1 Distributions of galaxies: The Local Group

1 Distributions of galaxies: The Local Group

M31: LMC: SMC :

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, satellites of Andromeda. The group has a size of about 3 million ly, and has a total mass of 5 x 10¹²M

Map of the Local Group……

Groups of Galaxies

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

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  Cluster are part of a continuous range of structures :
  

galaxies     groups     clusters     superclusters     large scale structure

However, clusters are rare extremes in the galaxy distribution, with / <> 10³ :

Clusters of Galaxies: poor and rich

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  Galaxies are not distributed at random in the sky:
  
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    Poor clusters contain ~ 10 - 100 galaxies of all types (spirals, ellipticals & irregulars).
    
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      See, for example the local group.
      
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    Rich clusters contain ~ 100 - 1000 galaxies
    
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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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      Here, for example, is a picture of the centre of the Coma cluster, the nearest rich cluster of galaxies, which is dominated by two giant ellipticals:
      

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  Clusters are supported against collapse by the random motions of the galaxies:
  

(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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  For a rich cluster, the size scale r ~ 1 Mpc and typical random velocities, v ~ 1000 km s^-1, give a mass estimate 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!
    

3 Processes: Collisions and Mergers

After their formation, galaxies can still change their appearance and star formation rates by interactions with other galaxies. Galaxies orbit each on in clusters. Those orbits can sometimes cause two galaxies to pass quite close to each other to produce interesting results.

Solid objects, like planets, can pass near each other with no visible effects. Stars are generally spread far apart in galaxies – the chance of a stellar collision is quite small. Instead, interactions between stars will be gravitational in nature.

However, galaxies are not solid, and can undergo inelastic collisions, which means some of the energy of the collision is transferred internally to the stars and gas in each galaxy.

Galaxies are also not widely separated but occupy a considerable fraction of the volume of a cluster.

The tidal forces will often induce star formation and distort the spiral pattern in both galaxies.

If enough energy is transferred internally to the stars, then galaxies may merge. Galaxy mergers are most frequent in dense environments, such as galaxy clusters.

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

The Mice

Arp 188 The Tadpole:

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

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  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.
  

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

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  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 (rapid encounters)
  

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  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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  How do the giant "cD" galaxies found at the centres of some clusters form?
  
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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:
    
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      The motion of a galaxy creates an enhanced "wake" of galaxies behind it
      

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  The excess gravitational pull of this wake slows the motion of the galaxy --- it is a frictional force.
  
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  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).
  
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  Once there, it will merge with all the galaxies that have preceded it.
  
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  Evidence for this scenario comes from the large number of "multiple nuclei" seen in cD galaxies:
  

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  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 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

• merging time, or time for a satellite galaxy of speed v to spiral in from an initial radius R, is t = 2pvR² / aGM (a is a constant).

  • 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. Stars and gas are stripped.

• tidal interactions = galaxy harassment

• 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,

ELLIPTICALS: formation

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  Old view – (ellipticals are boring, simple systems)
  
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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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  New view
  
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    Some ellipticals have hot x-ray gas, 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 a hierarchical process of mergers (bottom-up)
    

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  Vrot/s 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?
    

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  Fundamental Plane:
  
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  The missing parameter is effective radius (discovered in 1987). There are four observables (but only 3 independent parameters):
  
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    Luminosity
    
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    Effective radius
    
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    Mean surface brightness
    
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    Velocity dispersion
    

it a FUNDAM