Galaxies II: distances, clusters etc

How big is the universe?

Distances to Galaxies: Steps out from earth,

• Astronomical unit≈1010m

  10 billion m or 8 light-minutes

• Parallax of Individual stars

  1018m ∼ 100pc

• Moving cluster method (flock of birds seem to be moving towards a point on the horizon)

  1019m ∼ 2kpc

• Novae: smaller stellar explosions: this is Nova Cygni 1991 (expansion of shells)

  2kpc

  Nova Cygni 1992 Credit: NASA, ESA, HST, F. Paresce, R. Jedrzejewski (STScI)

• Cepheids: supergiant stars which pulsate regularly, can be seen in M100

  20 Mpc

  Credit: NASA, HST, W. Freedman (CIW), R. Kennicutt (U. Arizona), J. Mould (ANU)

• Type 1a Supernovae Mv = -20

  3000Mpc

• Brightest galaxies in clusters

  1000 Mpc

Various methods overlap, but still some problems Uncertainty increases at large distances.

Clustering:

This is the VIrgo cluster: over 1000 galaxies: 3 big ellipticals, including M87 at the bottom. Closest big cluster

Galaxies Of The Virgo Cluster Credit & Copyright: Matt BenDaniel

This is the core of the Virgo cluster: M 84 and M 86 are the big ellipticals: also some small ellipticals and spirals

Credit & Copyright: Jean-Charles Cuillandre (CFHT), Hawaiian Starlight, CFHT

Coma cluster contains at least 104 galaxies

The Hickson cluster is a very small compact one

Local group contains Milky Way & about 20 others LMC & SMC 50 kpc M31 and satellites (M32) 690 Kpc M33 2.7 mpc in Triangulum

• Remainder of local group are almost all dwarf elliptical. Closest component may be a dwarf galaxy "only" 20 kpc from us, in plane of Milky Way in Pegasus. Maffei I and II, seen as faint galaxies in I.R., but probably giant elliptical and spiral, probably at distance of 1-2 Mpc, and form their own group

Velocity of galaxies

Found in 1920's (Hubble, Humason, Slipher) that faint galaxies show a red shift: fainter the galaxy, faster the recession.

• Hubble was able to measure distances to closer clusters and found that velocity ∝ distance

v = H d H is Hubble constant: As measured by Hubble H = 550 km s-1/Mpc: Now we know H ~ 65 km s-1/Mpc : i.e. the average galaxy at 100 Mpc is receding at 6500 km/s

Can look at clusters in z: unfortunately random ("peculiar") velocities distort picture

• For a single galaxy, can only measure vel. in radial direction, so radial component of peculiar motion gets added on.t

• This distorts picture Coma cluster is close together in space, but velocity dispersion spreads it out in z. Clusters always point towards us: Finger of God effect!

Clusters appear to form

super-clusters:

local cluster together with Virgo and others form local super-cluster 100 Mpc. across

Do superclusters cluster?
Or how is mass distributed on the largest possible scale?

Might expect Stars ⇒ galaxies ⇒ clusters ⇒ superclusters

becoming smoother at each stage

Instead of super-clusters being approx. spheres of clusters, we seem to see voids separated by clusters

Voids are very empty: 1/100 th of the density of galaxies that might be expected
Space is like a foam: voids are like empty bubbles

and we can go deeper

And further: this is a cluster of galaxies at a redshift of .5

and further: this is a cluster of galaxies which is fairly close, but there the most distant galaxy known is buried in the picture

• Why is the universe expanding?
• What is it expanding into?
• Do superclusters cluster (or is the universe smooth?)
• and to start answering these we'll go on to Physics as a Creation Myth