The Cosmic Web by Gott J. Richard;

Author:Gott, J. Richard;
Language: eng
Format: epub
Publisher: Princeton University Press
Published: 2015-03-11T04:00:00+00:00

Chapter 7

A Slice of the Universe—the Great Wall of Geller and Huchra

When I was at Caltech, I met Ed Turner, a graduate student working on his thesis on binary galaxies. I found Ed’s work with binaries especially interesting because of my own interest in groups of galaxies. We decided to team up and make a catalog of groups of galaxies, using the Zwicky catalog of galaxies down to magnitude 14. This included galaxies up to 1,600 times fainter than the faintest naked-eye stars. We wrote a computer program to find groups of galaxies by locating regions of the sky where the counts of galaxies per square degree were statistically above average. This was one of the first instances of using a computer program to pick candidates rather than doing it by hand. One knew the rules the computer had used and, therefore, what the results meant. We found about a hundred groups. Many of these were well known, like the famous Virgo cluster, but many were new.

Many of these groups already had redshifts for a few of their galaxies taken by observers. As we have seen, the redshift of a galaxy is the shift in the galaxy’s spectral lines toward the red, indicating its recessional velocity from the earth. This is proportional to the galaxy’s distance, according to Hubble’s law. But, in addition, galaxies can have peculiar, or individual, velocities due to their orbital velocities within clusters. For galaxies within an identified group, there would be a mean velocity of recession for the group as a whole, plus random velocities associated with the orbital velocities of the galaxies within the group. Having the orbital velocities of the galaxies within a group and knowing the size of the group, we could determine the group’s mass. Overall, we found the groups’ mass-to-light ratios to be about 140 solar masses per solar luminosity, similar to but a bit smaller than what Zwicky had found for the Coma cluster (about 500 solar masses per solar luminosity). This was understandable, since the Coma cluster contained many galaxies whose new-star formation had stopped, making them less bright for their mass than most spiral galaxies in small groups where star formation was still going on. We found a lot of dark matter, as Zwicky had, but definitely not enough to cause the universe to recollapse in the future: that would require 1,500 solar masses per solar luminosity. We concluded that the mass associated with groups of galaxies was not enough ever to halt the overall expansion of the universe.

Our group catalog provided an opportunity for young observers. Alan Sandage of Palomar Observatory had taken the redshifts of a multitude of galaxies for his many projects; it was hard to compete with him. But a young observer could pick some of our groups that had no measured galactic redshifts, measure them all, and determine massto-light ratios for these groups. Bob Kirshner did just this, measuring the redshifts for galaxies in six of our groups. He found a mass-to-light ratio for these groups of 204 ± 60 in solar units.

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