DATE | Lecture 21 |
TITLE | Radio Astronomy and Galactic Structure |
READING | Chapter 15.5-15.8 |
MAIN CONCEPTS | Radio Astronomy, Spiral Stucture |
The problem with living in the middle of the Galactic disk is that the
interstellar medium blocks our view of most of the Galaxy. We are very
fortunate that there is a way around this, provided by radio astronomy.
This comes by way of the "21-cm" line, which arises from neutral hydrogen.
It is a special sort of energy transition, involving a "spin flip" of the
electron relative to the proton. The atom is in a lower energy state when
the spins are not aligned - the energy difference is equal to a photon
with wavelength of 21-cm. This transition is very weak ( a typical interstellar
atom can stay in the higher energy state for millions of years), but since
most of the Universe is hydrogen this radiation arises everywhere. Thus,
the Galaxy is both transparent and visible in 21-cm radiation. The transparency
makes it difficult to tell where along a line-of-sight the detected emission
is coming from. Because it is a spectral line, one can make use of the
Doppler shift to help with this problem. In general, the inner part of
the Galaxy rotates at a different speed than the outer parts, so there
will be some projected velocity difference between gas at different distances
along the line-of-sight. Given a model for the Galactic rotation, one can
use it to assign each velocity bin in an observed broadened spectral line
to a different location. In this way, we can build up an overall map of
the neutral hydrogen (Fig. 15.18). This map can be supplemented for nearby
locations by observing star forming regions containing massive stars; we
know how to assign distances to stars based on their main sequence location.
Another useful molecule is carbon monoxide (CO), whose rotational changes
can also give rise to radio photons. This is a good tracer of molecular
clouds.
The picture which arises is that we live in a flattened disk of stars
and gas, which is organized into spiral arms. We see many examples of similar
structure for other galaxies. Indeed, the nearest major galaxy, M31 in
Andromeda, we think looks very much like the Milky Way. The spiral structure
may at first seem sensible, since the Galaxy rotates at different rates
at different radii, it would take radial structures and wind them into
spirals. The problem is that the spirals are never too tightly wound, even
though the galaxies have had time for up to 100 rotations. The solution
is that these arms are not composed of particular stars and clouds; they
are "density waves". As on a freeway where there is an accident, the density
of cars behind and up to the wreck is much higher than in front of it,
but individual cars move through the density wave and then speed up away
from it. So stars, including our Sun, find themselves moving a little slower
while in the spiral arms and faster between them. The arms maintain themselves
by their own extra gravity, and serve the function of moving angular momentum
to the outer regions of the Galaxy, allowing it to slowly collapse. Due
to the higher density in the arms, this is where molecular clouds and star
formation tend to be concentrated. In other galaxies where we can look
down from above, the spiral arms are clearly traced out by the young bright
stars.