Category Archives: galactic history

astronomy, galactic history, galaxies, galaxy evolution

Big Galaxies Get Bigger

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Galaxies Extended by Supernova Action

A new CU-Boulder study indicates spiral galaxies like our Milky Way, and the M74 Galaxy shown here, are larger and more massive than previously believed. Image courtesy NASA

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.

 

 

 

 

galactic history, galaxies, Herschel Space Telescope, starbirth

Cosmic Fireworks

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Big News in Distant Galaxies

You know that saying about how time is the universe’s way of keeping everything from happening at once? Well, there’s a lot happening in astronomy news today, almost all at once. So, the universe is flinging cool new stuff at us.

First, take a gander at this image. It’s an artist’s concept of what galaxies in the early universe were doing about 13 or so billion years ago.

 

Galaxies in the early universe grew fast by rapidly making new stars. Such prodigious star formation episodes, characterized by the intense radiation of the newborn stars, were often accompanied by fireworks in the form of energy bursts caused by the massive central black hole accretion in these galaxies. This discovery was made by a group of astronomers led by Peter Barthel of the Kapteyn Institute of the University of Groningen in the Netherlands. (Credit: ESA/NASA/RUG/MarcelZinger)

Yep, they were making stars at a prodigiously fast rate, more rapidly than many galaxies do today. By comparison, our Milky Way’s star birth factories create at an average rate of one new star a year. Ours is a pretty quiet galaxy in that regard. And, while we do have a black hole at the center of the galaxy, compared to other galaxies’ very busy black holes, ours is pretty tame. Only occasionally does it capture a star or gas cloud and gobble it up.

Now, if you look at more active galaxies, you see more  star formation. And these busy galaxies were much more common in the early universe.  So, it makes sense that astronomers would find galaxies at that time busily baking up stars. Quasars and radio galaxies are prime examples of these active galactic denizens.  And, observing them is easy due to their bright radiation, which can be detected over huge distances. Essentially, these active galaxies are easily detected through their luminous radio, ultraviolet or x-ray radiation, which results from steady accretion on to their massive central black holes.

These exotic galaxies are getting a lot of attention from the Herschel Telescope, which is sensitive to far-infrared wavelengths of light (which indicate heat radiation). A group of astronomers in the Netherlands has used it to study star birth in distant galaxies.  Basically, it looks for heat radiation generated by star and planet formation in our own galaxy, and also studies the same radiation from complete galaxies.  If a distant object is emitting strong levels of far-infrared radiation, then it’s a sure bet that the galaxy is undergoing massive amounts of star formation. And, by massive, I mean creating hundreds of stars each year.

These busy galaxies also have strong signals in radio frequencies, emanating from their central black holes. The black holes are busy growing (accreting mass and perhaps even merging), at the same time their host galaxies are creating whole batches of hot young newborn stars.  And all of it is happening billions of light-years away, showing us galaxies in some of the earliest epochs of the universe.

The take-home message here is that these kinds of active galaxies existed early in cosmic history.  They’re among the largest, most distant, most powerful and most spectacular objects in the universe. And, they give astronomers a look at what massive normal galaxies may have looked like in their infancy as they balanced the action of growing black holes at their hearts with the demands of star birth in other regions.  These are the kind of “baby pictures” of infant galaxies that give astronomers a deeper understanding of what happened “way back when” at a time when the universe was a baby.