So far, we looked at the observations in visible light, which also corresponds to
the rest, red shifted rest frame ultraviolet light in galaxies.
In other words, what you just see directly in stellar surfaces.
However, we do know that galaxies contain dust.
And, likely they did so from very early on.
The dust absorbs ultraviolet and visible radiation and remits it in fiery infrared
or some[UNKNOWN). Because there has to be a obscured star
formation component involved in galaxy evolution.
And indeed as we aquired ability to observe deep universe on these wave
lengths it was discovered that there are new sources appearing in the sky that are
simply not visible at all in, in visual light.
Here we see a composite in the upper left. Visble light image from Hubble And the
median infrared image from the Spitzer Space Telescope.
As you can see in the upper right, there isn't a trace of the thread source.
In the lower left, you can see it just beginning to show up, zero to near
infrared. And in the lower right, it's pure mid
infrared image and source is very obvious. So sky would look different on these
wavelengths, and there could well be a previously unaccounted population of
sources. And this is indeed the case.
The first observations that uncovered such a population of sources were done at James
Clerk Maxwell telescope in Hawaii. It is a submillimeter telescope.
It is equipped with a barometer array measuring submillimeter radiation from
sources in the sky. These are very difficult observations to
make. This is why it took so long.
And they looked at a couple of the deep fields, the Hubble Deep Field, and another
one called Lockman Hole. And found that there are resources there.
The resources whose nature at the time was unknown but were of surely obscured star
forming galaxies far away. The reason why they look so bloppy is the
poor resolution of these telescopes. You may know that the angular resolution
of telescope is proportional to its diameter divided by the wave length.
So for optical, this is a very high number.
There are many wavelengths of photons stretched over, say, the mirror of the
Keck telescope. Not so in radial or submillimeter.
In there you have very low resolution. Nowadays, we have new inter therometers/g
like alma/g and sheila that will actually produce optical light resolution in these
wavelengths. But back then, this wasn't the case.
Now here is a really nearby example of what we might expect.
This is the galaxy M8, which is a nearby starburst galaxy.
The picture shown here combines radio or visible light.
In the purple is ionized hydrogen emission.
What we see here is a intensely star forming this galaxy obscured in the middle
but, the supernovae exploding pushed the gas out.
Th, they drive a galactic wind. Expelling it into intergalactic space.
Now if we take a broad-band spectrum of M82, we see that there are two bumps.
There is one, the optical, which corresponds to a sort of quasi-black body,
sum of all stellar, photospheres, and the bigger one in five red, which is thermal
emission from dust that was heated up by these young stars.
In this particular galaxy, there is actually more energy emerging in the form
of[INAUDIBLE] radiation than visible light.
But on average, in this part of the universe, there is roughly equal amount of
obscured and unobscured star formation The same turns out to be true at higher
redshifts. You may recall the concept of
K-corrections which means that as you observe in some stationary instrument on
planet Earth, you're looking at different parts of the spectrum.
For sources of different redshifts. Now by and large for galaxies, this makes
them dimmer because galaxy spectra tend to be redder, more energy in, in the red part
of the spectra. It turns out that for sub-millimeter
sources it's exactly the opposite. You are redshifting into the plank curve
is climbing from the inside. And so even though sources get further
away, and therefore should be dimmer, you're sampling a brighter part of their
intrinsic spectrum. And so some of those k corrections
actually become negative. The upshot is that for many of these
sources The brightness almost does not change with red shift.
And therefore if you can reach a flux level, you can see very far away.
This is what SCUBA sources turn out to be. Which also means that we can do fairly
deep source counts in these wave links. And here are some examples of it.
They can be used then to constrain directly the contribution of these sources
to the overall star formation history in the universe.
And say something about their evolution. There is one difficulty, however.
The poor angular resolution of these submillimeter observations means that
optical counterparts, which we need in order to measure the redshifts, are hard
to guess. In some cases there is an obvious
counterpart, but in many cases, there are many faint galaxies inside the There was
circle of a sub-millimetre source. It's not clear which one, if any of them,
is the actual counterpart. So it took a while to get radius, measured
for these objects. The trick that was used is that the radio
measurements can affect some of them. And radio measurements have precision that
is perfectly good to, to match to optical observations.
Then the spectra we're taking of those, and in fact may turn out to be strong
emission sources, where that blind emission could be powered by star
formation or quite simply by an active nucleus in those galaxies.
Possibly a little bit of both is happening.
And the predictions from fitting the source counts, where many of these sources
will be around redshifts 2 or 3 and that exactly turn out to be the case.
So now that we've seen how we can detect both obscurred and unobscurred component
of evolbing galaxies. We'll, we'll look into the overall star
formation history of the universe.