We'll now talk about high energy diffused backgrounds, which are in x-rays or gamma
rays, and their origins, which are mostly from active galactic nuclei.
This picture is an x-ray image of the moon from the ROSAT satellite, and you can see
that the illuminated side of the moon is reflecting x-rays from the sun.
Sun doesn't need sun. And there is a dark side of the moon, with
some x-ray events even from there, they're, could be in the foreground of a
world between the Earth's atmosphere and the moon.
But there is also a notable excess background.
Behind the dark side of the moon, which shows clearly that their origin is beyond
Earth moon system. And those are x-ray photos of the diffused
x-ray background. The cosmic x-ray background was actually
discovered before the cosmic microwave background, with the very first
astronomical x-ray experiment, which was rocket born test led by Ricardo Giacconi,
who many years later got Nobel Prize for this.
And they detected first souces, such as Scorpius X1, but also diffuse x-ray
emission that seemed to be coming from everywhere.
And at that time it simply wasn't known, where does it come from.
For comparison it is about a percent of the diffused optical infrared background
that we talked about when we talked about galaxy evolution, which essentially, is
integrated starlight ever emitted from galaxies, and that itself is a percent
level relative to the cosmic microwave background.
So in terms of, sheer energy density, these backgrounds are relatively
negligible. But, their astrophysical interest is, very
considerable. We now, believe, and, actually very,
certain of this, that most of the, X-ray background, originates from, active
galactic nuclei. It's been resolved individual sources.
Some of it comes from star formation in galaxies, say, Super Nova or [unknown] and
such. But most of it comes from active nuclei.
There was however, one major puzzle, and that was the x-ray spectrum of the
background. So, here is, a very wide bandwidth plot of
various cosmic backgrounds, from radio through almost gamma rays.
And you can see that microwave background, the black body curve on the left,
dominates the picture. This is log-log plot.
And so then there is optical, near infrared back, or far infrared background.
And then there is x-ray background, schematically illustrated with that wiggly
line. The black dot in the lower right is the
cosmic gamma ray background. So what about the spectrum, the cosmic
x-ray background? Here is a plot of measurements from a
variety of different satellites that have been doing that.
Sometimes overlapping in energy, sometimes not.
You can see that there's a little bit of spread between them, and that's just
issues of cross calibration. It's not so easy to calibrate these
things. Now this shape, of an x-ray spectrum is
typical of the very hot gas. So thermal Bremsstrahlung radiation.
You have a hot plasma electrons are, passing by protons, all ionized.
They're decelerated by, electron, electric interaction, they emit photons then.
So it's not like a Planck black body, but it's, thermal emission nevertheless.
And, that would, imply, on the face value of it, that it's, the universe is somehow
filled with hot plasma and this is where it comes from.
But that doesn't work, because we simply haven't seen it.
Instead of that, we see a lot of active galactic nuclei and x-rays that are
identified optical radio sources. But the problem is that their spectra
don't look like this. So how can you have collective emission,
with a spectrum that looks like thermal emission from plasma, composed of a lot of
non-thermal sources? It turns out to be a combination of their
evolution, in time and red shift, and slightly different ways in which x-rays
are being released. Some of it comes directly from [unknown]
electrons. Some of it comes from reflected, and
continuing emission from, dust surrounding the active nucleus.
The way to find this out was to obtain really deep images in x-rays of selected
patches in the sky. And then try to identify the sources.
This was done in Hubble deep field, and other deep fields that are done.
Chandra telescope had its own deep fields, one which are, Hubble, the other one in
the south, and so on. Typical exposures for these were in
millions of seconds. So a lot of invested time in these
observations. And as you can see, there are plenty of
x-ray sources. They look sharp in the middle, and they
look big on the outskirts, and that's purely the effects of the x-ray optics.
It's not so easy to make x-ray mirrors, and so the image quality deteriorates in
radius. They're not really bigger they're just,
images are fatter on the detector itself. And this is precise enough, that we can
actually seek optical or radio counterparts.
We can also count the sources, and see if the total emission, from all of the
sources, counted to the faintest level, adds up to the overall integrated
background. And the answer is, it pretty much does.
And so, in different energy bands, you get counts to a different depth, but they
follow more or less the same kind of curve, which is some sensible evolutionary
models. And, if you extrapolate to fainter source
counts, because they're reasonably well behaved, just like in the optical they're
reasonably well behaved counts of both optimum play and galaxies, you can account
for, reasonably for 90% of the total observed x-ray background, and the
remaining 10%, could be just uncertainties of extrapolation.
So what are the x-ray sources? Here's a collection of, whole bunch of,
Chandra sources, identified in the optical.
And, a lot of them look point-like. These are active galactic nuclei,
sometimes you can see the galaxy, those are all to near by ones.
But, by and large, those all seem to be associated with no thermal activity in
quasar-like objects. Interestingly enough, some of these do not
show any signs of non-thermal AGN in the middle, in optical.
So the visible ray, UV ray, for red light is completely hidden, but x-rays do
penetrate the dust, and we see them. It turns out that there is not a good
correlation between x-ray and optical fluxes.
