We saw what are the generic expectations in terms of observating galaxy formation,
but now let's look at the observations themselves.
How would we go about it? First we call the, most of the energy from
young, forming galaxies, say... Brought the ellipticals would come from
conversion of hydrogen to helium in stars adding up to about ten to the 60th ergs
overall. And the question is then, what is the time
interval over which this energy's released because what's observed is not energy but
luminosity. So if we confine ourselves to the
lifetimes of typical star bursts which are some tens of millions of years or maybe
free fall time scale[UNKNOWN] ten to the eight years or maybe merger time scale ten
to the nine years. We get roughly a range of luminosities
that are between ten to the 11, ten to the 12 solar luminosities.
And at the redshifts of interest that maps to apparent magnitudes of the order of say
25 to 30 give or take depending on exact redshift and the luminosity of the actual
protogalaxy. Few percent of that energy will emerge in
form of emission lines, most notable lyman alpha if it's unobscured.
And that will form a good way of finding galaxies.
However, the big question is, is this luminosity obscured by dust in which case
we have to look in the sub-millimeter, or just make a stellar photospheres in which
case it emerges in rest from UV and will be observable in visible or near infrared
light today. Seems that first there is no dust.
At the very first stages we must be looking at unobscured galaxies.
Emmission lines would then form a very good indicator of star formation at high
redshifts and there are several ways in which we can do this.
The simplest one is putting a dispersing element, like prism or grism, in front of
the telescope optics, and so every source gets dispersed into its spectrum.
That's called a slitless spectroscopy. This works for relatively bright objects
from the ground at least because at any given pixel for a given spectrum you only
get. Light from that wavelength, but you get
light from all wavelengths from the background or foreground.
So for really faint objects this will not work from the ground, although it has been
done from space using Hubble Space telescope.
An alternative is to use standard, long slit spectrograph.
In which case a dispersed image of a narrow but long slit is being detected and
that will work fine over a very small area in the sky but over a large area in a
redshift. And because the light has been dispersed
properly into sky subtraction, that's pretty good.
Finally you can take a narrow band images, which band pass is very small.
It will select emission lines of particular redshift.
So it can go fairly deep, over reasonable area in the sky, as much as detector
covers, but only over tiny redshift range. All of these have been used and all of
them have produced some results. Narrow-band imaging has been very
effective especially for looking for line and alpha line, of neutro hydrogen, the
strongest unobscured[UNKNOWN] line. This is an example of the first high
redshift galaxy found using that technique.
It is a quasar companion redshift 3.2 and you can see in the bottom right image what
it looks like in the light of line and alpha line[UNKNOWN].
The quasar in the middle, and its companion galaxy, off to the side, are
very clearly seen. Yet, on the continuum image, just above
it, the companion galaxy's just as faint as many others.
So this is a good way to select emission line galaxies of high red shift, and it
has been very productive so far. Another possibility is to just look on the
long slit taking spectra of perhaps other objects.
In this case here is a spectrum of quasar, that's the black streak in the top left,
dispersed over some range of wavelength and off from it, away from it you seem an
emission line that's attached. This is a background galaxy richer, 6.4,
so, which is unrelated to the quasar. So it's pure chance that one finds objects
like this. However, there is so many galaxies in the
sky that this is not so crazy, and in fact a number of galaxies have been found using
this technique. This is maybe not very efficient, but it's
completely unbiased. Except of course for blind flux itself.
Here is an example of one of the most distant if not the most distant galaxies
selected using narrow band imaging, which are also confirmed using slit
spectroscopy, this the [inaudible] of 7. The three images show 2 continuum images
the galaxy light is diluted completely and the narrow banding which, you know, which
it stands out because of it's strong Lyman alpha emission.
The plot at the bottome shows the specturm that confirms that's indeed what we're
looking at. A very popular technique that goes deeper
then spectroscopy is the Lyman-Break imaging.
We talked about that. In the context of galaxy evolution.
Remember this works as follows. There is a strong continuum break at Lyman
lifa or maybe a Lyman limit due to the intergalactic hydrogen or gas in galaxies
themselves. And so if you have one filter blueward of
that break, and at least two others redward.
You can look for objects that look blue, or flat spectrum in the red, and then
suddenly, very red, that is with a strong drop, the blue end.
This is illustrated on the bottom with actual image from Hubble Space Telescope.
Now, you can see the galaxy is well detected in three filters redward of the
Lyman break. But is completely absent from the one
that's lower than the Lyman break. This is a very effective way of finding
high redshift objects. However, one has to trust that this is
indeed the correct break and, usually, one would like to get confirming spectra to
make sure. Here is an example of an object or
actually two objects seen in Hubble ultra deep field where this technique has been
applied. There are three detections, the huddled
black points, and everything else is upper limits.
What's drawn through this are couple model spectrum involving stellar populations,
and it's probably obvious that more than one kind of spectrum can be used to fit
these things. The authors go through certain arguments
why should be in particular the one that corresponds to high redshift Lyman-Break,
although obviously there is some uncertainty involved.
If you believe that it's the high redshift object with a break that's been detected
here. Then you estimate photometric redshifting,
that's what's given here. At this time all of the galaxies beyond
redshift seven or so have been detected using this technique.
There are no spectroscopics confirmations so far because they're just to faint.
We have to wait for the next generation of telescopes, say 30 meter telescope.
And almost surely some of these really are at high redshifts, but it's possible,
given the ambiguity of fitting models that some of those really are at lower
redshift, maybe highly reddened galaxies, and so these have to be taken with a grain
of salt. Here is a selection of such high-retro
galaxy candidates from Hubble ultra-deep field and you can judge by yourself just
how reliable these detections really are. They're estimated photo-metric redshifts
are shown on the right, and indeed, they seem to always make an appearance in some
red band but disappear in blue side. On the other hand, there is in addition to
measurements of things, signals. There is a question of interpretation of
fitting the correct stellar evolution model.
Now assuming that this is actually correct, one can extend the modal plot.
The history of Star formation and now we see that finally past about red shift four
or five, the star formation density declines at high red shifts, which is what
you expect. You begin with no galaxies whatsoever, you
build them up, as we already know there is a fairly broad maximum or plateau of star
formation history Down to redshift one or two and then decline towards the present
day. So this makes sense and almost surely some
of this is right although there may be some contaminants in these ostensible high
redshift galaxy candidates. Another interesting thing happened, that
some of these ostensible high redshift objects when observed in mid infrared
shows some signal, which would correspond to somewhat older stellar population,
maybe up to billion years old, meaning that they have to start very early on at
very high redshift in order to have this evolved stellar component added to the
brand new young stars that we normally look for.
And finally how do we connect formational galaxies with that of large scale
structures. You remember the idea behind biasing, that
the first objects that form, form at the highest peaks of the density field.
And so you expect that first galaxies will be strongly clustered in what will be
cores of future clusters of galaxies. This is indeed seen we've seen companions
of galaxies around quasars as well, it's in the field.
And here is a set of Lyman alpha emission line galaxies that are on redshift of
five. What's been seen in the cluster fairly
strongly. Also the most distant quasars now past
redshift of six or so, seem to show excess number of galaxies around them.
Not always, and this is a very difficult obsrvation, but at least in some cases.
And that fits very well with the idea of biased galaxy formation.
Next we will turn to the Reionization Era at the frontier of modern observational
cosmology.