Transcript Document

Cosmology and
extragalactic astronomy
Mat Page
Mullard Space Science Lab, UCL
14. Galaxy counting and the
evolving Universe
Slide 2
14. Galaxy counting
• This lecture:
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The distribution of galaxies on the sky
What type of galaxy is found where?
How many galaxies in a cubic Mpc
Luminosity functions
Slide 3
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 4
Distribution of galaxies in South:
The galaxy distribution
Slide 5
• 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 6
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”.
What sort of galaxies do we see in
clusters?
Slide 7
Slide 8
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 9
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.
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
Slide 11
Recent map from 2dF
Slide 12
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...
More reasons to count galaxies
Slide 13
• 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?
Galaxies are evolving!
Slide 14
• 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.
Evolution of AGN
Slide 15
• Before we knew much about the evolution of
galaxies:
Slide 16
We can also use bright AGN to study clouds of
gas (galaxies?) out to very high redshift as
absorption line systems.
Slide 17
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
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!
Slide 18
CDM
observed
Galaxy counting: key points
Slide 19
• 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.