Cosmology and extragalactic astronomy Mat Page Mullard Space Science Lab, UCL 14. Galaxy counting and the evolving Universe

14. Galaxy counting This lecture: The distribution of galaxies on the sky What type of galaxy is found where? How many galaxies in a cubic Mpc Luminosity functions Slide 2

Origin of galaxy counting Remember we postulate an homogeneous, isotropic Universe? That means we should see the same number of galaxies in all directions. First galaxy counters were looking to check this, like the star counters before them who eventually showed we were in a galaxy. They found that the galaxy distribution is reasonably (but not exactly) isotropic. Slide 3

Distribution of galaxies in South: Slide 4

The galaxy distribution At first, things like dust had to be worried about. But even 50 years ago, cosmologists saw evidence for “large scale structure” from the Lick survey and others. Maps made by counting how many galaxies in a square degree of sky to some limiting magnitude. –Done by eye/hand with photographic plates. –Must have driven the astronomers insane. Slide 5

Clusters Galaxies are not uniformly distributed they prefer to cluster together. Our own galaxy has a group of companions, gathered together in the “local group”. Astronomers like Abell and Zwicky noticed there were some areas on photographs that were very dense with galaxies compared to the mean. They called these “galaxy clusters”. Slide 6

What sort of galaxies do we see in clusters? Slide 7

Galaxies in clusters Some of all types of galaxies seen in clusters However, the majority are ellipticals Spiral galaxies make up a larger proportion of galaxies outside clusters. Why should this be the case? Slide 8

Several possible reasons Discs are fragile. Easily damaged by many encounters in clusters. The hot intracluster gas strips the gas out of the disc as the spiral galaxy moves through the cluster. Velocities of the galaxies in the cluster are too high for them to accrete discs. Slide 9

What about the galaxy distribution in 3 dimensions? Clearer picture of large scale structure than 2 D projection on the sky. Classic redshift survey by John Huchra et al: Note fingers of god pointing radially back at us. Slide 10

Recent map from 2dF Slide 11

The galaxy distribution The galaxies shows a very particular distribution. They show walls and filaments. The clusters are at the intersections. They also show voids. Like foam or soap bubbles. Very similar to the cold dark matter simulations –But the modelers already knew what they had to make... Slide 12

More reasons to count galaxies For at least 30 years, astronomers were counting galaxies on photographic plates, then CCD images for another reason. If you count the number of galaxies per square degree as a function of magnitude, it is related to the number of galaxies per square degree as a function of distance. The number of galaxies per square degree as a function of distance tells you how volume and distance are related. Different cosmological models predict different amounts of volume for a given distance, as they have different expansion rates. In principle you can distinguish the cosmological models by this method. But why is it fundamentally flawed? Slide 13

Galaxies are evolving! The so-called “faint blue galaxy problem” meant that no cosmological model could account for the number of faint blue galaxies. The dominant effect being measured is the evolution of the galaxies, not the cosmological volume. Not until the big redshift surveys that people realised that this form of galaxy counting was futile. Much more useful than galaxy counts are luminosity functions. Can also use the {V/Vmax} test. On average, objects which are not evolving with time should lie half way across the volume available to them. Slide 14

Evolution of AGN Before we knew much about the evolution of galaxies: Slide 15

We can also use bright AGN to study clouds of gas (galaxies?) out to very high redshift as absorption line systems. Slide 16

And in the last 20 years it has been possible to measure their evolution directly with luminosity functions. Lilly et al., 1995 ApJ 455, 108 Slide 17

The luminosity function The galaxy luminosity function is something that cold dark matter models ought to be able to reproduce. Instead, they do a very poor job. Cosmologists blame this on the very complex nature of star formation and AGN in galaxies. They have a trick to fudge it using feedback from star formation and AGN At present it is a fudge! CDM observed Slide 18

The galaxy distribution shows ‘large scale structure’. Clusters of galaxies are the densest parts. Galaxies and AGN populations are evolving with time. More blue galaxies in the past - consistent with hierarchical model (they have merged and become ellipticals now) However, cold dark matter doesn’t make the right galaxy luminosity function without a fudge. Galaxy counting: key points Slide 19