[4097] Physics 316: Extragalactic Astronomy and Cosmology (Summary of Lectures)
Lecture 30

[L.S.Structure] General Summary

In class there was a general discussion of the large-scale structure (observed) in the universe
(much repeated from the last lecture). 		The large-scale structure is also discussed in some detail in Bothun Chapt 3,
which I expect you to be familiar with.

Item discussed (but not in Bothun Chapt 3):
Movies shown in class
From the movies & discussions you have a good feel for the characteristics associated with the various types of clustering (galaxies, clusters, superclusters, "filaments", "voids").

However I also want you to know the typical size-scales involved and the problems & difficulties associated with determining these characteristics, size-scales etc.

Size "Structure"
1 pc Separation between stars
100 kpc Scale size for a large Galaxy
1 Mpc Scale size for a typical Group of Galaxies
10 Mpc Scale size for a large Cluster of Galaxies
100 Mpc Scale size for a typical Supercluster
100-300 Mpc Scale size for walls, filaments & voids
5 Gpc size of our horizon c/H0
(assuming 1/H0 of ~1.5x1010 yrs)

[L.S.Structure] Summary of Bothun Chapter 3

When reading about the various structures, keep in mind that the various terms are usually used in fairly imprecise or "fuzzy" mannor. There are no strict definitions for most of them.
  • eg. what exactly constitutes a "void"; where a "void" ends, and a "wall" begins; when does a "filament" become a "sheet" etc
This is because there is little point having such rigourous definitions - the universe itself is "fuzzy"

[Sect3.1]
  • There is no time dependence on the gravitational potential which is associated with a mass fluctuation
  • Initial density fluctuations at the time of recombination appear as fluctuations in the CMB
  • Galaxies can be used as "tracers" of the mass distribution (at least the visible [baryonic] mass)
    • Must be careful concerning any "bias"
  • Effect of neighboring galaxies is that a galaxy will not move in a pure Hubble flow. i.e. "pure" expansion is perturbed by the mass concentrations
    • the resultant "peculiar velocities" makes measuring distance more difficult.
    • However, such motions can be used as a diagnostic of the the mass distribution (eg. within a cluster etc)
[Sect3.2]
  • The velocity distribution is a poor predictor as to whether a structure is gravitationally bound (ie. whether it can be considered a "destrete entity")
    • The presence of an X-ray emitting ICM is a better means of determining whether a "structure" is bound
      • (plus physics & diagnostics of cooling flows of course)
  • There is clustering on a variety of scales
    • groups & clusters
    • chains of clusters (superclusters, "walls" & "filaments")
    • associated "voids" on various scales
  • Various timescales for several of these structures (eg groups & clusters) are less than the age of the universe
    • such structure can evolve significantly
  • Most superclusters appear to be flattened structures
    • Plus alignment of superclusters often seems to match the alignment of smaller scale structures.
      • chance ?
      • memory retention of structure collapse mechanism ?
      (gravitational collapse & dynamical relaxation ought to have erased these)
[Sect3.3]
  • (does not exist !)
[Sect3.4]
  • the "light" (ie. electromagnetic radiation incl X-rays etc) we detect from galaxies is not necessarily representative of of all the baryonic matter in the universe
    • as we have already discussed some (& will again in a later lecture) Omegab < 1
      • Omegatot = 1 is is strongly suggested by the CMB results and the observed abundances of the light elements
[Sect3.5]
  • 2-D images of the sky are a good start, but the 3rd dimension (redshift) is obviously preferable to fully characterize and understand the structures
  • Redshifts are difficult to measure for many faint/distant objects
    • astrophysical problems associated with a lack of strong, easily detected narrow features (with which to measure the redshift)
      • in the optical band, stellar absorption lines are often used - however these are relatively weak & difficult (require long observations) to measure with all but the largest telescopes
      • in the radio, the 21cm emission line due to hydrogen is used. However, again large radio telescopes are required (very few of these)
    • "logistical" problems associated with the observing time required and large number of sources to be observed.
    • (recent & ongoing technical developments & dedicaded survey programs have increased the rate at which these studied can be performed)
  • Redshift surveys generally take one of two approaches
    • stripe
    • pencil beam
    There are uncertainties plus pros & cons with both approaches
  • An artifact of the peculiar velocities is that virialized structures (such as the core of a cluster) appear as "straight line" when plotted in angle-redshift space (same angle, range of redshifts).
  • The (radial, towards/away from us) components of the peculiar velocities (due to gravitational attraction) breaks the direct (Hubble) redshift-distance relation. (Thus "sizes" are often quoted in velocity space).
  • There are filaments & sheets/walls that appear to be at the intersections of voids
  • The voids are devoid of bright galaxies
    • there could be a population of much fainter galaxies
    • (and/or dark matter)
[Sect3.6]
  • Some of the issues and results regarding the motion of the Local Group with respect to Virgo, and in the general direction of Hydra-Centarus Supercluster were discussed in Lecture 14 (see also Homework 4).
    • There is some form of large-scale "flow" that "our" cluster/supercluster is participating in !
  • Note: Sect 3.6.4 will be discussed briefly next time (since there have been recent developments)
  • The appearance of structure can/could be misleading (optical illusions, and statistical fluctuations)
[Sect3.7]
  • voids appear to be preferentially inhabited by spiral galaxies
    • can use the Tully-Fisher relation to estimate the distances and hence use the redshift (velocity) to calculate the inflow/outflow
  • Voids do indeed appear to be real "structures" - they are not an obvious artifact in redshift-space
  • walls indeed appear to have a "thickness" (ie "depth")
  • walls appear to be relatively unevolving structures
[Sect3.8]
  • There have been suggestions of a "really big flow".
    • However results not confirmed using other techniques
    • In disagreement with the Dipole Anisotropy of the CMB
    Current feeling is that it unlikely
[Sect3.9]
  • Peculiar velocities on large scales obviously must be kept in mind when determining H0
  • However there are obviously techniques for taking these into account.
  • An example (and test of the method) is provided by the relative distance between the Virgo & Coma clusters.
  • These values are indeed generally consistent with the estimates we have discussed before.

End of lecture

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