Now let's see what the observations say about galaxy evolution.
The simplest thing to do is take images of different wavelengths that is
observationally much cheaper than taking spectra, which require longer integration
times. And this can be used to do galaxy counts
as a function of their brightness, color etc., but that can only go so far.
And to really understand what's going on, redshifts really are necessary.
So it was only in the recent years that we have obtained sufficiently large samples
of. Galaxies in deep fields to really reach a
good observational understanding of galaxy evolution.
There is one important dichotomy. You can think of star formation as coming
two flavors: the simple, unobscured star formation where you see the light from
stellar photospheres as they are, or light absorbed and re-radiated by interstellar
dust, which now moves all of the energy into infrared or submillimeter regime.
So there are two regimes in which we can observe galaxy evolution and star
formation in them, and each of them has its own tools and selection effects and
limitations. You may recall, when we first mentioned
Source counts as a potential cosmological test.
That the evolutionary effects really mess thing up.
And they always work in the sense of making counts higher than they would be in
the absence of the evolution. Even the galaxies that are evolving in
brightness say due to the fading of stellar populations that were more
luminous in the past and therefore they would be at the higher magnitude level.
But there would be many more of them thus, they'd be moving to the left, and
producing a less declining curve. Likewise, if galaxies are assembled from
smaller pieces, there are more smaller pieces in the past than exactly the same
observational effect appears. To disentangle these we need redshift
theories. Still here at the deep galaxy counts from
the Hubble deep fields and these days we do this down to about twenty-ninth
magnitude or thereabouts which is really spectacular and by extrapolation over the
entire sky, there may be a couple of hundred billion galaxies within the
observable universe. You may also recall that evolution is
expected to appear stronger in bluer wavelengths, and less so in redder, and
that's indeed exactly what's seen here. Now, this is now.
Infrared galaxy counts and they show roughly minor discrepancy compared to the
much stronger ones that are observable in the, in the bluer parts of the spectrum.
But those galaxies evolve as their stellar populations evolve.
Their colors evolve too, generally going from bluer to redder.
However that's also complicated by the redshift.
The whole spectral entry distribution is moving from bluer filters to the redder
filters. So, any given time you can look at color,
color space and see what galaxies will do in there.
Now, you can Make use of their complex trajectories and use those to evaluate
redshifts, from colors alone. Those are so-called photometric redshifts.
You can think of those as a really low-resolution spectroscopy.
And this seem to work remarkably well, typically in at least four or five filters
and here are examples of some of the measurements, those are dots with their
bars with models of stellar populations drawn to them.
This actually looks too good, and there could be many different models that can
fit the same set of measurements. But that can be also about a statistic.
So here's a typical plot. Usually there is a really excellent
agreement between spectroscopic redshifts. Done as a control and predicted
photometric redshifts. The state of the art is that maybe down to
a few percent. However there always out-layers, galaxies
for which gross error is made. And that's usually due to something like
presence of an active nucleus, or some other peculiar happening like that.
A particular form of photometric redshifts relies on the presence of deep jumps in
the spectrum of the galaxy. There are a couple of those.
There is the limit of the Balmar series of hydrogen which then results in a step of,
of what is magnitude around 3,600 angstroms in the red stream.
So you can use that by measuring flux in filters bluer than the gem, in redwood of
the gem, an even stronger effect occurs at the limit of the alignment series, and
those are extremely useful to find galaxies of very high riches.
Moreover, for the reasons we'll be discussing later, intergalactic medium
absorbs light. Blueward of Lyman alpha line.
So it's really the variant of the Lyman alpha line that serves as an interesting
conjunct point. This has been used to great effect, in
particular by Steidel and collaborators, who discover large numbers of galaxies of
high redshifts and then study their evolution and properties.
But again colors and magnitudes have their limitations and redshifts are needed.
So that the advent of eight, ten meter class telescopes like BLT in Chile, or
[unknown] telescopes in Hawaii, in Subaru and so on, we can begin possible to
actually do this in an effective way. Also, with the development of multifibre
spectrographs, which you may recall also revolutionized redshift series of low
redshift, and nowadays. Thousands if hundreds of thousands of
faint galaxy redshifts have been obtained. A good winning strategy is to obtain
really deep images from space where you don't have to worry about the effect of
Earth's atmosphere. Images taken from Hubble can go much
deeper than those taken from the ground, with a better resolution.
