5 Clusters 1 Distributions of galaxies: overview

5 - Clusters

5.1 Distributions of galaxies: overview

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.

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

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.

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

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  See, for example the local group.
  

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

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  So, we can estimate their masses using the same formula as for elliptical galaxies:
  
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  M = K s ² r / G
  

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.

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  A typical rich cluster contains ~ 1000 galaxies each with the luminosity of the Milky Way (~ 10¹⁰ L).
  
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  So the mass-to-light ratio of the whole cluster is M / L ~ 20 solar units.
  
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    ~ 95% of the material in a cluster of galaxies is dark!
    

5.3 Processes: Collisions and Mergers

made ~ a few orbits of the cluster.

The Mice:

Arp 188 The Tadpole:

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.

cD Galaxies and Cannibalism

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  Perhaps by repeatedly merging with other cluster members.
  
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  Why do these mergers occur at the centre of the cluster?
  
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    Because dynamical friction makes galaxies lose kinetic energy:
    

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 t_dyn = R/V = (R³ / GM_cluster) ^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

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  Ellipticals contain no gas & dust
  
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  Ellipticals are composed of old stars
  
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  Ellipticals formed in a monolithic collapse, which induced violent relaxation of the stars, stars are in an equilibrium state
  

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  Some ellipticals have hot X-ray gas,
  
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  Some have dust
  
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  Ellipticals do rotate (speed varies)
  
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  Some contain decoupled (counter-rotating) cores
  
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  Some have weak stellar disks
  
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  Ellipticals formed by mergers of two spirals, or hierarchical clustering of smaller galaxies
  

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  V_rot/s anti-correlates with luminosity
  
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    Lower luminosity ellipticals have higher Vrot/s -- rotationally supported
    
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    Higher luminosity ellipticals have lower Vrot/s -- pressure supported
    

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  In 1976, Faber & Jackson found that:
  
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    Roughly, L µ s⁴
    
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    More luminous galaxies have deeper potentials
    
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    Can show that this follows from the Virial Theorem
    
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    There is a large scatter – a second parameter?
    

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.

• 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!



r_e µ s^{1.24 -0.82}