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Welcome back, everyone. OK, now that we have looked at one galaxy in detail, our own.
And we looked at the different classes of galaxies – spirals, ellipticals,
and irregulars – and we've seen that dark matter really dominates in all of the cases,
we're ready now to sort of begin asking questions about
the history of galaxies and the particular features that we
need to be conscious of when we think about how
galaxies must have evolved with cosmic evolution as well.
And that brings us to a topic called active galactic nuclei.
And this is a topic that's quite rich in the sense that
what people have found is is that if you look at often distant sources,
but even relatively near,
cosmologically speaking, galaxies, in some of them, if you look at the center,
you'll see enormous amounts of power being
generated by the center of the – at the center of the galaxy.
So that's why we call them active galactic nuclei.
There is enormous amount of energy coming out in radio,
in X-rays, in other wavelengths.
And it really took a long time for people to understand what this was about,
but I just want to go through a little briefly the different kinds of
active galactic nuclei because there really is a zoo – this is a phenomenologically,
there's a very rich phenomenon,
rich set of categories about...
Some show beams of plasma being ejected, some don't.
Some are very bright in the radio, some aren't bright in the radio.
It was really, it took a lot of work to untangle all of
these classes and try and unify them into a single physical model.
But the first kind of active galactic nuclei that were discovered were
so-called quasars or quasi-stellar radio sources,
and these were objects that appeared on the sky as being point sources.
And yet, when you looked it on the radio they had
enormous amounts of power coming from them.
And when people first looked at them,
they saw spectral lines in them,
absorption lines that were – it's really hard
to understand where the absorption lines were
coming from or what kind of elements these were.
And it wasn't until people recognized that these were
actually extremely red-shifted spectral lines, that they were just normal,
old elements that we see in stars,
but the lines had all been shifted very far to the
red because the object was moving away from us,
and it was moving away from us due to the expansion of the universe,
so it was basically telling us that these things were very,
very distant from us.
And once it was recognized that these things were very distant,
then you could calculate it's brightness and that's how we recognize that
there was enormous power coming out from them.
So quasars are a – and only later on it
was recognized that these were actually associated with galaxies,
that these were, you know, the brightest parts of fairly young or very young galaxies.
So quasars are the earliest form of active galactic nuclei that we recognized.
Later on, there was another type of – actually,
this actually enormous predated what are called Seyfert galaxies,
which also have very interesting properties associated with their nucleus.
One of the things we see about Seyfert galaxies is we see evidence for
enormous or very fast material being
ejected from the centers of the galaxies in Seyfert galaxies,
up to 10,000 kilometers per second of material being blown outwards.
Other types of active galactic nuclei are what are called radio galaxies,
and they're called that because when you make radio observations with them,
often what you'll see are very – first of all,
you'll see a lot of radio emission coming from them,
but when you resolve that radio emission,
what you find is their jets.
There are enormous kiloparsec-scale jets,
beams of plasma, coming from the center of the galaxy,
coming from that galactic nuclei.
And these jets are really phenomenal objects,
and when we study them what we find is that they are probably
being ejected at speeds that are close to relativistic,
close to the speed of light.
So whatever is happening at the center of the galaxy,
we've already seen that there are very fast material,
around 10,000 kilometers per second,
but there's even material that's being pushed right up to the speed of light.
So whatever is happening at the center must involve relativity,
must have be able to boost things to relativistic speeds.
And then finally, there's another class called blazars,
and these are very, very compact high-energy quasars as well.
They show rapid variability,
that's the other thing that's important,
that in almost all of these classes,
we see variability: the brightness changes with time,
the spectral lines can change in time.
So, that is also telling us something about the nature of the central source.
So OK, so the takeaway here is that at the center of many galaxies,
there is extremely – something going on that leads to extremely active phenomena;
it pumps out huge amounts of energy,
and astronomers really struggled for a long time to understand what this mechanism is.
In the next lecture, we'll sort of talk about
the unified model for active galactic nuclei,
and then from there we're gonna be able to see how it connects to
our ideas of galactic evolution.