5 Clusters 1 Distributions of galaxies: overview



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5 - Clusters

5.1 Distributions of galaxies: overview


The Local Group

The biggest and brightest Local Group members: the Milky Way Galaxy and the brightest Messier objects: M31 (Andromeda) and M33.

Next in line would be M32, the elliptical companion to M31, and the two Magellanic Clouds (the LMC and SMC). The Clouds are big and close, so we have good detailed studies of them.

The rest are smaller objects, either irregular galaxies or dwarf ellipticals.

M31,M32,IC10: LMC: SMC :

In fact, our own galaxy undergoes a gravitational interaction with the LMC and SMC.





NGC 6822 (``Barnard's Galaxy'') and IC 1613: Two other more distant and less luminous irregular galaxies. They are providing new insights for both the distance scale and the evolution of galaxies. Both have Cepheid variables, -still the best way of determining distances within the nearest few Megaparsecs, and both have current star-formation activity.



NGC 6822 (left) and IC 1613 – both are Irregular galaxies



The Local Group – dominated by the two giant spirals, Andromeda (M31) and our own Milky Way.

In addition to Messier 33, an intermediate mass Sc galaxy, there are 15 ellipticals and 13 irregular galaxies in the cluster, including the Magellanic Clouds, our Galaxy's satellites, Messier 32 and NGC 205 (M110 – added to Messier later), satellites of Andromeda.

Mass: The group has a total virial mass of 5 x 1012 M

Size: M31 is about 0.78 Mpc away (visual magnitude of 4.4)

Number: What is the criterion for inclusion in the Local Group? Proximity. If we grant membership to all galaxies within 1.2 Mpc, we have about 30 members.

Structure: dominated by Milky Way and M31 (binary galaxy system + satellites)

5.2 Groups of Galaxies

Almost all galaxies are found in pairs, groups, and clusters

Groups have < 50 galaxies, sizes ~1-2 Mpc, s ~ 100-500 km/s

In contrast, clusters have 50 – several thousands of galaxies, sizes ~ few Mpc, s ~ 700-1200 km/s



Groups Clusters

Core radius 250 h-1 kpc 250 h-1 kpc

Median radius 0.7 h-1 Mpc 3 h-1 Mpc

Velocity dispersion 150 km s-1 800 km s-1

(line of sight)

(h is Hubble’s constant in units of 100 km s-1 Mpc-1)

(Radial or line-of-sight) ) Velocity dispersion, s :

f(vr) = A exp ( -vr 2 / 2s2)

Nearby groups:

    • The Local Group,

    • M81 group (3.6 Mpc away),

    • Sculptor group (3.9 Mpc away)

  • Groups have HI gas and X-ray halos. X-ray luminosities ~ 1042-1043 erg/s

  • M/L ~200 !!!

  • Groups show morphological segregation:

spirals and irregulars tend to lie on the edges of the groups,

dE’s and dSph’s are companions to massive galaxies

  • Groups are dominated by spirals and irregulars

Compact groups pose some special puzzles.

Consist of 4-7 galaxies within an area of only a few hundred kpc diameter. . Stephan's Quintet:

They contain more spirals than expected from the usual morphology-density relation, and have very short predicted lifetimes against merging.

They are the dregs of a once-rich population, constantly forming from more diffuse group environments, long filaments seen lengthwise, and fictitious chance alignments.

A crucial role for bound or captured high-velocity members in pumping the group's energy and keeping it from merging.

Stefan’s Quintet is actually a Group of 4 at 100 Mpc + NGC7320 at 12 Mpc.

Remarkable shock wave (green arc). Red: Spitzer/IRAC 8 microns; Green; H alpha; Blue: visible. (Calar Alto Obs.)


The Hierarchy of Galaxy Clustering

1. Loose groups contain the majority of all bright galaxies. Our own Local Group is a good example of a very loose group – three fairly conspicuous spirals and many more small galaxies.

