Let us now turn to the shapes of elliptical galaxies.
They're called elliptical for a really good reason.
Their projected surface brightness profiles look pretty close to ellipses.
And here are 2 intensity maps. Lines of equal surface brightness.
In the image on the left, the little knots are actually stars in the foreground,
because their brightness gets measured too.
And you can obviously see that elliptical galaxies really do look like ellipses,
although, ellipticity may be changing as a function of radius.
Generally, they tend to be a little rounder in the middle.
Originally, people thought that ellipticity is due to rotational
flattening. Elliptical galaxy spins, centrifugal force
stretches it in two directions, orthogonal to the spin axis but turned, that turned
out not to be the case. And more modern studies indicated that in
fact, ellipticals, first of all, don't rotate very much and, second, their shape
is not to do to rotation. It is due to the velocity and isotropy.
As you would recall, the stars in elliptical galaxies move randomly.
Sort of like molecules in gas. That's why they call them pressure
supported. But, it's possible in a dynamical system
like that, that velocity dispersion is different along x, y, and z axis.
A galaxy may be a little hotter in one axis than the other.
And so that means the stars will go further out along that axis so be longer.
And now we think that the shapes of elliptical galaxies are due entirely, to
this anisotropy. The, their temperature if you will is
different, in different directions. We can use statistics of observed shapes
to try to decompose, to project what's going on.
What's shown here in the histogram is the distribution of through ellipticities,
through dimensional ellipticities, if indeed ellipticals were simple, flattened
or elongated prolate ellipsoids. But in reality, they can have three axis
and therefore to flock, through axis ratio's, and that plays a little more
complicated. So, the simplest case is if they
spherical. There aren't many galaxies like that.
Another case is that if one axis is equal to another And, those are bigger than a
third one, than it's a football, it's a prolate ellipsoid.
The other hand, if it's short one is actually shorter than the other or equal,
we have oblate ellipsoid, flattened thing. Flying saucer.
And more generally, we have three axes of different shapes.
So, on average, the actual ratios are shown here.
In fact ellipticals are fairly close to be oblate ellipsoids, flattened along one
direction, but not perfectly so. In addition to the dynamical evidence for
anisotropic velocities which I'll show you in the next module, we can actually tell
this from pictures themselves. And this little tricky to envision but
works like this. If you look at the set of nested triaxial
ellipsoids from[UNKNOWN] you will in fact see that they're apparent projected major
axis seems to move, rotate in the sky. And that is called the isophote twist.
You can again see here, you can look from same[UNKNOWN] from different directions,
and the regions of firedance will project in certain.
Major axis erection at lower densities, you will see it from slide to different
angle. An ellipse will look like if it's turning,
and so that's exactly what's observed. Even in our immediate neighbor Andromeda
Galaxy, Andromeda has two elliptical dwarf elliptical companions and if you stretch
the picture at high contrast you can see that For one of them NGC205, the isophotes
are twisted and actually a little boxy, which is another interesting question.
What's shown on the lower left is set of isophotes for some elliptical galaxy and
major minor axis are drawn. You can see as you go to ever larger
radii, the Axis seemed to rotate. So this is still elliptical shapes.
But actually, ellipticals are not purely elliptical in shape.
Pretty close, but not always exactly. And the first deviation that you can
quantify is, if there is an extra, harmonic if you will, around given
isophote. You can think of an elliptical galaxy
isophote as say single period wave as you go in the azimuth.
But suppose that there is a, twice as high frequency component, then you will see two
waves as you go around and there are 2 possibilities here.
One is that they can be co-aligned with major axis and minor axis.
In which case they're just going to bump out the isophotes along the axis and the
thing would look like its got little lemon like shape.
The, these are called disky galaxies because that's exactly what you get if you
are to project together very thin disk on top of an elliptical isophote.
The other possibility is that they're out of phase.
And so then you get kind of bumps between the major and minor axis.
Those are called boxy ellipticals, and we believe that their shape is due more to
anisotropy and less to rotation. Of course, disks are always supported by
rotation, even if the larger elliptical component is supported by anisotropy.
And so here are a couple examples. One of, disky, elliptical and now we think
that in fact most elliptical galaxies those originally classified as elliptical
showless kind of isophotes lemon shaped, actually you have disks and there may be
close to zeroes than elliptical sproper but there is a continuum of properties
there are no sharp distinctions. The other one shown is also boxy galaxy.
Some galaxies have both. A different radii.
As you go up in radius, what looks in inner part is a box elliptical maze start
looking like a disky one because there is a disk you see at large radius or, the
other way around. And so this is not a fundamental
distinction like. Pure spirals versus pure ellipticals.
It's more of a na, nature of gradual change of the mix.
There are some trends that support that there is physical and meaningful thing
going on which has to do with velocity and isotropy.
The boxy galaxies are more an isotropic. They also tend to be more luminous ones
and more and, and also have higher extra luminosities.
Those can be understood in terms of merging, random merging of pieces into an
elliptical galaxy will both anisotropize/g its velocity dis-, dispersion.
And, it will contribute to the mass, making then bigger, and can also heat the
gas. And so there is a trend sometimes people
overstate that, that boxy galaxies are products of mergers were the disky ones,
the larger products of disks about to collapse.
Neither is the case in reality. There is a mixture of those forming
mechanisms and it's a question of a degree.
Next time we'll talk about internal kinematics of ellipticals.