Transcript The Mass of the Galaxy - University of California, Berkeley

The Mass of the Galaxy
We can use the orbital velocity to deduce the mass of the Galaxy
(interior to our orbit): vorb2=GM/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”.
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…
Sources of Radio Emission -1
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
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.
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, there’s 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.
Optical and
Radio Sky
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.
Radio Data
Imaging and
velocity maps
in CO.
Composite image of
Perseus region in
hydrogen.
Finding the Galactic Structure
21-cm map
Molecular Clouds
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.
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.
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.
Tracers of Spiral Arms
In addition to radio maps, you can use HII regions or O&B stars to
try to locate spiral arms. The Sun is near the Orion-Cygnus arm,
but that is a “recent” occurrence. It’s been around about 18 times.
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
The Heart of the Galaxy
Infrared
X-ray
The Galactic Center
The Monster Lurking at the Center
Recent adaptive optics pictures in the infrared at
the Galactic Center show stars orbiting a central
invisible mass. Kepler’s 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…)
The Multi-wavelength Milky Way
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 < 100 million years
Blue giants
Core collapse (Type II)
Open (e.g., Pleiades)
Yes
Yes
2%
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.
Population II
Bulge and halo
Random, high velocity
Only > 10 billion years
Red giants
White dwarf explosions (Type I)
Globular (e.g., M3)
No
No
0.1 - 1%
Galactic
Structure
Disk (I) and Bulge (II)
(stars, ISM, open clusters)
Halo : Pop II
(stars, globular clusters)
Dark Matter Halo
Formation of the Galaxy