The Mass of the Galaxy

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The Mass of the Galaxy
We can use the orbital velocity to deduce the
mass of the Galaxy (interior to our orbit)
vorb2GM/R. This comes out about 1011 solar
masses. We can also get a mass estimate from the
integrated light of the Galaxy (corrected for
interstellar absorption). This comes out
substantially lower. There must be some dark
matter.
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Mapping the Galaxy Radio Astronomy
We can only see our local neighborhood because of
interstellar dust.
To penetrate this, we can use radio wavelengths
(much longer than the size of dust particles). Of
course, something has to be producing radio
emission
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Sources of Radio Emission -1

• Thermal emission from cold interstellar clouds
• At a few 10s of K, blackbody emission will be in
  the radio, or somewhat hotter clouds have a long
  wavelength tail

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Sources of Radio Emission -2
2) In a strong magnetic field, spiraling
electrons will produce non-thermal synchrotron
radiation. This can happen near stars or compact
objects, or from cosmic rays in the galactic
field.
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Sources of Radio Emission 21 cm radiation
Neutral hydrogen has a very weak radio spectral
transition. So the Galaxy is transparent to it.
On the other hand, theres a lot of neutral
hydrogen. So we can see it everywhere. There are
also molecular lines from CO and other molecules.
The transition occurs because electrons and
protons have spin. Having the spins aligned is
a higher energy state. So in about 10 million
years it will decay to the ground state
(anti-aligned). Or a 21-cm photon can be absorbed
and align the spins. Because the Galaxy is
transparent, it is hard to tell where the
emission is coming from along the line-of-sight.
But because we know its precise wavelength,
Doppler shifts in this line can tell us how the
gas is moving.
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Optical and Radio Sky
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Deciphering 21-cm maps
With a rotation model of the Galaxy, you can sort
of figure out where different parts of the
emission are coming from.
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Radio Data
Imaging and velocity maps in CO.
Composite image of Perseus region in hydrogen.
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Finding the Galactic Structure
Molecular Clouds
21-cm map
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Spiral Arms in Galaxies
Since inner orbits are faster than outer orbits,
you might think that is why one sees spiral arms.
But these would rapidly wind tightly galaxies
have had 100 rotations since they formed.
Instead,
the spiral arms are density waves apparent
patterns where stars are denser due to slowing
down from mutual gravity.
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Density Waves
Traffic jams are good examples of density waves.
Certain parts of the freeway may have a high
density of cars, yet individual cars do not stay
with the pattern, but flow through it. They move
slowly when at high density, and move quickly
when at low density. The site of an accident
might produce a stationary density wave (but
again, cars are always moving through it).
Thus, the spiral arms of a galaxy are just a
pattern that may rotate slowly or not at all
individual stars will be passing through it all
the time.
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Spiral Arms and Star Formation
When the ISM passes through it, it gets
compressed, and star formation is enhanced. This
makes bright hot young stars, and the pattern
stands out.
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Tracers of Spiral Arms
In addition to radio maps, you can use HII
regions or OB stars to try to locate spiral
arms. The Sun is near the Orion-Cygnus arm, but
that is a recent occurrence. Its been around
about 18 times.
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Spiral Tracers from Outside
In other galaxies, the arms are easy to see
because their ISM does not hide optical
diagnostics from us. There are always only a few
arms (often 2), and they are never too tightly
wound.
O B stars HII
regions 21-cm radiation
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The Heart of the Galaxy
Infrared
X-ray
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The Galactic Center
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The Monster Lurking at the Center
Recent adaptive optics pictures in the infrared
at the Galactic Center show stars orbiting a
central invisible mass. Keplers Laws yield a
mass inside one light year of 2.7 million solar
masses! It has to be a black hole (but apparently
it is napping at the moment)
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The Multi-wavelength Milky Way
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Stellar Populations
Stellar Population Location Star motions Ages
of stars Brightest stars Supernovae Star
clusters Association with gas and dust? Active
star formation? Abundance of heavy elements (mass)
Population I Disk and spiral arms Circular, low
velocity Some lt 100 million years Blue giants
Core collapse (Type II) Open (e.g., Pleiades)
Yes Yes 2
Population II Bulge and halo Random, high
velocity Only gt 10 billion years Red giants
White dwarf explosions (Type I) Globular (e.g.,
M3) No No 0.1 - 1
Population II stars are old and metal poor, found
in large orbits in a random spherical
distribution. Population I stars are young and
metal rich (including hot stars), all orbiting in
the disk in the same direction.
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Galactic Structure
Disk (I) and Bulge (II) (stars, ISM, open
clusters) Halo Pop II (stars, globular
clusters) Dark Matter Halo
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Formation of the Galaxy