Welcome to the galaxies and cosmology. Yeah, for today's lecture we can't make a break

because I have another meeting at two, so I'll just do the lecture in one sitting until 1 45.

Okay, so last time we spoke about the contents of the local group and we looked at one of the types

of constituents in there, the dwarf spheroid galaxies. Today we'll be mainly talking about

the larger galaxies, the spiral galaxies. We start with Andromeda and then we'll be looking

in general at the physics of the interstellar medium in spiral galaxies because that's something

that we are going to need when we talk about the different types and classes of spiral galaxies.

And we start here with a nice picture of Andromeda. This is what it looks like. You see here a nice

bulge in the center. You see dark areas where there's a lot of dust that blocks the light of

the underlying stars, which you can see here in this bluish tint. And you also see an elliptical

galaxy which is gravitationally bound to Andromeda here. So Andromeda, similar to our own Milky Way,

also has smaller galaxies nearby. It's 60 kiloparsecs away. It has a diameter of 60 kiloparsec

and a distance of 780 kiloparsec. Depending on whom you trust, it is about 50% more luminous

than our own Milky Way. There is a big caveat to this in that recent measurements about the

luminosity of the Milky Way have corrected its luminosity and mass upwards. So it might well be

that Andromeda is only comparable to our galaxy and not much more luminous, but right now the jury

is still out there. We do see that it apparently has a different structure by looking at the number

of globular clusters that are bound gravitationally to Andromeda, which is about twice the number of

what we have in the Milky Way, again with the caveat that there is a certain volume around our

Milky Way that we cannot observe. We don't know what happens behind the bulge of our Milky Way

because we can't look through this thing because there are too many stars in it. And it has also

similar to our galaxy many satellites. I already mentioned this one down here, but there are more

than 20 additional galaxies gravitationally bound to Andromeda. We look at Andromeda because

Andromeda is a galaxy that is very nice to study because it has the right orientation towards us

so that we can really observe its structure in a nice way. And it's the most nearby spiral galaxy,

so we also have the highest resolution in this system. And this is summarized in this image here,

which is a composite multi-wavelength image of Andromeda. We start here at the top left with

the optical image of the galaxy. Top right, so this traces mainly the stars. Top right is the

infrared image, which as I also mentioned last week already traces mainly dust. And down here is the

X-rays, which traces the very hot gas and the compact object populations. It traces the neutron

stars and the black holes in this galaxy. And here in the middle, this shows you how things go

together. So for example, if you look here at the black band in the optical image, you see that this

is where the galaxy's infrared bright. So this is dust that is so cold that in the optical light,

it blocks the light of the stars that are behind it. But in the infrared, it thermally radiates,

and so we see it bright here. You also see that the dust distribution here in the infrared is not

distributed evenly, but you have very clear bands there, which are reminiscent of spiral arms. So we

see a big structure of the galaxy. You also see down here now, if you look at the X-ray image,

you see that the distribution of neutron stars and black holes, so the outcome of the evolution

of very massive stars, is not uniform over the galaxy. There's a much higher population of these

objects in the bulge, in the central region of the galaxy, than outside. So you see really that the

bulge is a physically separate thing from the rest of the galaxy here. So if we just take a better

look at this image, and since there is a camera here, it's unfortunately not very good to see

everything here. But if we look at the image, essentially all of the points here are white

dwarfs, neutron stars, and black holes. You also see here in the center that there is diffuse

radiation that surrounds these point sources. Okay, so this is hot gas that you would produce,

for example, in supernova remnants. You also see here some reddish objects. These are more absorbed

sources and colder sources. And if you look at the online slides, you will see that this diffuse gas

really goes approximately around this region here. Outside here we have additional objects,

so background sources, galaxy, active galactic nuclei, supermassive black holes, which we'll

be looking at in some galaxy clusters. I think this is a galaxy cluster and does not belong to