So we have the disc in the center of the bulge, and around this we have a halo.
The halo contains about 150 global clusters, typical masses of up to about a
million solar masses and it turns out lots of field stars, so there are
population 2 stars orbiting in a high inclination orbits that are not part of
globular clusters, in fact we now know that 99% of the mass of the stars in the
halo is outside the globular clusters. Globular clusters are very visible
minority. furthermore about half of all these, 40%
of orbit with inclinations larger than 90 degrees, which makes them retrograde
orbits, so if the stars in the disc are orbiting one way.
These stars are orbiting in a, by and large in the other direction so the
angular momentum of the halo in net is about zero.
In terms of size remember Shapley had a large halo of 100 kiloparsecs taking into
account absorption and correct distance measurements, all but about less than 10
of the globular clusters and most of the field stars are within 50 kiloparsecs.
So typically one says, that the halo's sphere of radius 50 kiloparsecs.
You'll notice this is twice the size of the radius of the disc, so the halo is
larger and envelops everything. Of course, some of these objects are in
the disc, we detected them first as population 2 stars in the solar
neighborhood. That just happened to be moving with
wrong velocity and all of the stars in the halo, as we said, are old between 11
and 13 billion years. What else is out there? Well, we knew
that the halo was permeated by warm gas, we could obeserve that because warm gas,
by warm I mean up to 100,000 or a few hundred thousand Kelvin absorbs visible
light, and so we could see the visible light absorption from the halo.
some of the hydrogen atoms are excited, some of the other atoms are inexcited,
are, are there and have lower excitation energy than hydrogen so we could track
the gas in the halo, and there's not that much.
It's not a significant mass. a new discovery from September this year
so I'm putting it in there with an asterisk because I don't know how
verified it's going to be, is that there is another gas component in the halo.
This is hot gas with temperature of a million Kelvin which makes it transparent
to a visible light because there are no electrons with that excitation energy but
it has been absorbed through x-ray absorption.
By oxygen highly ionized oxygen atoms in this gas and if the Chandler result
holds, then this halo might extend as far as 100 kiloparsecs so that's more than
300,000 light years. And that's past to give you a sense, the
Large Magellanic Cloud, and they contain as much mass as the entire stellar
population of the thin disk. So, 60 billion solar masses again, over again of
the mass of the thin disk might be have been lost until recently in the form of
this halo of hot gas. Note, that this is twice the radius of
the globular cluster halo. So we're beginning to construct our
model. I mentioned the large Magellanic Cloud.
Indeed the Milky Way is not alone, there are satellite galaxies, a few dwarf
satellites as well as the Magellanic Clouds orbit the Milky Way.
I should point out that there are it's not quite clear whether the Magellanic
Clouds have a positive, just positive or just negative Kinetic total energy
relative to the Milky Way. In other words, are they inbound orbit or
are they moving off and at some point will disappear, but not in my lifetime?
in any event they are certainly, currently very near the Milky Way and we
can see as we'll talk about the evidence of their tidal interactions with the
Milky Way. Some other galaxies in the other some
other of these dwarf galaxies are currently in the process of being ripped
apart by the tidal forces of the Milky Way which are breaking them up into
rings. The objects in the ring are being slowed
down and eventually these dwarfs are merging to some extent with the Milky
Way, the Milky Way gobbling up dwarfs and growing.
this will see is part of the process of galactic dynamics so it pays to at least
enjoy a look or two of our neighbors. To the left obviously the spectacularly
beautiful Magellanic clouds. If you've never seen them and have a way
to get to the southern hemisphere by all means do.
It's a mind blowing experience, I've done it once and I can't forget it.
to the right the Leo one dwarf. This is what a dwarf galaxy looks like.
Of course, there's a long exposure picture,
you probably would completely miss it and look right through it.
The way that we identify these objects as being part of a separate galaxy is by
tracking both their chemistry, their metallicity, and their proper motion, and
radial motion we see a group of stars in a given direction.
They all seem to be moving in concert, then we can study their random motion and
see that they are in fact gravitationally bound to each other.
And then this if the group is large enough dignifies them not as a star
cluster, but as a small galaxy.