Big Galaxies Get Bigger

Galaxies Extended by Supernova Action

Most spiral galaxies that I’ve seen are big. Really big. The Milky Way, which is the stellar city WE live in, contains hundreds of billions (or perhaps even trillions) of stars bound together in spiral shape that stretches across 100,000 light-years. But, it also contains clouds of gas and dust, at least one supermassive black hole, and countless stellar black holes. I’m talking a massive amount of …well… mass….all wrapped up in and between our galaxy’s spiral arms. And, the same for many other spirals such as Andromeda and 74 (at left).

For a long time, one of the big mysteries about the Milky Way—and other spiral galaxies—involved knowing the actual amount of mass they contained. You could try to figure it out by looking at all the stars and nebulae and estimating their masses.

However, there are some problems with that: one is that you can’t see all the stars. Some of them are hidden by the clouds of gas and dust that thread through the galaxy. Another is that some are so dim you can’t see them. And, it gets tougher when you try to do the same thing for more distant galaxies. In addition, galaxies all seem to have black holes, and how do you estimate those without knowing how many stellar mass black holes there are and whether or not there is a massive one in the core?

You could try to figure out how much mass there is in a galaxy by observing the rotation rates of material in the galaxy and from that figure out the mass. If all the mass in a spiral galaxy was simply bound up in the stuff you could see, then it would be relatively straightforward to figure out how much mass it had. Which is what astronomers tried to do. But, the galaxy kept throwing other problems in the way.

If you measure the motions of stars as the galaxy rotates, there should be bigger differences in the rotation rates of stars in different places in the galaxy. Those on the “outskirts” of the galaxy should be rotating at a slower rate than those closer to the center, following Kepler’s law of motion that says the farther out something orbits, the slower it moves and the longer its orbit is.

Well, a funny thing happened. The Milky Way and other spirals didn’t show a huge difference in rotation rates. Puzzling, yes.  And it took a while for astronomers to figure out why: there is more mass to the galaxy than we can see, and that mass is affecting the rotation rates of the stars in the galaxy. This led to the suggestion that some unseen dark matter was affecting the rotation rates of stars in a galaxy. Today we know that dark matter exists, and astronomers are busily mapping it by the effects it has on the matter we CAN see (the so-called “baryonic matter”).

While some astronomers search for dark matter and try to chart its distribution in the universe, others are continuing to measure galaxies for the matter that CAN be detected directly (that is, through the light it radiates). And, it turns out that the Milky Way and other spirals are much larger and more massive than astronomers thought.  They’re not only surrounded by dark-matter halos (think of them as shrouds of dark matter), but many galaxies also have gas halos enveloping them. How do astronomers know this?

They use instruments such as the Cosmic Origins Spectrograph aboard the Hubble Space Telescope to look at light from distant quasars as it streams through relatively nearby spirals. University of Colorado-Boulder Professor John Stocke and a team of researchers used distant quasars, which are active regions at the centers of distant galaxies, as cosmic flashlights to illuminate the gas clouds surrounding the closer galaxies. It turns out that ultraviolet light from the quasars gets absorbed as it passes through these extended gas halos of galaxies. Imagine shining a flashlight through a dust cloud or fog; some of it would get absorbed or bounced back. If you could study the light from the flashlight through a special instrument called a spectrograph (which breaks up light into its different wavelengths),you might be able to tell what the fog or dust cloud was made of simply by looking for gaps in the spectrum where light was absorbed.

The Cosmic Origins Spectrograph detected the light  from distant quasars and measured the amount of ultraviolet radiation that was absorbed by the gas surrounding the galaxies in the team’s survey.  The amount of UV that was absorbed allowed them to detect the gas clouds and start to make some estimates about how massive those clouds are.

So, where do the gas clouds come from? Supernova explosions. When very massive stars die, they eject huge amounts of hot gases out to the distant reaches of the galaxy. Eventually the gas gets recycled back into the galaxy, where it is used to create new stars. So, not only do the gas clouds add to the mass of their galaxies, but they play an active part in galaxy evolution as old stars die and get their material recycled into succeeding generations of stellar newborns. This adds a new wrinkle to the ways that galaxies, especially spiral ones, evolve over time.