1 Distributions of galaxies: The Local Group

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1 Distributions of galaxies: The Local Group

The biggest and brightest Local Group members are the Milky Way Galaxy and the brightest Messier objects: M31 (Andromeda) and M33. Next in line would be M32 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: LMC: SMC :

In fact, it seems that 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 that have been extensively studied with large telescopes. 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 10 million light-years, 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, 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……

What is the criterion for inclusion in the Local Group? Proximity. If we grant membership to all galaxies within 4 million light-years, we have about 30 members.

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

  • Nearby groups:

    • The Local Group, M81 group, Sculptor group

  • 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

  • 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.

The Intergalactic Gas

There is tantalizing evidence for intergalactic neutral hydrogen in the form of so-called “high-velocity clouds”.

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.

Compact groups pose some special puzzles. They consist of 4-7 galaxies within an area of only a few hundred kpc diameter. They contain more spirals than expected from the usual morphology-density relation, and have very short predicted lifetimes against merging.

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. Stephan's Quintet:

2 Galaxy environments

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

Compact groups are moderately rare systems typically containing a few bright galaxies (e.g. Hickson 1997). 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.

Galaxy (rich) clusters containing hundreds to thousands of members span a range of morphologies. Some are irregular systems lacking definite centers, 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”.

galaxies     groups     clusters     superclusters     large scale structure

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

Very roughly (depending on definitions) the total galaxy content of the universe is divided :

1-2% in rich clusters
5-10% in clusters
50-100% in "Local Group"s &/or looser group

Clusters of Galaxies: poor and rich

  • Galaxies are not distributed at random in the sky:

    • 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.

      • 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:

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

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

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

    • 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.

    • 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!

3 Processes: Collisions and Mergers

Galaxy Mergers/Interactions :

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?

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 (rapid encounters)

  • 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

  • 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:

      • 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 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

  • 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 a hierarchical process of mergers (bottom-up)

Rotation of ellipticals: Vrot/s v. luminosity

  • Vrot/s 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?

  • Fundamental Plane:

  • The missing parameter is effective radius (discovered in 1987). There are four observables (but only 3 independent parameters):

    • Luminosity

    • Effective radius

    • Mean surface brightness

    • Velocity dispersion


re µ s1.24 -0.82

  • Any model of galaxy formation has to reproduce this relation

  • Can also define the D-s relation for use as a distance indicator (standard candle!)

The Milky Way:

1. Proto-Galactic fragments: 1 - 100 x106 Msun

2. Mergers into spheroidal distribution, rapid evolution near centre: chemical gradients

3. Collisions and tidal encounters between fragments; globular clusters + field stars in halo.

4. Disrupted gas clouds collide, dissipate energy. External torques provide a systematic angular momentum; thick disk

5. Later: remaining gas sinks to mid-plane to form a thinner disk of present star forming molecular gas.

6. Further mergers fuels the bulge


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