Now between elliptical and spiral
galaxies, Hubble introduced the immediate
type S0, which are galaxies that obviously
had a disk but he couldn't see any spiral
arms.
Here is Sombrero galaxies, one of the
famous examples.
And indeed, physically they aren't
traditionally in that sense that, it's a
large bulge with relatively small or other
unimportant disk, and disk that doesn't
have a lot active star formation.
And just like other spiral galaxies, these
S0's or lenticulars because their shape
resembles a lens, could also contain bars.
Among this spiral sequence which really is
2 spiral sequences, 1 with bars, 1
without.
There is a trend from earliest types, SAs
to the latest types, SCs, sometimes people
call SDs in even more extreme cases.
In the sense that as you go along that
sequence, the bulge becomes less
important, the disk becomes more
important.
Spiral arms become more prominent and more
ragged as you go further.
Here is a typical spiral galaxy, our
immediate neighbor, Andromeda galaxy or
M31 which has two big companions M32,
small elliptical, and NGC 205 which is
circle dwarf elliptical.
As well as a lot of even smaller
companions dwarf [unknown].
Now in this picture, all of the little
dots you see are actually stars in our own
galaxy.
M51 is another well photographed spiral
galaxy.
It has seems to have something at the end
of its spiral arm.
This is actually another galaxy with which
M51 is interacting.
Sort of its own magellanic cloud except
bigger and in a more spectacular state of
merging.
So about half of all disk galaxies contain
these bars, and including Milky Way.
Bar is hard to see because of the dust in,
in the disk but now thanks to the infrared
observations we are pretty sure that we do
have a bar.
And what bars really are, they are form of
dynamical instability in rotating disks
that form spontaneously.
Presence of dark halos actually can
regulate against formation of bars so
spiral disks are just borderline unstable
to formation of bars.
Now bars are not static patterns they
actually do rotate as the disk rotates,
unlike spiral arms which really are not,
they're density waves rather than
physical, distinct physical sub systems,
about which we'll talk about later.
Bars can also serve as a means of
transferring gas from the outer disk to
the middle, where it can ignite star
formation or feed an active nucleus if
there is one.
The reason for this is that this rotating
triaxial potential creates instability and
can soak energy around your momentum of
the surrounding interstellar medium,
feeding it to the middle.
You see often, spiral arms seems to
originate at the ends of a bar.
And that too, has to do with dynamical
instabilities, and mechanism of formation
spiral arms.
And then there are dwarf galaxies, which
is something of a heterogeneous grab bag
of things.
Hubble put them at the end of his
sequence, but really there several
completely different families in there.
Dwarfs can be gas poor or gas rich.
Gas rich would be things like a very small
spiral or, or generally galaxies with no
specific shape, but plenty of gas igniting
star formation.
But by and large at, unless they have a
lot of star formation they have very low
surface brightness, and so they're very
hard to spot.
And just like the majority of all stars
are dwarfs.
The majority of all galaxies are also
dwarfs.
Numerically, they are more important than
traditional galaxies of the Hubble
sequence.
But they actually don't contain a very
signifigant amount of mass or luminosity.
So you can think of these four galaxies as
food for larger galaxies as they keep
merging into the bigger ones.
And even in halo in the Milky Way we not
see remnants of at least one dwarf galaxy
being consumed through tidal interaction,
the so called Sagittarius dwarf.
Dwarf galaxies themselves are probably not
the product of any significant merging of
even smaller pieces.
In some sense, they are sort of the
original building blocks of larger
galaxies.
So here is what Hubble called a dwarf
irregular.
This is large magellanic cloud, does look
a little bit irregular, but actually what
it is, it is a dwarf barred spiral in
process of tidal disruption with
interacting with Milky Way, and the
obvious part is the bar.
There are also generally young
star-forming dwarf galaxies, One Zweiki
18, Zweiki's catalog is shown here.
You can think of them as almost like
puddles of gas, which are just beginning
to form stars for the first time, or at
any rate, they haven't formed many stars
in the past.
So these are the closest thing to young
galaxies that we see near us.
Now what Hubble thought those Dwarf
Elliptical or Dwarf Spheroidals are
completely different kinds of this.
There are small elliptical, scaled down
version of the elliptical and that's a
separate story, M32 is an example of
those.
But even among the Dwarfs there seems to
be two class of things.
One is Dwarf Ellipticals which really are
probably a different family of objects all
together.
And Dwarf Spheroidals so called, because
they belong to the galactic halo usually
they do have elasticity.
And those look just like little peppering
of stars at top of the Background they do
not contain much in terms of stars, but
they turn out to contain a lot of dark
matter and they are interesting from that
point.
More recently thanks to the more sensitive
detectors it was realized that there were
a lot of spirals out there which are very,
very low surface brightness disks, but
nevertheless, they do have disks.
And the question was, how many of those
did we miss?
Until the ice bergs in the sky that you
just see the bright central portion, but
not the disk.
Which you really have to work hard to pull
out of the noise.
And the interesting thing about these
galaxies is that, they seem like normal
spiral in any other way in terms of the
amount of gas, in terms of rotation
curves, total masses and so on, except
they haven't made many stars and disks.
And the reasons for this are still
unclear.
One of the first large but low surface
brightness disks called, Malin 1.
The two panels on the lower right show
normal contrast picture and a really high
contrast picture.
So if you look at a distribution of
surface brightness of disks, you see
something like that.
At the bright end which here is on the
right are the traditional Hubble spirals.
But then there is an extension to the
lower surface brightness of these newly
found low surface brightness disks.
In the past, because of selection effect,
there exist and just wasn't known, the
heart of the plot of the noise, and it
appeared as if there was a Gaussian
distribution with the characteristic
central surface brightness and that was
called, Freeman's law.
So this again, turn out to be more or
less, a selection effect.
A different kind of irregular galaxies are
what we now know are really mergers of
galaxies in progress.
And, we talked about those when we talked
about numerical simulations.
Here are a couple of famous examples,
Antennae you see tidal tails, and Tadpole
galaxy where again, has a long tidal tail.
And obviously, since they represent least
two galaxies involved they do not fit in
any of the traditional morphological bins.
But we know that they're really important
in terms of galaxy evolution because
galaxy merging is one of the key processes
that are contributing to evolution of
galaxies.
A subset of those are so called polar ring
galaxies, where , erger of two spirals is
not yet complete and there are, there are
angular momentum or orthoginal.
So you see a ring, remnant of one of them,
orthogonal to the disk of the other.
This was puzzled at first until people
realized what was going on.
So next time, we will actually.
Look into the trends of physical
measurable properties along Hubble
sequence and where they really come from.