So we have seen what stars do in spiral galaxies, but what about gas?
This is a composite picture of a lot of neutral hydrogen images of a lot of
different spiral galaxies, and you can see that in many cases you see spiral arms
just fine. Well, there are many components to the gas
in spiral galaxies. There is cool, neutral hydrogen, which we
denote as H-Roman One. Then there is molecular gas, usually, most
of that is actually molecular hydrogen and then carbon monoxide.
But there are hundreds, if not thousands, of others species of molecules out there,
some of them, very complex. And the importance of, of the gas, is
that's the fuel for star formage. You need to have cold gas, cold,
interstellar material clouds to collapse and be able to ignite, nuclear fusion in
the core. Now there is ionized gas as well ionized
by the ultraviolet radiation from young stars.
And that's what makes those pretty pictures of nebulii and star formation
regions. And that, we can observe, observe it
through optical spectroscopy or other forms of spectroscopy, but the neutral
hydrogen is best observed through the spin-flip line which I'll explain in a
moment, at the wavelength of 21 centimeters.
At that wavelength, radio waves go through the dust.
And so we can see all the way through. So just as there are stellar disks, there
are gas disks, and they also can show spiral arms.
But one important thing is that hydrogen disks extend further than the optical
ones, sometimes by a substantial factor. And because this spin flip line is so
sharp and well defined. It can be used as an excellent tracer of
velocities of gas, including rotation of, of galaxies the, ta, the Doppler shift.
This is how it works, in a hydrogen atom, you have a proton and an electron.
And there are 2 possibilities for their spins to be Aligned or anti-aligned, one
has a slightly higher energy level than the other and that transition corresponds
to the photons of the wavelength of 21 centimeters.
So, this is, where it counts. Now, this is not something that's easily
achieved in a lab but can happen easily in densities and temperatures of interstellar
medium. So there's plenty of it.
And this 21 centimeter line has been main tool to probe interstellar material in
these galaxies, near and far. So here is a contour map of neutral
hydrogen superposed on an optical image of this galaxy and it's obvious that hydrogen
goes much further in visible stars. Now by observing Doppler shifts of the 21
centimeter line we can see the velocity field.
And that looks like this. These are the lines of equal radial
velocity and they look like this because on one side The disc is coming towards us
and the other is going away from us. Because the gas goes so much further out
than stars, we can measure rotation curves much further using this method, than say
through optical spectroscopy. So here is a simple comparison of optical
and hydrogen images of famous spiral galaxy M81, in the Virgo cluster, and you
can see that gases nicely concentrated in spiral arms, which is, of course, where
stars are made. Although it tends to be a little more
extended. So that was the atomic gas.
What about molecular gas? Aside from hydrogen molecule, h2.
The next most prominent one is carbon monoxide.
Incidentally, all of the different molecular species tend to be found in same
places, by and large. Now in the case of molecular clouds, which
are colder, they get to be closer to the dust lanes and they're closer to the
regions of star formation. You can see spiral arms outlined even
better. But there are also new features, there are
rings of Molecular gas around galaxies. There are bars and other things.
As you probably learned in astronomy before, the interstellar medium has
multiple phases. I touched upon that a few minutes ago.
The, first there is a cold interstellar medium and that's mostly the neutral
hydrogen gas, temperatures ranging from typically 10's to 100 degrees Kelvin.
So that's all of the cold gas, both atomic and molecular as well as dust.
This is what makes stars in, in this cold galaxy.
Next, there is warm interstellar material. Typically with temperatures of thousands
or tens of thousand of degrees. And, it's warmed up and ionized by the
radiation by young stars. And that's where all these pretty nebula
come from. And finally, there is hot interstellar
medium, which really mostly belongs to the galactic halo.
It's the gas that's expelled from the disc through explosions of supernovi and the
shock waves of supernova remnants are what heats it up to this particular range.
Here, here is the typical spiral galaxy rotation curve, with it's decomposition
into the bulged disc, and dark matter components.
We spoke about those extensively when we talked about dark matter, so, I will not
dwell on this much further. And next, we will look at where does the
spiral structure come from.