Something can be a very luminous x-ray source and very faint in the optical or
vice versa. And, they come in different nature if we
plot here x-ray fluxes, essentially. Optical flux, you see there is no
correlation whatsoever. And sources of different kinds appear in
different way. There are some galaxies which are powered
by star formation. And there, those tend to be lower
luminosity x-ray sources, whereas active nuclei or quasars tend to be higher
luminosity x-ray sources. Redshifts were measured, for a whole lot
of them, they turn out not to be particularly high ratchet objects.
Most of them are to ratchet two or so, typical for quasars.
And, here are the, central Hubble diagrams of sorts, for the x-ray sources.
Although it's not, apparent flux, it's, absolute luminosity.
The reason you don't see faint sources of high red shift is that, they're just too
faint to detect. But, you can see they do extend to very
high luminosity's. And, essentially all of the ones at higher
edges, which are solid squares, are active galactic nuclei.
Well, what about gamma rays? The story there is very similar.
Being active galactic nuclei, those red-shifts are, ought to be the principal
extra-galactic gamma ray sources, aside from gamma ray bursts, which, are
spectacular, but do not contribute much energy overall.
This is a sky map from Fermi satellite after first year of operation, and, can,
plotted in galactic coordinates, and see Milky Way plane has a lot of gamma ray
emission from variety of sources. But, outside the plane, there are point
sources and essentially all of them are associated with active galactic nuclei and
specifically with blazars. After few years of work with Fermi, they
actually could add up light from non-blazars and turned out that, they do
not, add up to the full observed gamma ray background.
So those are, the obvious sources, but there could be extra kinds of sources that
might be contributing. Well, some of the caplogs of these beamed
AGN are probably incomplete on a factor 2, probably, because when measurements were
done, sources in low states, were never made it into a caplog.
But even so, there isn't enough in the known active galactic nucleus population.
Additional sources could include some star formation where gamma rays come from
shocks and super nova remnants, or even shocks and, and gas, and it was also
proposed that some component of dark matter may be decaying and, and emitting
gamma rays, although so far no evidence was seen for that whatsoever.
So there could be also fainter populations of being active nuclei that somehow didn't
catch our attention earlier, perhaps because they are too faint.
So this is still and area of ongoing work, but, still being active nuclei, are
probably the main contributors. Something else interesting, is that beam
nuclei blazars can produce photons of tera electron volt energies.
Typical gamma ray observations would be of giga electron volt energies, or mega
electron volt energies. But, these things go up to tera electron
volts. They can be actually observed from the
ground. Earth's atmosphere is opaque to the
photons themselves, but these high-energy photons can knock out particles from,
atoms and molecules in the upper atmosphere, and gain some kinetic energy,
and then you have charged particles moving at a very high speed.
Which can emit, so-called [unknown] radiation, and those can be seen with
large telescopes on the ground. So here are the light curves of several
blazars in tera-electronvolts, and they've been also detected in the optical in terms
of their Lorentz factors, or axis of 50, and, that was interesting itself.
This, gives us some confidence that probably the highest energy cosmic rays
also come from these beam active nuclei. Remember, those are accelerators in the
sky. And, interestingly enough, the highest
energy cosmic rays are 100 million times more energetic than particles in Large
Hadron Collider, in Geneva. So there are many uses for these cosmic
accelerators. First of all, they probe the demographics
unification of active nuclei. They, they're obvious contributor to the
cosmic gamma ray background. There is lot of physics of relativistic
jets that can be learned from them. And there may be even the ultimate sources
of high energy particles. So there could be the future of particle
physics because we'll never have an accelerator that is hundred million times
more powerful than large hadron collider. They do other interesting things.
For example, they're very compact radio sources, and they are the only important
radio source population, foreground population, to the measurements of cosmic
microwave background at high angular resolution, so they have to be accounted
for precisely before, better cosmological conclusions are drawn.
And, they can also serve as a probe of star formation, even though they
themselves have nothing to do with star formation.
And the reason for that is that, intergalactic starlight came from all
different galaxies, as well as the microwave background, are opaque to
high-energy photons, and, this sounds a little weird.
That gas of photons is opaque to other photons, but in the center of energy, of
the two photons, one of which can be infrared and the other is really high
energy gamma ray, they're both high energy, and they can exceed [unknown]
production energy for positrons, and electrons.
So they can, so the two photons interacting can turn electron-positron
pairs, which later, an islet somewhere, but essentially, intergalactic star light
forms a fog for high-energy gamma ray sources.
And so, if you can measure the cutoff in their energy, as a function of red-shift,
you can show energy density of intergalactic star light, without recourse
to any red-shift survey, or any deep counts.
Which is a nice independent way, of constraining the history of cosmic star
formation. And here is another plot.
It's likely to express in a slightly different way of all the different cosmic
diffused backgrounds. They're diffused when you don't have a
good resolution, but all of them except cosmic microwave background, break into
individual sources, once good enough observations are obtained.
And, roughly speaking, in radio, it's mostly from active galactic nuclei, but
there's some star formation. Microwave background is cosmological
origin, sub millimeter, far infrared, visible, and near ultraviolet all come
from star formation. Obscured or not obscured.
X-rays mostly come from active galactic nuclei.
Some small component comes from star forming regions, and gamma rays as far, as
we can tell, mostly, maybe entirely, come from active nuclei, even though we haven't
accounted for all of them yet. Next we will turn to the study of the
evolution of active galactic nuclei.