidence of profile
evolution from z = 0.1 to z = 0.3
Lx-T relation

analytic and numerical simulations
of cluster formation => total X-ray
luminosity Lx~T2
◦ L is dominated by thermal
bremsstrahlung L ~ n2 T1/2Rvir3,
◦ mean gas density n ~ M/Rvir3 is
constant and
◦ T = M/Rvir
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however, observations show Lx ~ T3
◦ gas has additional heating?
Cooling flow clusters
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ICM in the centres of galaxy clusters should be rapidly cooling at the rate
of tens to thousands of solar masses per year
this should happen as the ICM is quickly losing its energy by the emission
of X-rays
X-ray brightness of the ICM is proportional to the square of its density,
which rises steeply towards the centres of many clusters
typical timescale for the ICM to cool is relatively short, less than a billion
years. As material in the centre of the cluster cools out, the pressure of
the overlying ICM should cause more material to flow inwards (the
cooling flow)
Cooling flows

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it is currently thought that the very large amounts of expected cooling are
in reality much smaller, as there is little evidence for cool X-ray emitting
gas in many of these systems = the cooling flow problem
theories for why there is little evidence of cooling include
◦ heating by the central AGN possibly via sound waves (seen in the Perseus and
Virgo clusters)
◦ thermal conduction of heat from the outer parts of clusters
◦ cosmic ray heating
◦ hiding cool gas by absorbing material
◦ mixing of cool gas with hotter material
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The problem appears to be widespread,
from the most massive clusters to the
centers of individual elliptical galaxies
Perseus cluster:
Distribution of galaxies
Galaxies of all types gather along filaments and in galaxy
groups and clusters
 Elliptical and S0 occur preferably in regions with high galactic
densities
 Spiral and irregular in less dense regions
 Morphology-density relation

Cluster\galaxy
type
E
S0
Sp
Regular
35%
45%
20%
Intermediate
20%
50%
30%
Irregular
15%
35%
50%
Field
10%
20%
70%
S0 galaxies: smooth feature-less
disks, larger bulges than in Sp,
stars are old and red as in E
Morphology-density relation

Dressler 1980:
Sp+Irr
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Virgo cluster:
S0
E
Fornax cluster:
Effects of environment

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only about 1% of galaxies are isolated
most are found in groups, ~5% in rich clusters
◦ => environment may play an important role

in dense clusters
◦ relative distances between galaxies << rel.dist. between stars in galaxies
◦ up to 100 galaxies/Mpc^3, i.e. a mean distance of 150 to 300 kpc.
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Tidal interactions
◦ merging
◦ galaxy harassment
◦ galaxies vs. cluster potential
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tidal stripping
Ram pressure stripping, starvation
Viscous stripping, thermal evaporation
Pre-processing of galaxies in groups
galaxies may lose
material
Tidal effects – dynamical friction
As a massive galaxy moves through a “sea” of stars, gas, (and the dark halo), it
causes a wake behind it increasing the mass density behind it
 This increase in density causes the galaxy to slow and lose kinetic energy
 The galaxy will eventually fall in and merge with it’s companion
G2M 2
f dyn  C
vM2
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C … depends on structure of both galaxies
M … mass of galaxy falling in
v … velocity of galaxy falling in
ρ ... density of stars (surrounding material)
◦ the slower the galaxy’s speed, the stronger the dynamical force, the more intense
the interaction
◦ the more massive the object, the greater the effect
Galaxy interactions
Terminology:
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Major merger – two similar mass galaxies, gives rise to tidal tails
Minor merger – a satellite (dwarf) galaxy merging with a larger massive galaxy,
makes bridges, also tidal stripping
Retrograde – galaxy is rotating in opposite direction of velocity of “intruder”
Direct – galaxy is rotating in same direction as velocity of “intruder”
Impact radius – distance between center of galaxy and intruder
Inclination angle between galaxy and intruder
Viewing angle – our line of sight to the merger
In 1940s, Holmberg predicted the giant tides developed in the interaction and
merger of the galaxies
◦

Simulations on an analogue computer – two systems of 74 movable lamps whose
intensity decreases with the square of the distance simulated the stars
Observations of long filaments around interacting galaxies (flux tubes of the
magnetic field?, explosions in galaxy centers?)
“Galactic bridges and tails”

