We now turn our attention to the subject of galaxy evolution, which is at the core
of most of modern cosmology and extragalactic astronomy.
First of all, galaxies must evolve for two reasons.
They're made out of stellar populations and stars evolve and also galaxies merge
into assembling ever larger objects through hierarchical structural
information. So, therefore, there are two aspects to
galaxy formation. One is the assembly of the mass and the
other one is conversion of gas into stars and energy.
The former is relatively well understood. This is something we can model very well,
in numerical simulations and it's driven by the dark matter.
The later is much more difficult, because it involves messy dissipative processes
including star formation itself, the feedback of stellar populations,
supernovae active nuclei and so on. And there we can model processes up to
some limit, using modern hydrosimulations because the analytic theories of star
formation do not exist. So even though dark matter dominates the
process of galaxy assembly because it's the dominant mass component That's not
what we see. What we see is the light.
And from that we have to infer about the assembly of the mass as well.
First let's take a look at the relevant time scales for galaxy evolution.
One timescale will be typical starburst scale which we now think is in or, of the
order of 10 to 100 million years. That also happens to be the timescale of
the life of the very massive stars which are energetically the dominant ones and
which also provide most of the photons that ionize the interstellar medium.
It also turns out that this is the estimated time scale of active galactic
nucleus episodes. And it's not obvious whether this is
coincidence or not. The internal time scales within galaxies
are commiserate with their freefall time scales, which would be.
Few hundred million years. And the time scales needed for merging an
assembly are typically of the order of a billion years.
And finally, there is Hubble time of the order of 10 billion years, through which
galaxies evolve. There are different observational
approaches to this problem. First of all we can look at our own Milky
Way and try to deduce from it's constituents and properties how it may
have form. Next we can look at nearby galaxies and
study the systematics of their properties as we discussed earlier through Hubble
sequence, through use of scaling relations and so on.
And finally, we can observe it directly by looking deep, since light travels at a
finite speed, the further out we look, the deeper in the past we look.
And so we can see galaxy evolution unfolding on our path light con.
To do this we need a good combination of theoretical tools and observational tools.
On theory side we can use numerical simulations of structure formation to tell
us about mass assembly. We can also predict behavior of the
stellar populations because we know a lot about stellar evolution and so making some
assumptions about Initial mass function of stars and the actual history of star
formation rate in galaxies, we can predict what their spectra would look like as a
composite of these evolving star populations.
And there are also hybrid semi-analytical schemes that combine these.
On the observational side, there again are three approaches.
First, we can take deep images, the deeper the better if you want to look in the past
over a full length of wavelengths. And from those we can infer a lot about
star-forming histories or galaxies that may be merging as well.
More important approach is to spectroscopy.
Because this is what reveals not just retrusive galaxies, but physical
properties that happen in them such as star formation rates, presence of an
active nucleus, if there is one, etc, etc. And finally, we can look at the diffuse
backgrounds, the collective emission of all galaxies ever.
Now that has a drawback of not knowing which source is where until you actually
resolve the background. But, it bypasses selection effect because
you get everything, whether or not individual galaxies are affectable or not.
A very important thing is to beware of, of the selection effects.
And they come in multiple gui, guises. The most obvious one is the flux limit.
Any given astronomical observation runs into the signal to noise problem at some
faint level, and that's how deep you can go.
But more subtly, there is a surface brightness selection limit that, for
extended objects, it's the number of pixels above a certain threshold, that is
not just how much light there is, but how diffuse it's distributed.
The more compact galaxies would be much easier to detect than the diffuse ones.
And so we know that, for sure, the deeper we look, we are getting an ever more
luminous, or higher surface brightness objects and we're missing the intrisically
faint ones or the intrinsically diffuse ones.
If we don't know what we are missing, then it will be very hard to correct.
But we can make reasonable extrapolations from nearer parts of the universe, and see
how that works. Let's refresh our memory of the dynamical
evolution galaxy merging. We've discussed this earlier, and seen
how. Hierarchal assembly of galaxies is a
process the keeps going on from the earliest days of the universe and we see
it even today in major mergers of galaxies.
This obviously arranges the mass but Thanks to the dissipation of all gas to
the centers, it can also trigger a burst of star formation, as well as feeding
active nuclei Thus stellar population evolution of galaxies and their denomical
evolution can be very deeply interconnected.
An important physical process here is dynamical friction, and it works as
follows. If you have a massive body, in this case
say a galaxy, moving through a sea of mass points, stars of another galaxy, say.
It will accelerate them. And as it keeps moving, you'll be
encountering more and more stars to which it will exert its acceleration.
Those stars acquire a certain velocity. And that kinetic energy has to come at the
expense of the perturber. So, the.
Stellar population or population of test particles in which this perturber plunges,
gains kinetic energy usually in the form of random motions.
Whereas the perturber loses it's orbital kinetic energy and this why galaxies that
they're initially in parabolic orbits can merge.
Incidentally, a good local example is the Magellanic Clouds, and they are being torn
apart by tidal interaction with the Milky Way.
In a few billion years the will merge with the Milky Way.
This has already happened to numerous dwarf galaxies in the past, and we see
fossil evidence of that in[UNKNOWN] Halo. A time scale for dynamical friction can be
expressed as follows. You start with the velocity, relative
velocity, of the observer, divided by the rate in which that vel-, velocity is being
diminished. And deceleration due to the energy loss.
After some theoretical computation, you obtain a formula like this.
It has several interesting features. The denser the medium, which has been
perturbed. And the higher the mass of the perturber.
The shorter time scale, which is intuitively clear.
But also, the time scale gets to be longer if the velocity of the observer is high.
And the reason for this is that. Perturbers that zip by very fast simply
don't have enough time to deposit enough kinectic energy in the perturbing galaxy
so it will take many different passes for the dynamical friction to actually do it's
thing. This is why the time goes up.
We've seen numerical simulations of merging galaxies earlier and this is a set
of snapshots from one of those showing what dark matter particles do.
In these simulations, always, eventually, the merger product looks something like an
elliptical galaxy. But if you follow the evolution of the
gas, you find out that the gas is reacting much more to the encounter and it's losing
kinetic energy very quickly and very effectively meaning it has to sink to the
bottom of the potential well. Which also means it will achieve high
densities. This is exactly the kind of conditions
that can lead to burst of star formation or they can also feed a giant black hole
if there is one there. Next we will talk about the models of the
stellar population evolution.