There is tantalizing evidence for intergalactic neutral hydrogen in the form of so-called “high-velocity clouds”. The local group is still collapsing from a point of maximum expansion. Other loose groups, such as the M81 system, are less extended and have undergone some dynamical evolution.



2. Compact groups are moderately rare systems typically containing a few bright galaxies. Galaxies within a compact group are separated by only a few galaxy diameters, and in many cases display evidence of tidal interactions. Intergalactic gas in compact groups is sometimes detected in HI or X-rays.

3. Galaxy clusters containing hundreds to thousands of members span a range of morphologies.

Some are irregular systems lacking definite centres, while others appear regular and symmetric. The regular systems contain large amounts of hot ( 107 K) gas; this material is polluted with significant quantities of “metals”.

Coma in the optical Coma in X-rays


The Intergalactic Gas

Intergalactic temperature: a balance between expansion (adiabatic) cooling and ultraviolet heating, around 10,000K.

In denser groups, we observe diffuse, ionized gas in the X-ray (free-free or Bremsstrahlung radiation) with temperatures of ~107 K

Gas is probably a mixture of material

  • Gas that never formed into galaxies

  • Metal enriched gas that escaped from galaxies

Gas is confined by the gravitational pull of the group

The virial theorem then implies that groups must have mass to light ratios of M/L ~ 150-500

  • More dark matter!!

  • Either the individual galaxies halos extend much farther out

  • Or (more likely) there is mass between the galaxies forming a group gravitational potential.

Chandra observations of clusters……bow shocks, filaments:


1E 0657-56 A 1795

High resolution observations with Chandra show that many clusters have substructure in the X-ray surface brightness

Clusters of Galaxies


Poor clusters contain ~ 10 - 100 galaxies of all types (spirals, ellipticals & irregulars).

      • See, for example the local group.

Rich clusters contain ~ 100 - 1000 galaxies

      • Mostly ellipticals

      • Often dominated by 1 or 2 giant ellipticals - designated "cD" galaxies - near their centres:



Below: the centre of the Coma cluster, the nearest rich cluster of galaxies, which is dominated by two giant ellipticals:



Clusters are supported against collapse by the random motions of the galaxies (Virial theorem: KE + PE/2 = 0)



(c.f. the motions of stars within elliptical galaxies)



    • So, we can estimate their masses using the same formula as for elliptical galaxies:



  • M = K s 2 r / G

where K is of order unity (3 – 7).

For a rich cluster, the size scale r ~ 1 Mpc and typical random velocities, s ~ 1000 km s-1, give a mass of M ~ 2 x 1014 M.



    • A typical rich cluster contains ~ 1000 galaxies each with the luminosity of the Milky Way (~ 1010 L).

    • So the mass-to-light ratio of the whole cluster is M / L ~ 20 solar units.

      • ~ 95% of the material in a cluster of galaxies is dark!

5.3 Processes: Collisions and Mergers


Time for a galaxy to cross a cluster is:
tcross = 1010 (d/10 Mpc) (v/1000 km s-1)-1 yr
…galaxies on the outskirts of a cluster have only

made ~ a few orbits of the cluster.


Approximately 0.3% of galaxies are currently in the process of merging. When two galaxies collide, they initially create long tidal tails and plumes, but ultimately settle down to systems which look very like normal elliptical galaxies. Could this be how the ellipticals formed?


NGC 4676 –

The Mice:

Arp 188 The Tadpole:
Below is a sequence of images of various real galaxies which we see at progressively later stages in the merger process

The system shown below, NGC7252, is a system at a very late stage of merging – this is shown in successively deeper images of the system – which appears with the short integrations, to be a single galaxy.



NGC7252 above provides the "smoking gun" which shows that mergers between galaxies can produce elliptical galaxies.

This HST Image below is of the ‘Cartwheel galaxy’, and is a particularly impressive example of what can happen when two galaxies collide face-on.

It is likely that the Milky Way will collide and merge with the Andromeda Galaxy in about 3 billion years from now.