Tommre & Toomre (1972) – their numerical simulations established that
gravitational interaction with another galaxy could be the origin of the
filamentary structures
◦ encounters on parabolic orbits
◦ disks of test particles
◦ all self-gravity of the disk neglected
The Antennae
The Mice
Galactic tides – observation examples
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Disrupted spiral galaxy Arp 188 with a long tail
featuring massive, bright blue star clusters seen by
HST:
◦
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probably a more compact intruder galaxy crossed in
front of Arp 188. During the close encounter, tidal
forces drew out the galaxy’s stars, gas, and dust forming
the spectacular tail.
The tidal action allows the formation of four arms if
the two companions are disk galaxies. When their
masses are comparable, the two internal spiral arms
join up to form a bridge that disappears quickly, the
two external spiral arms are drawn into two
antennae:
The Antennae – interacting
galaxies NGC 4038 – 4039
Arp 188
The Mice – interacting
galaxies NGC 4676
Tidal action
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The tidal force experienced by an object of diameter d in interaction with a mass
M at a distance D: the parts closest to M are more attracted than those away.
The order of magnitude of the force: Ftide ~ GMd/D3
If the distance between two galaxies is greater than their individual radii, the
main term in the tidal forces varies as cos2θ in the plane of the galaxy
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In the case where the companion galaxy’s orbit lies in an inclined plane, the
azimuthal dependence of the tidal force is no longer bisymmetric but contains
the Fourier term m=1 => excitation of oscillations observed in warps
The principal effects are purely kinematic, which explains the success of the
simple restricted three-body simulations
◦
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There exist two poles of perturbation rotating with an angular velocity Ω => formation
of two spiral arms in the galaxy
Especially the selfgravity of the gas is negligible. It is far more perturbed than the
stellar component due to its small velocity dispersion and its greater extension into the
external regions
The tidal interaction can be very violent and collisions between clouds in the
spiral arms give rise to starbursts
There exists a certain correlation between the presence of bars and companions:
interacting spirals yield a greater fraction of barred galaxies than field galaxies
Formation of filaments and rings
Galaxy mergers
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Major mergers of galaxies generally lead to elliptical-like remnants, with some
irregular structures in the outer regions
Depending on the orbital geometry of the merger, the remnant can either be
prolate or oblate.
In general, mergers of two equal-mass disks lead to rounder remnants if the spins
of the merging progenitors are more tilted relative to the orbital angular
momentum.
Highly flattened remnants can be produced in prograte and retrograde encounters
Mergers affect both stellar and gaseous content
Dynamically cool stellar disks warm up
Compression of gas => shocks => star formation
Formation of bars
Nuclear inflows – nuclear star burst, AGN
Some material ends up in long tails and bridges (see Toomre &Toomre 1972)
Most of material stays bound
Mergers

Slow interactions important in galaxy groups
◦ smaller velocity dispersions than in clusters

Long-living tidal tails in clusters destroyed by
potential of the cluster
The Milky Way is warped by the
passage of the Magellanic Clouds
 The Magellanic Clouds may eventually
merge with the Milky Way.
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Tidal dwarfs
Galaxy harassment, IC light
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Cumulative effect of frequent close high-velocity encounters
◦ once per Gyr,
◦ relative velocity of ~ 1500 km/s
◦ Impact parameter of ~ 50 kpc
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+ tidal effect of cluster potential
Produces distorted galaxies with enhanced star formation
rate
Low vs. high surface galaxies
Intracluster (IC) light
◦ forms as galaxies collide and interact gravitationally within the cluster
◦ gravitational forces strip stars out of their parent galaxy => diffuse web
of faint ICL throughout the cluster
Ram pressure stripping (RPS)
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Momentum transfer process
Gunn & Gott (1972) assumed that after the formation of a galaxy cluster,
the remaining gas is thermalized via shock heating to virial temperature (~
107K)
As spiral galaxies move through this hot plasma at densities of ~ 10-3 cm-3,
the ISM in disks can be partly or totally removed by the ram pressure of
the ICM
Gunn & Gott (1972) predict that the ISM is removed from the disk if the
ram pressure exceeds local gravitational restoring force:
 ICM vg2 
 ( r , z )
 ISM ,
 z max
where ρICM … ICM density, vg … relative velocity galaxy-ICM,
ΣICM … ISM surface density, Φ(r,z) … total galaxy potential
RPS, cont.

