>> So, once you have 3-D distribution, you
can project it onto the supergalactic
plane.
And here is a density landscape, high
peaks corresponding to high projected
density from one of the IRAS-based
redshift surveys.
So, the big set of mountains off from
center is the local supercluster.
We are always in the middle, so we're sort
of on the slopes of it.
But why project on the supergalactic plane
if you have 3-D picture?
And so, here is the highly-smoothed
density distribution of galaxies in 3-D
from the Redshift Survey.
Now, you see that they're really all these
blobs that really are in complex relation
to each other.
I mentioned a second center for
astrophysics surveyed by Huchra and
collaborators and what they did is they
chose a slice in the sky.
Very thin in declination, long in right
ascension, fan-shaped like that.
They did, they did this simply in order to
have manageable numbers of galaxies to
observe, went deeper than the previous
survey.
And when, past the coma cluster of
galaxies, and obtain this famous stick man
diagram.
The structure in the middle is the coma
cluster, and it actually doesn't look like
that, it is elongated in radial direction
by so-called finger of God effect.
Namely, remember, that measured velocities
of galaxies are vector sum of the Hubble
expansion velocity and a peculiar velocity
the galaxy may have on its own.
Now, in rich clusters of galaxies,
galaxies have high velocity dispersions.
They have to move fast in order to balance
the gravitational potential of the, of the
dark matter.
So, if you look through a cluster of
galaxies, suddenly some galaxies will
appear to be closer to us because they're
moving towards us relative to Hubble
expansion.
Some are moving away from us, and there
will be an elongation in what we call the
redshift space, as opposed to actual 3-D
space.
So, this is a two-dimensional slice
through a three-dimensional structure.
And you can see that galaxies actually
form a kind of mesh of filaments, and
there are big voids that had been noticed
previous to this.
Because this is a two-dimensional cut,
then it's reasonable to assume that, in
fact, voids must be really more of a
spherical nature, or at any rate, not
flat.
And the sheets might intersecting through
this plane, may actually look like
filaments.
In reality, they both act sheets in
filaments of a large scale structure.
So, there are actually two important
effects about redshift space.
And remember, redshift space has the
radial coordinate that is associated with
total measure velocity of the galaxy.
And a priori, we do not know which part is
due to Hubble velocity, which would be the
real radial coordinate, and which part is
due to peculiar motion.
Statistically, we can decompose that
layer.
Then what happens, in rich clusters of
galaxies, as I already mentioned, there
will be an elongation towards the
observer, due to the radial velocity
dispersion of galaxies moving fast inside
a cluster.
On the other hand, if we're looking not at
the dense structure like cluster but, say,
an intersection with, with one of the
sheets or walls or, or filaments.
Galaxies will be falling towards it.
It's not virial a structure but it's a
density excess and so galaxies fall to it.
So, those, which are on our side, are
falling towards it, acquiring velocity
that makes them look like they're a little
further out.
Those on the other side are falling back
towards that filament and that will make
them look like they're a little closer.
So then, this will squish the structure
along the line of sight, the opposite of
the finger of God effect.
So, the dependence theory is how dense or
how virialized is the density that we see.
In case of higher densities like clusters
of galaxies, we see the elongation, the
finger of God effect.
In case of low density enhancements, like
filaments, we see the opposite.
So, these effects are well-understood and
they have to be deconvolved out of
observations when performing the full
analysis.
So, the full compilation of CfA2 redshift
survey by Hucher et al., when, well, past
10,000 kilometers per second and they
noticed that there is an even larger
structure they called the Great Wall,
which contains, in part, the Coma Abell
1367 cluster, or maybe supercluster.
And it was basically filling up the space
of, available to the Earth's survey, up
until then.
And every time a redshift survey was done,
a structure was seen that was as large as
it can be fit in the survey volume.
Which, on face value of it, doesn't bode
well for the assumptions of homogeneity.
The resolution of this came with last
comparison redshift survey.
But before I tell you about that, let me
just give you some idea how these
measurements are done.
In the early days, spectrographs were
doing on galaxy at the time.
This was a tedious work and that's what
CfA survey was.
But then, astronomers designed
multi-object spectrographs, and they come
in two varieties.
First, we have to know, of course, where
the galaxies are in the sky.
In one type, you have a metal plate that's
positioned for coplanar telescope, and a
little open indoor slit was made where
each galaxy should be.
Then, instead of having one spectrum with
lot of blank space on each side, you have
a whole lot of little short spectra with
object in middle.
The other way is to use optical fibers.
Again, the fiber head is positioned to
where the galaxy should be, using some
kind of device, usually a robotic arm.
And then, these fibers are grouped
together to enter the slit of the
spectograph, and then the spectra are
taken together.
Both approaches work fairly well.
And this is what really moved the redshift
surveys to industrial strength of hundred
of thousands and now millions.
Recall that second CfA survey was a slice
in the sky.
A fan-shaped slice thin declination along
the right ascension, and they saw all
these voids and filaments and stuff.