Minor mergers between galaxies of very different masses are much more common than major mergers.

cD Galaxies and Cannibalism


How do the giant "cD" galaxies found at the centres of some clusters form?

  • Perhaps by repeatedly merging with other cluster members.

  • Why do these mergers occur at the centre of the cluster?

    • Because dynamical friction makes galaxies lose kinetic energy:

Dynamical Friction
Why does the orbit of a satellite galaxy moving within the

halo of another galaxy decay?

Stars in one galaxy are scattered by gravitational perturbation

of passing galaxy.

Stellar distribution around the intruder galaxy becomes

asymmetric - higher stellar density downstream than upstream.

Gravitational force from stars produces a `frictional’ force which slows the orbital motion.

The motion of a galaxy creates an enhanced "wake" of galaxies behind it



The excess gravitational pull of this wake slows the motion of the galaxy --- it is a frictional force.

The net effect of this force is to make a galaxy slowly spiral in toward the centre of the cluster (the point of lowest energy).

Once there, it will merge with all the galaxies that have preceded it.

Evidence for this scenario comes from the large number of "multiple nuclei" seen in cD galaxies:

These secondary condensations of light leftover mergers – M31 is another example:




Dynamical friction timescale:

  • As a massive galaxy moves through a “sea” of stars (and the dark halo), it causes a wake 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

Merging time tdyn = R/V = (R3 / GMcluster) 1/2

Merging time is typically Gyr ( We can’t watch mergers happen.)

Other effects of galaxy interactions:

  • When two galaxies interact, the energy sapped from their motion via dynamical friction is transferred to the random motions of the stars

  • Stars that acquire the most KE escape, the rest remain loosely attached, “puffing up” the disk

  • Tidal stripping: Consider a small galaxy of mass m and radius r orbiting a larger galaxy of mass M at a distance D.

  • The stars on one side of the satellite galaxy feel an acceleration that is different from the stars on the other side of the galaxy, this sets up a tidal force and energy is no longer conserved.

  • tidal interactions = galaxy harassment

  • Ram Stripping. Consider a spiral galaxy moving through a cluster. As it interacts with the intracluster gas, gas from the spiral will be swept out. This phenomenon is called ram pressure stripping. This phenomenon is probably the cause of the observed HI gas deficiency in spirals in clusters and from the transformations of spirals to SO’s in cluster environments,

5.4 ELLIPTICALS: formation


Old view – ellipticals are boring, simple systems

    • Ellipticals contain no gas & dust

    • Ellipticals are composed of old stars

    • Ellipticals formed in a monolithic collapse, which induced violent relaxation of the stars, stars are in an equilibrium state

New view

    • Some ellipticals have hot X-ray gas,

    • Some have dust

    • Ellipticals do rotate (speed varies)

    • Some contain decoupled (counter-rotating) cores

    • Some have weak stellar disks

    • Ellipticals formed by mergers of two spirals, or hierarchical clustering of smaller galaxies

Rotation of ellipticals: Vrot/s v. luminosity



  • Vrot/s anti-correlates with luminosity

    • Lower luminosity ellipticals have higher Vrot/s -- rotationally supported

    • Higher luminosity ellipticals have lower Vrot/s -- pressure supported

Faber-Jackson relation:

  • In 1976, Faber & Jackson found that:

    • Roughly, L µ s4

    • More luminous galaxies have deeper potentials

    • Can show that this follows from the Virial Theorem

    • There is a large scatter – a second parameter?



The Fundamental Plane:

Recall that for an elliptical galaxy we can define an

effective radius Re - radius of a circle which contains half

of the total light in the galaxy.


Measure three apparently independent properties:
• The effective radius Re

• The central velocity dispersion 

• The surface brightness at the effective radius Ie=I(Re)
Plot these quantities in three dimensions - find that the points all lie close to a single plane!



re µ s1.24 -0.82

Any model of galaxy formation has to reproduce this relation; core surface brightness is not constant and M/L ratio increases with luminosity.



Can also define the D-s relation for use as a distance indicator






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