Let's now turn to two different extreme ends of early type galaxy properties.
One is the super massive black holes and their nuclei and the other one is the
dwarf family of galaxies. As it turns out super massive black holes
Measured in millions and billions of solar masses, or even more, are ubiquitous.
They are present in essentially every galaxy of substantial size near us.
In most cases, they don't do very much; but sometimes they create/g material From
outside and that causes burst of great luminosity and activity.
Those are the active galactic nuclei, which we will discuss in more detail later
in class. The super massive black hole paradigm for
active galactic nuclei quasars and such is now very well established.
And it's interesting to figure out Those super-massive black holes come from.
If they're not doing anything very special, being recsys sources or a radio
or something. One thing that we can do is probe their
masses using stars as test particles. We can do this by measuring kinematics of
stars in the very centers of early type galaxies.
When this was done for the Milky Way, we found out that there is a 3 or 4 million
solar mass black hole there, which is not very active, just sputters occasionally.
But it could have been a luminous active nucleus in the past.
And as it turns out, masses of these large black holes in cores of ellipticals or in
fact all galaxies correlate remarkably well with a whole number of other
properties of galaxies, and that is telling us something about formative and
evolutionary mechanisms. We now, in fact, think about crawl
evolution of galaxies and their super massive black holes.
So here is one of the first cases, the small, elliptical satellite of Andromeda
and it'll shown on the right is profile of its velocity dispersion.
You can see there is a sharp spike in the middle, which is what you would expect if
you were to embed a large point mass like a black hole in an otherwise normal
galactic core. This will measure for many, many more
galaxies, and several interesting trends were found.
The first one was that the mass of the black hole is proportional to the total
stellar mass of the host galaxy, amounting to something like 0.1%.
That alone suggests that there is some sort of common formative mechanism.
A more interesting correlation is between mass of the black hole and the velocity
dispersion of its host galaxy measured at large radii, where the influence, dynamic
of influence of black hole is completely negligible.
So somehow properties of galaxies on scales of kilo-parsecs are related to the
black holes in their cores which are micro microparsecs.
The, there is something that couples them through 9 orders of magnitude in size.
Another approach to this is by considering all of the quasar light ever emitted.
We have now a reasonably good understanding of the evolution of active
galaxy population as a function of time. And we can assume certain efficiency of
accretions, say that maybe 10% of all matter that falls into black holes con, is
converted into luminous output we can discuss that later, and then simply add up
how much mass should have been accumulated through the history of universe.
And if we do this, we find out that on average, you expect that typical luminous
galaxy today would have about 10 million solar mass black hole in it's core.
And Milky Way has a 3 or 4 million solar mass one so it's perfectly sensible.
Andromeda one is maybe a little more massive.
So this, can be compared directly to the measurements from kinematics in senses of
super mass in black holes in galaxies. And we find out the two agree very well,
and they correspond to the local average black hole density of about 500,000 solar
masses per Cubic mega-parsec, which is about three orders of magnitude less than
mass density of stars. An even more interesting relation was
found by Laura Ferrarese and collaborators.
And she estimated masses of dark halos of galaxies, from their kinematics.
And it turns out that those are correlated with super massive black holes as well.
Superbly well. But interestingly, in a non-linear
fashion, whereas the masses of black holes were proportional to the luminous stellar
mass or at least for the bulge component. Here we find out that they're proportional
to a steeper power of halo mass. Meaning that more massive halos, more
massive galaxies, therefore, are more efficient in making black holes.
You could understand that by the more massive ones being more efficient in
obstracting merging fuel. And, maybe that's what's going on.
But what's remarkable about these relations is that they have such a small
scatter. We think that merging is a fairly random
[inaudible] process efficiency will vary and yet somehow, after Hubble time or so
There is remarkably sharp correlations. So we can qualitatively understand where
they come from but their quantitative understanding, why they're so sharp is
still a mystery. And here they are, all on same plot.
Top left is proportion between black hole mass, and the luminous stellar mass.
Then there is proportion between black hole mass and velocity dispersion.
Which looks a little bit like Tally-Fisher or Faber-Jackson relation.
Again, circular velocity, and against the halo mass.
Proportional to the halo mass to roughly 1.6.
And now for something entirely different, dwarf galaxies.
In Hubble's days, and for some time after people thought there is one kind of thing
called dwarf ellipticals and they're just small ellipticals.
Now we know this is not the case. They're a very different family of objects
and in fact they may be two different families of objects.
In addition to simple division of being gas poor or gas rich, in making stars.
The reason why we think they're very different is that they follow very
different correlations between their Fundamental properties which I'll show you
in a moment. And those correlations a product of
formative evolutionary processes for galaxies then that suggests that they're
two different paths and therefore two different families.
As it turns out, dwarf galaxies, dwarf spheroidals in particular are totally dark
matter dominated. They have higher mass to light ratios then
Any other galaxies, and we think we understand why this is.
Again, remembering the scenario where supernova explosions can expell gas from
galaxies. They can do so in shallow potential wells
thus removing variance. But super nova shocks would not effect
dark matter at all. So dark matter will stay.
So, lower luminosity galaxies will be more efficient in losing their luminous mass
while retaining the dark matter. And therefore you expect them to be more
dark matter dominated. Which is exactly what's observed.
So here is a set of correlations produced by John Kormendy that shows some
properties of elliptical galaxies, dwarf spheroidals, and globular clusters, which
really don't belong in this diagram at all, but they're there just for symmetry's
sake as all stellar systems. And they show central surface brightness
versus radius in the top left. The central surface brightness versus
luminosity, top right. The velocity dispersion versus radius in
the lower left and velocity dispersion versus luminosity on the lower right.
The two families with thicker symbols are elliptical galaxies and dwarf ellipticals
and dwarf spheroidals. The little dots are globular clusters.
And you can see that obviously they separate very cleanly in this parameter
space. So let's look at this in a little more
detail. This is just plot of mean surface
brightness, with an effective radius, versus luminosity.
And, whereas for normal ellipticals, which is the upper right set with the red line
going though them There is a trend that the more luminous ones have lower surface
brightness because they have more diffuse surface brightness profiles.
The exact opposite trend happens for the dwarf galaxies.
Not only is the trend opposite, but the intercept is different as well.
So in the region where they overlap the difference or rather the ratio between
surface brightness at the given luminosity implies the ratio of 3 dimensional
luminosity densities by about the fact of 1,000 or more which is like between
uranium and air. So these are not dwarf elliptical they're,
a different kind of thing, they're is just like calling cotton puffs, dwarf canon
balls and here is really telling diagram of mass to light ratio versus luminosity.
I did not plot globular clusters and elliptical, non ellipticals as individual
points, just indicated where are they in this diagram.
And I plotted dwarf spheroidals as solid dots and.
Dwarf ellipticals like those around Andromeda has the open symbols.
And you can see that there is this branch of dwarf spheroidals that just shoots up,
reaching mass to light ratios of the order of 100 at very low mass end.
This has been confirmed by many, many subsequent observations now we know more
of these galaxies. This will lead into the next discussion,
about how we can use scaling relations in correlations for galaxy families to learn
something about their internal physics information.