So we have started talking about the results on galaxy evolution studies in
general field. First we found out how the luminosity
function seems to be evolving slightly. But now let's see some of the other
results that have been obtained. A simple thing to do is to look at colors
of galaxies as a function of redshift. And if we then predict what galaxies of
different Hubble types would look like if there was no evolution, just a k
correction, which you may recall from way back in Hubble diagrams, what measurements
would look like given the redshifted spectrum.
This is what it's like, the points are the actual measurements and the three lines
labeled with different Hubble types correspond to the colors that those
particular Hubble types would have with a given redshift.
And, as you can see, the observations pretty much follow the band.
So there isn't very much, in terms of color evolution for the Hubble type
sequence galaxies all the way after redshift affinity.
Hubble space telescope has also afforded us another possible measurement, which is
to look at galaxy sizes. Say, looking at their half light radii or
some other form of objectively defined radius.
And so this is the result of what. The radii, average radio galaxies do as a
function of redshift, namely galaxies grow in time.
If the radii were fixed in proper coordinates, if there was no evolution at
all, then solid line shows what f would look like.
But instead of that the points show that there is a substantial growth of the
galaxy. This is consistent with our understanding
of how this galaxies seem to form from the inside out.
As you'll recall the bulge is all the spark in the middle.
Then you have stellar disk. Then hydrogen extending beyond it.
That hydrogen gas has to turn into stars. And here we'll probably see the collective
effect of that. Modern spectra of high redshift galaxies
can be used to measure not just the redshifts, but also velocity broadening.
In other words, get a velocity dispersion. With radii measured from Hubble space
telescope, we can infer dynamical masses, or from their, from fitting of the
spectral energy distribution we can infer their stellar masses.
So that is shown here. The sizes of symbols correspond to the
magnitudes. We can see that the most massive galaxies
seem to be already in place at redshift one or two.
This is sort of the upper envelope of this distribution.
Whereas the lower masses keep seem to be evolving.
This is known as galaxy downsizing. Naively you would expect a hierarchical
formation scenario. That you make small ones first, and then
you gradually build the big ones. But that seems to be the opposite to
what's observed. The solution to this is probably due to
biasing effect we talked about late, earlier.
We cannot wait long enough to see galaxies merge.
But we can. Assume that some number of close projected
pairs will eventually merge. So by doing statistics of close pairs of
galaxies, allowing for projection effects and things like that, we can infer the
likely merger rate as a function of redshift.
And here it is. It's a power of 1 plus z, and it's more or
less exactly what's expected from the modern models of hierarchical structure
formation. You will recall that when we talked about
scaling relations, like the fundamental plane, I mentioned that they can be used
as a sharp probes of galaxy evolution. Luminosity function is a very broad
distribution, but these correlations are by construction the sharpest we can have.
And so if we can follow, say the evolution of their intercept or maybe slope as a
function of redshift, we can gain new understanding in the evolution of
galaxies, in this case ellipticals. What's shown here in the open points
through which lines fit Is the essentially zero red shift from the mental plane edge
on. The solid dots are ellipticals in one of
the most distant cluster now known. And there are too few points, but you can
see that they're consistent with being on a shifted version of the fundamental
plane. Numerous studies have been now done, both
for field and cluster ellipticals. And it was always seen that ellipticals at
higher edge show a shift in the intercept to the fundamental plane, the surface
brightness, that would correspond to the fading of stellar population in time, as
expected from evolutionary stellar populations.
And we can even say something about the star formation histories.
We can fit different evolution models. And find out which ones seem to describe
the data the best. And the answer is that models in which
elliptical galaxies form. Relatively early on and don't evolve very
much since then, just passively fade away, seem to fit the data fairly well.
That is shown in the plot on the left. The plot on the right shows fading, or if
you're look[UNKNOWN], brightning of surface brightness which remember is
related to luminosity density of sterllar populations.
As a function of red-shit and you can see that it is systematically increasing in
red-shift for both field and cluster ellipticals.
So these studies are consistent with what we've seen earlier that as far Hubble
sequence galaxies are concerned there seem to be pretty much in place buy about
red-shit of unity. The ellipticals are evolving in a way we
will expect from early burst of star formation and relatively modest star
formation history after that. And we also begin to see changing the tilt
of fundamental plane. Which really means that galaxies of
different masses evolve at slightly different pace.
And it goes in the sense that those at the lowest mass end evolve fastest, which is
a, again another example of the galaxy downsizing.
That's a first from the mental plane's concern.
What about Tully-Fisher? Well, that has been done as well, but it
turns out to be much more difficult and complex to do it for spiral of high red
shifts. And these results are still not a hundred
percent clear but at least broadly consistent with this picture.
So far we've looked at the evolution of galaxies in the field.
What about clusters? This is a dense environment, you expect
galaxy interactions to play some role. The first hint that something interesting
is going on in clusters, the so-called Butcher-Oemler effect established very
early on. These astronomers found out that clusters
at larger etchers seems to contain larger proportion of bluer galaxies, for whatever
reason. Generically you expect the galaxy
evolution leads from bluer galaxies to other galaxies so at least qualitatively
this seem about right. Remember that in clusters, some merges
will occur, but vast majority of interactions will not lead to merging,
however it will disrupt galaxies, may remove vially some of their gas or starts,
dark matter, and that's the process called the galaxy harassment.
There'll be a lot of cumulative small encounters that would maybe remove gas
from galaxies little by little. This would tend to transform Lay type,
spirals and dwarf irregulars, into S0's and ellipticals in time.
With the Hubble space telescope, it became possible to look directly at a morphology
of galaxies in distant clusters, and here is One of them were different symbols
correspond to galaxies of different morphological types.
With that and spectroscopic measurements we can try to disentangle what's going on.
An interesting finding was made. There is a novel type of galaxies found in
these evolving cluster populations. So called post-starburst galaxies.
Galaxies that are bluer than they should be at the zero redshift there was nothing
happening and have spectral energy distributions consistent with having
undergone a burst of star formation maybe up to a billion years earlier.
This could have been caused by some interactions, and these galaxies will then
presumably fade into Hubble types. And so the summary of these results is
that, as far as we can tell, there is a conversion of light type spirals into 0s
in ellipticals in the clusters. This is related to density of clusters as
well. The more[INAUDIBLE] relation and that can
account for the observed[INAUDIBLE] effect and presence of things like E plus A or
plus starburst galaxies. So one possible scenario is that spirals
from field are falling into cluster potential well.
As they do that, they encounter dense intracluster gas, the extra gas.
Some of their own gas gets stripped away that quenches the star formation in them.
They may undergo a burst of star formation triggered by some of the interactions but
eventaully that fades too and in the end you may have an all disk like S0 galaxy.
So this is a plausible scenario. This is not necessarily the only way to
make zeros. But it can account for the observed
effects in clusters. Next we will talk about the obscured
component of star formation in history in galaxies.