Las Campanas Redshift Survey do the same
thing but they did three slices, and they
also went further out.
So, in their survey, they could see
structures of same size up to a hundred
megaparsec or so that were seen by CfA
Redshift Survey.
But they also went further and no larger
structures were seen.
In other words, homogeneity does seem to
apply on scales larger than hundreds of
megaparsecs.
And for cosmological purposes, that's
perfectly good enough.
Two huge redshift surveys really
transformed this field.
The first one was done by Anglo-Australian
observatory, Australian UK consortium, who
used one of those robotic fiber
spectrographs on 4 meter Anglo-Australian
telescope in Siding Spring in Australia.
They collected about a quarter million
redshifts and they did job first.
They also observed a lot of quasars and so
on.
Their analysis really started revealing
interesting new features of large-scale
structure.
At the same time and continuing beyond,
was the Sloan Digital Sky Survey.
They used a 2.5 meter telescope in New
Mexico, at Apache Point Observatory.
And they did both imaging survey, and
spectroscopic survey.
They did the imaging of large areas of the
sky first, selected galaxies, then they
used multifiber spectrographs to obtain
redshifts of galaxies as well as quasars
and so on.
This was spectacularly successful, an
earlier release of the Sloan Digital Sky
Survey had something like, 800,000 galaxy
redshifts, but they did continue since
then.
And now, they have of the order of 1 and a
half million galaxy redshifts, and
comparable number of velocity measurements
for stars, as well as couple hundred
thousand quasars.
This is truly a fundamental data set on
which many of the modern studies of the
large-scale structure are based.
So, here is the redshift slice of the 2dF
survey.
And it looks sort of like Las Campanas
Redshift Survey, only with much better
resolution, because they had an order of
magnitude, more galaxies.
You can see the master filaments and
voids, and, and so on, but you can also
see that on scales larger than about 100
megaparsecs, we do not see super voids or
super filaments or anything like that.
Here is a picture showing sky coverage
from the latest incarnation of Sloan
Digital Sky Survey.
They have these strange belts in the sky
because they operate in drift scanning
mode.
Telescope moves in large circles in the
sky as they collect the data.
And then they obtain redshifts along, in
the same areas.
This may look a little strange at first.
But if you know exactly where you are
pointing, you can certainly take this into
account in your analysis.
And here is the projection of the Sloan
Sky Survey.
Now again, this squishes 3D information
into a plane that cuts across the sky.
And we see the same qualitative features
as before.
A frothy kind of large scale structure,
composed of filaments or walls or sheets
intersecting and more or less
quasi-spherical voids inside of them, sort
of like sponge-like topology.
There is always a tradeoff, of covering
large area of the sky, and being complete,
versus going deep.
If you have finite amount of observing
time, there are only so many galaxies you
can observe.
And so, you can trade one for the other.
These large scale surveys that we
described covered large areas, but, they
did not go very deep on cosmological
scales.
Complimentary to them were surveys done
over small areas in the sky, but going
very deep, so-called pencil beam surveys,
because of the narrow cone.
The first of those were done in 1990s, and
they saw presence of large-scale structure
out of high ratchets, spikes in the
retrodistribution.
At first, it seemed like those are
periodic but that turn out not to be the
case and now it's believed the
characteristic scale that they saw there
was really due to the barionic acoustic
oscillations that scales 100 megaparsecs
or a little more.
Here is the little redshift slice
distribution from one of these modern
survey.
This one is called DEEP and it was
[unknown] telescope reaching out the
redshift 1 and a half or several times
deeper than the large area surveys from
before.
Now, if you make a histogram of
redshiftsalong the line of sight, you'll
see these spikes.
They correspond to intersection of
clusters and filaments and, and walls
along your line of sight.
And the interesting thing is that
structure keeps resisting out to the
largest distances we can measure.
This leads to an interesting new
phenomenon called biasing, which we'll
discuss in a little more detail.
So, here is a comparison of some of the
redshift surveys.
There are many ways in which you can
compare redshift surveys.
In this particular case, it is number of
objects measured versus the volume of
space that's been covered.
Obviously, the deeper you go, you get more
volume at the expense of taking more
spectra, having more objects.
Now, as the SDSS-Main is the actual Sloan
redshift survey, they had few others.
For one of them, they didn't really
actually use spectroscopic redshifts.
They used multicolor photometry to derive
statistical estimates of redshifts, the
so-called photo z survey.
This turns out to work reasonably well if
you have good photometry at least several
filters.
And also they can look at spectra of
quasars, very far away.
They look at absorption line clouds along
the line of sight.
Those are very numerous and they provide
means of probing large-scale structure out
to the highest redshifts.
Another way to compare them is to look at
area coverage versus number density of
objects that are being covered.
And so again, there is a trade-off.
Now, in this plane, diagonal lines show
the total numbers of objects.
And you can see progressing first from
thousands to now millions.
Next time, we will talk about how we
actually quantify galaxy clustering using
the two-point correlation functions.