And so there is a set of selected fields in the sky, where very deep observations
have been obtained. Hubble deep field was the first one,
followed by the ultra deep field, and Chandra deep field, and extremely deep
field. So these are the deepest windows in the
universe we obtained so far. Once you have these images in a number of
filters you can deploy large telescopes to measure[UNKNOWN] of as many galaxies as
you possible can. And that's still very much an ongoing
enterprise in observational cosmology. Here is the first one of those, the Hubble
Deep Field with it's characteristic B2 bomber shape and the histogram of
redshifts obtained with the Keck telescope.
So even those where the deepest observation is.
Until then, the bulk of these galaxies is actually at the very high redshifts, about
redshift 1/2. They go beyond redhift of 1, but not by a
lot. In subsequent work, pushing ever deeper,
galaxies were found at redshifts of 5 or even almost 6, but still the bulk of
these. Even at the limit of the present day
observations with eight and ten meter class telescope is of the order of unity
or less. So we do not actually probe evolution of
galaxies very deep through direct measurements.
Small numbers we do see, but then one has to beware of selection effects.
The complete understanding is really a thread use less than one.
This was done now by numerous groups, both in north and south, and tens of thousands
of feign galaxy redshifts would be obtained.
Results are usually in a really good, mutual agreement.
This one is from an SL Beam survey in Circle Good Field, which is where Hubble
Deep Field was, plus other observations surrounding it.
And so here are a couple interesting diagrams.
On the left, you see the redshift histogram.
And it's very spiky. It's not spiky because it's noisy, it's
spiky because of the large scale structure that the line of sight intersects
filaments or even clusters and voids, and that's what produces the observed
distribution. On the right you see absolute magnitudes
in the rest frame of galaxies plotted versus redshift.
And you see there is a sharp cutoff that corresponds to magnitude limit.
People who do these surveys decide that they can only go down to same magnitude
level, say 24 magnitude, and that maps into different absolute magnitudes with
different redshifts, so this is a built in but well understood selection of facts.
Nevertheless, one has to be aware of it. Naiively, if you looked at this, you would
conclude that galaxies of higher redshifts are more luminous.
No, you simply only see the luminous ones with high redshifts, you are not seeing
the fainter ones. So this was done for deep fields and the
result is as follows. Individual galaxies cannot be really
compared. You need to look at the whole population.
And the simplest description of the entire population is the luminosity function.
Distributional galaxy luminosities. So if you look at that in different
redshift shells, you find out that the observed galaxy luminosity function is
very similar to the one we seen in yours. It revolves very slowly but you begin to
see couple of interesting effects, by about refuge of half or so, the faint end
steepens. We see more evolution in intrinsically
fainter galaxies. And second thing is that as you push deep
enough, you start to see brightening at the bright end, which is what you expect
from fading of stellar populations. The steepening was a little bit of a
surprise. It was thought before that galaxies
responsible for the excess counts in the skies, in the imaging are evolving
galaxies with larger redshifts. The bulk of them turn out to be dwarf
galaxies at modest redshifts. And The evolution of galaxies depends very
much on their intrinsic luminosity. This is known as the downsizing.
In face value, it's exactly opposite what you expect from a hierarchical structure
formation where you slowly build up large galaxies, you expect the larger galaxies
to be evolving fastest, but it's the, exactly the opposite.
Moreover, the evolution of the lumosity function depends on the galaxy morphology.
And if you can split them in morphological types, say early and late type spirals and
ellipticals, you find out that the later type galaxies, those further to the right
in Hubble sequence star forming disks and irregulars have the strongest evolution.
Galaxies done earlier part to Hubble sequence pretty much done their evolving
by about a redshift of one. They do evolve since then, but both of the
change appears in the faint population and also the late Hubble Types, it is only
when we reach redshifts of the order of two and beyond that we start to see clear
effects of strong evolutional stellar populations at the very bright end.
So the generic conclusion these days is that most of the Hubble sequence was
pretty much in place by about ratchet point one.
And things have been evolving a relatively modest space since then.
We'll see some other approaches to this a little later.
Next we will talk more about some of the observed results as well as the evolution
in clusters as opposed to field, which is what we just talked about now.