NGC 4522 (in Virgo cluster)
normal stellar disk + truncated HI disk
RPS, cont.
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NGC 4569 (in Virgo cluster)
Galaxies caught in the act
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Truncated gas disks
One-sided off-plane gas
Tail
Bowshock on the opposite side
e.g. NGC 4522
◦ rot.vel.~130 km/s, ~ 1Mpc from M87,
los velocity ~ 1300 km/s
◦ stellar disk undisturbed
◦ Hα truncated to 3 kpc (-> even molecular gas
is stripped?)
◦ HI similar to Hα
◦ RPS to low?
 bulk motions and density enhancements
due to subcluster merging?
Caught in the act
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NGC 4569
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NGC 4402
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Highly HI-def.
shows central starburst
soft X-ray emission at one side
HI arm
~500 kpc from center, 1100
km/s
◦ also dust is stripped
Caught in the act …
HI tails
In central region of Virgo cluster
 110 x 25 kpc
 Material from NGC 4388
 Gas can remain neutral for about 108 yr
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X-ray trails
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E.g. galaxy C153 in cluster A2125
Viscous stripping

Nulsen 1982
◦ outer layers of a spherical galaxy travelling through the hot ICM
experience a viscosity momentum transfer that is sufficient to drag out
some gas at rates depending on the character of the flow (turbulent:
drag force ~ ram pressure force)
occur simultaneously with RPS
 may dominate in edge-on motion of galaxies
 mass loss rates can be comparable to RPS rates
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Starvation/strangulation
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Estimate:
◦ typical galaxy of mISM ~ 2 109 Msol and SFR ~ 2 Msol/yr consumes its gas
within ~1 Gyr
◦ even if stars returned half of the consumed material back to ISM, the
gas would be exhausted after few Gyr =>
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Larson et al. (1980): galaxies are surrounded by reservoirs of
gas => gas supply
The reservoirs however can be stripped quite easily
◦ => starvation (strangulation)
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Bekki et al. (2002): about 80% of the halo gas can be stripped
during few Gyr even from galaxies in cluster outskirts
Thermal evaporation
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Galaxies are surrounded by the hot ICM
effects of heat conduction and consequent evaporation of
the ISM in contact with the hot ICM
At the interface between the hot ICM and cold ISM the
temperature of the ISM steeply rises and the gas evaporates
and is not retained by the gravitational field
Thermal evaporation depends on the ICM temperature and
on the magnetic field, and to a lesser extent on the density.
analytical estimates of the time-scales of the evaporation in
clusters, subclusters, and groups
◦ about 1 - 3 Gyr, 2 - 7 Gyr, and 10 Gyr, respectively
Galaxies in different environments

From large surveys like 2dF and SDSS
◦ hundreds of thousand galaxies allow comparison between
different environments
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Colors, morphology, and star formation rates
◦ In dense environments blue, late-type, and star forming
galaxies are less common than in low-density environments
◦ Color-density relation
◦ Morphology-density relation
◦ Star formation-density relation
Galaxies in different environments, cont.

In the nearby universe (z<0.1)
◦ Sparse regions (ngal<1 Mpc-2 or R>Rvir)
 Values converge towards the field population
◦ Intermediate regions (ngal=1–6 Mpc-2 or R=1–0.3 Rvir)
 Fraction of late-type’s decreases
 Galaxies with strong SF vanish
◦ High-density regions (ngal>6 Mpc-2 or R<0.3 Rvir)
 Early-type fraction increases
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With increasing z
◦ Butcher-Oemler effect = increase
of the fraction of blue galaxies in
clusters with z
Virgo cluster

all types
dE+dS0
E+S0
Sp+Irr
Virgo cluster
◦ Distribution of galaxies of
all types follow that of X-ray
◦ Late-types are more
extended
◦ Early-types more
concentrated
◦ 52% of bright spirals have
truncated Hα disks
Gas content of late-types
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HI-deficiency
Galaxies closer to cluster center have smaller HI disks (with
normal central surface density)
Fraction of deficient galaxies is correlated with the X-ray
luminosity
Solanes et al. (2001): 1900 galaxies in 18 nearby clusters
◦ In 2/3 of the clusters the galaxies show HI deficiency
◦ Fraction of gas-poor galaxies increases towards the centre
◦ Gas-poor galaxies tend to be on more radial orbits
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Molecular gas not affected (?)
RPS enhances the star formation rate in the inner disk by a
factor of ~2
HI-deficiency
def = log
M HI,observed
M HI,expected
expected HI mass corresponds
to an isolated galaxy of the
same morphological type and
optical diameter
 Virgo core galaxies:
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◦ average deficiency of ~ 2.6
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Distribution of HI-deficient
Virgo galaxies:
def > 0.3