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Mapping the Galaxy with Radio Astronomy


[intro music] Behold, a photograph of our home galaxy, the Milky Way, in all of its glory. Just look at those spiral arms. But how was this photo taken? Down here on earth we’re stuck within the plane of our
galaxy’s disk. This is why the Milky Way appears as a band in the night sky:
we’re seeing our galaxy edge-on, like viewing a Frisbee from the side. Trying to map the Milky Way galaxy from Earth is a lot like trying to map MIT’s campus
(or your own neighborhood) while standing in one spot on the ground. You might be able to see some of the stuff around you, but much more is obscured, so you can’t
see what the area looks like beyond your point of view. The best way to get a good map is by viewing the place from above: From high up here, it much easier to see
how MIT’s buildings are positioned relative to
each other, to create the campus map. There’s a problem, though. If you want to take a photo of the
galaxy from above, you need to get a camera there, and “there” is a vantage point which is tens of thousands of light-years away. That makes it tens of millions of times farther away than the farthest spacecraft
we’ve ever launched. It also means that even if we did have a
camera there, it would take tens of thousands of years
for the signal to return to us. So-called “photos” of the Milky Way galaxy, then, are actually just artists’ conceptions. That doesn’t mean we have to give up, though. We have a clever tool at our disposal if
we want that picture of the galaxy: Radio Astronomy. Radio astronomy allows us to take advantage the natural properties the most common
thing in the galaxy, clouds of hydrogen gas, and piece together a
portrait of our home galaxy, the Milky Way. Hydrogen gas naturally emits low-energy
radio waves, which we can detect back here on Earth. If we aim a radio telescope into the plane in our galaxy, we collect
radio waves from the various hydrogen gas clouds which lie along our line of sight. Normally all of these radio waves would be near the frequency of 1420.4 megahertz but the waves from gas clouds moving away from us are stretched out and are thus at a slightly lower frequency. The opposite is true for waves from gas which is moving towards us. As a result, the data we receive from the telescope
tells us how fast the gas is moving in the direction we’re looking. If we can
predict how fast different parts of the galaxy are moving, then we can work out where the gas must
be concentrated along our line of sight to make the particular blend of
radio waves which we observe. A lot of gas all moving at one speed for instance indicates a denser region of orbiting gas, possibly as part of a spiral arm. If we apply a
heaping dose of trigonometry, we can plot these gas
concentrations along the line of sight we were using in the galaxy. If we repeat this process over and over again along slightly
different lines of sight into the Galaxy disk, we can start to
plot where the gas is concentrated in multiple areas, and thus begin to map out the shape of
the galaxy. Watch as we keep filling in the map. At the end of the day, we have something
looks like… this! Whoa! See those arcs of higher density? Those are sections of spiral arms in our
very own galaxy. We managed to paint a partial portrait of the Milky Way without even having to leave the MIT
campus. What’s even better is that we’re not the first people to have tried to
find a map of the galaxy this way. This approach was first
used by Kerr, Oort, and Westerhout in the 1950s. If we compare our map to theirs, we
find that we agree on the locations of several important features. Today, we know that this larger section
is part of the Perseus Arm, the shorter bit is part of the
Sagittarius Arm, and this more distant piece is known as
the Outer Arm of the Milky Way. We managed to make this map with
little more than a radio telescope and some trigonometric know-how. Cool! So, we’re done right? Well, not quite. One useful piece of information we’d
like to have about our galaxy is its rotation curve, which is a graph of how fast things orbit in the galaxy as a function of how far they are from
its center. What might the Milky Way galaxy’s rotation curve look like? Before we apply the data, let’s try to
figure out what we’re expecting. Thankfully, the physics of orbits is a field
of study stretching back centuries. Kepler’s third law states that there is
a relationship between the time it takes an object to complete its orbit how big its orbit is. If you rearrange
the math a bit, you end up with this relationship: we
expect that the velocity of objects in orbit will be proportional to
the inverse square root of the orbit’s radius. Going back to our telescope’s data, we already know the frequencies of the
radio waves tell us how fast gas clouds are moving. We can work out that the fastest gas
cloud we observe is the one which is the
closest to the center of the galaxy because its orbital velocity carries it
along our line of sight, so we see the most exaggerated speed. With a bit of trigonometry, we can then
find how far that point is from the center of our galaxy. The trick is to repeat this calculation
for different lines of sight into the galaxy. Doing so allows you to create the graph of the rotation curve. Since we already have a model of what we’re expecting the rotation curve to look like, let’s scan through the galaxy with our radio telescope and find the actual rotation curve. We gather a bunch of scans, crunch the numbers, and… Wait a minute, these don’t look alike at all! Something
must have gone wrong. Or did it? It turns out that this experiment
has been repeated all over the world many times over, and every time people find this shape for
the rotation curve! The rotation curve we observe doesn’t match how Kepler told us it should look. Was Kepler wrong? Not quite. We can also try flipping our reasoning and asking: What would have to be different about
our galaxy in order to see this kind of rotation curve? It turns out
that you expect exactly this kind of curve if the galaxy has a lot more stuff in it in a particular arrangement. But when we
look out where this stuff should be, we don’t see anything! This stuff doesn’t
interact other normal matter, or with light, both
which pass right through. But it does interact gravitationally and provides enough extra heft to the
galaxy that things orbit at the speeds we observe. This is indirect evidence for the
existence of the mysterious “dark matter”, whose composition and
properties are an open problem in science right now. We have as of yet never observed dark matter
directly, but we can tease out signs of its existence from work like these observations. It’s amazing that we can find indirect evidence for something like
dark matter, while trying to do something else entirely like taking a picture of the galaxy. These kinds of serendipitous finds
happen all over the place in science, but it’s particularly amazing here given that the tools used were just a telescope which looks a lot like a satellite dish, and the hydrogen gas floating throughout the galaxy. Radio astronomy, while simple, is powerful enough to save the day.

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