Our understanding of the Milky Way galaxy has come a long way.
What first looked like a dusty band of light in the sky turns out to be an
enormous spiral galaxy that is home to one hundred to four hundred billion suns.
But there's literally more to the Milky Way than meets the eye. It's invisible to
our telescopes, yet it makes up most of the mass of our galaxy. It's called "dark
matter", and by definition we have no way of directly observing it. That makes estimating
the mass of our galaxy very difficult. But thanks to new observations
by the Hubble and Gaia space telescopes, astronomers pinned down the mass of our
galaxy, setting the stage for a new probe into the matter we cannot see. Welcome
back to Launch Pad, I'm Christian Ready, your friendly neighborhood astronomer.
Understanding the mass of our galaxy is one of the most fundamental parameters
we can measure; it helps us to understand how our Milky Way formed and evolved and
how galaxies work in general. But even estimating the mass of our galaxy is
very difficult. To understand why, let's first take a quick look at the anatomy
of our galaxy. At the heart of our galaxy is a supermassive black hole four
million times the mass of our Sun. It marks the center of the Galactic Bulge, a
ten thousand light year bar-shaped structure made of mostly of older stars.
The Bulge gives away to the disk which features the spiral arms and mostly
younger stars. It spans anywhere from a hundred thousand to 150 thousand
light-years in diameter ,but is only a thousand or so light-years thick. We are
here, about 26,000 light years from the galactic center living just off the
Galactic "main line". Surrounding the Bulge and Disk is a halo of globular
clusters, each home to hundreds of thousands of older stars. But all four
hundred billion stars of the Disk, the Bulge and the Halo only amount to a few
percent of the mass of the galaxy. The rest is dark matter which extends in a
halo out to as far as a million light years from the galactic center. We don't
know what dark matter is but we know it's there because of its gravitational
influence. If our galaxy didn't have any dark matter, stars would travel slower
the farther away from the center they are. Instead,
stars in our galaxy orbit at pretty much the same speed everywhere in the disk
and halo. That means all of the matter that we can see, the stars, the planets,
the spiral arms, are just the tip of the galactic iceberg. But how do you weigh a
galaxy when you can't see most of it? Well we can't directly measure what we
can't see, but we can use globular clusters in the halo as tracers to
measure their velocities. The more mass the galaxy has, the faster those clusters
will move. Globular clusters make ideal tracers because they extend out to great
distances from the galactic center. If we can measure their true velocities, we can
weigh the galaxy - including its dark matter envelope - out to hundreds of
thousands of light-years. In order to determine a cluster's true velocity, we
need to know three things: its distance from the Sun, its velocity along our line
of sight - called "radial velocity", and its sideways motion called "proper motion."
With the proper motion and distance we can then work out the cluster's actual
sideways velocity or "transverse velocity" Then we can use the transverse and
radial velocities to work out the cluster's true velocity in space.
Measuring a cluster's radial velocity is relatively easy; all you do is measure
the Doppler shift of their spectra. The amount and direction of the Doppler
shift reveals how fast the cluster is moving toward or away from Earth.
But measuring a cluster's proper motion is very difficult and that's because
these clusters are so far away that their proper motions across the sky are
very tiny on short timescales. For this reason, the true velocities of globular
clusters had to be estimated based on measurements of the radial velocities,
but estimates for everything else. These estimates were then combined with
estimates of the amount and distribution of dark matter in the galaxy to come up
with another estimate for the mass of the Milky Way. Unsurprisingly, these
estimates ranged from as low as 500 billion to 3 trillion times the mass of
the Sun. That's a pretty wide range, but that's the best astronomers could do
because they didn't have any accurate proper motion data of
popular clusters - until now. An international team of astronomers led by
Dr. Laura Watkins of the European Southern Observatory used data from the
Gaia and Hubble Space Telescope's to make the most accurate measurements of
the motions of globular clusters. Gaia is the European Space Agency's mission to
create a three-dimensional map of the sky. It does this by making ultra-precise
measurements of the positions, distances, radial velocities, and motions of more
than 1 billion stars in the Milky Way and it's satellite galaxies. In 2018, the
Gaia mission issued its second data release which included distance and
proper motion measurements of 75 globular clusters. The team selected 34
pristine halo clusters that weren't obscured by the disc or contaminated by
metals. These objects measured by Gaia ranged in distance from 6,500 out to 68,000
light-years from the galactic center. Not only that, but the team used
the Hubble Space Telescope to extend their survey even farther. Hubble has a
smaller field of view than Gaia, but it can see much fainter and therefore more
distant clusters. Not only that but Hubble's been in space now for nearly 30
years so it's been able to track the motions of some of these clusters for up
to a decade at a time. The team added 12 clusters from the Hubble data archive,
bringing the total sample size up to 46 clusters ranging in distance from 6500
to 129,000 light-years from the galaxy's center.
After determining the individual velocities of the clusters, estimating
their masses, transferring their velocities from Sun-centered to galactic
centered, refining estimates of the distribution and the amount of dark
matter in the galaxy, and a whole host of other stuff, the team arrived at an
answer for the mass of the galaxy: 1.5 trillion solar masses; right in the
middle of the range of the previous estimates. Maybe that's a little
anticlimactic. It would have been really interesting if the result was something
completely outside of the range of previous estimates but it does set the
stage for further refinement of these
measurements. Gaia and Hubble are still monitoring the motions of the 157 known
globular clusters surrounding our galaxy. That means more data on more clusters
will be available in the coming years. That in turn will lead to even more
accurate measurements of our galaxy's mass, which will help us to better
understand the amount and distribution of dark matter. That in turn will lead to
a better understanding of how dark matter influenced the formation and
evolution of our galaxy, and indeed how it led to the formation and evolution of
the universe as a whole. If you want to learn a little bit more about the
galaxies that occupy our local region of the cosmic woods, I have some videos on
them that I'll list to here. And I'd like to give a special shout out to my
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next time, stay curious my friends.