\documentclass{article}
\title{Using SETI@home data to explore hydrogen distribution in the
Galaxy}
\author{Eric Korpela, Paul Demorest, Eric Heien, Carl Heiles, Dan Werthimer}
\begin{document}
\maketitle

The SETI@home data system presents a unique window on the universe.  It is
situated at the largest radio telescope in the world, which provides the best
available imaging resolution that can be acieved with a single telescope.
Because the frequency band it receives is centered near the 1420.4 MHz 
frequency emitted by hydrogen, the data tapes output by the SETI@home data 
recorder can be analyzed to extract information about hydrogen distribution 
in the Galaxy.

Like the stars in the Galaxy, hydrogen is mainly distributed in a large disk,
like the one shown (edge on) above.  
As you look along the disk you see more than you do when you look perpendicular
to the disk.  How much hydrogen there is along a line of sight corresponds with
how intense the radio emission is.  The plot below shows the spectrum along 
two lines of sight.   The red curve is the spectrum along a line of 
sight 5 degrees from the plane of the Galaxy.  The green curve is along a
line of sight 25 degrees from the plane of the Galaxy.  You'll notice that
there the intensity of the hydrogen emission is higher when you are looking
along the plane of the Galaxy,

The other thing you will notice is that the shapes of the plots is 
different between the two.  That is because the hydrogen in the Galaxy
is moving with respect to us.   Because it is moving, the radio waves it
emits will either be red-shifted if it is moving away from us, or blue shifted
if it is moving toward us.  In the red spectrum above we see four obvious 
peaks in the spectrum, so we are seeing hydrogen moving at four different velocities.  
In the green spectrum we only see a single obvious peak.  The reason for the
difference is that hydrogen in the Galaxy rotates about the center of the
Galaxy in a manner similar to the way the sun does.
When we look out of the plane of the Galaxy, our line
of sight leaves the dense hydrogen at a point relatively nearby.  So in the
green plot, we are seeing hydrogen that is nearby and is rotating around 
the Galaxy at about the same speed as the sun does.  When we look along the
line of sight along the plane of the Galaxy, we see hydrogen that is farther
away, and is moving at a different speed and in a different direction than the
sun is.  

That doesn't fully explain why there are peaks in the spectrum, though.  There
are peaks because hydrogen is not evenly distributed in the Galaxy.  Hydrogen in
the Galaxy is more concentrated in the Galaxy's spiral arms, and less
concentrated elsewhere.  The locations of the peaks can tell us where the
spiral arms are.

Now you know everything there is to know about hydrogen in the Galaxy.  Not by a
longshot.  Astronomers don't know a tenth of what there is to learn about 
hydrogen in the Galaxy.  That's why we're going to take the SETI@home data
tapes, and use them to generate 11 million spectra like the ones above.  We'll
use the spectra to make a map of hydrogen distributions in the areas of the
Galaxy visible to Arecibo.  We will make the map available to anyone who wants
to use it to study the Galaxy.

\end{document}
