Look Over Here…

It may come as a surprise to folks to learn that we on Earth don’t live in the middle of the Milky Way Galaxy. We actually live out in the suburbs, about 26,000 light-years away from all the action at the center of our stellar city. That’s actually a good thing, because from all accounts, the core of the Milky Way has a black hole or two, and a whole lot of starburst activity and other stuff going on, some of it not very healthy to be around. Those aren’t conditions conducive to a nice quiet life on a water-bearing world such as ours.

Nonetheless, like urban folk all over the world, sometimes we get an itch to see the “downtown” area with its bright lights and excitement. So, we try to look at the center of the galaxy, only to find that it’s hidden by dust clouds. In northern hemisphere summer, you can go out a couple of hours or so after sunset and look south toward the constellation Sagittarius (shown in the image above from Wikipedia). Just off the tip of the spout in the teapot shape of Sagittarius is where the center of the Milky Way is located. The bright clouds are stars that lie between us and the core of the galaxy, which is hidden behind dust clouds. For folks in the southern hemisphere, Sagittarius is going to be overhead or even north of overhead (depending on where you are). But, no matter where you live, if you can get outside and take a gander at Sagittarius, you’ll be looking toward the heart of our home galaxy.

Now, it turns out we can look through that dust if we use a telescope equipped with infrared detectors. Infrared light CAN get through the dust. The image at left is from the Spitzer Space Telescope, and it shows the core of the galaxy-the stuff we can’t see with our visible-light eyes. There are hundreds of millions of stars packed into that scene, along with dark dust clouds that even infrared light couldn’t pierce.

It’s kind of fascinating to go out and look up at that region of the sky, which seems rather placid in visible light. Yet, behind all those dust clouds are some fascinating events taking place. Think about it when you go out to check out the center of our galaxy when you’re stargazing over the next couple of months.

Learning More about the Home Galaxy

Wow. I miss ONE AAS meeting, and they lose two spiral arms from our galaxy! This is what I get for staying home and working on two big projects with massive deadlines. I did manage to log in and watch the press conferences from the comfort of my overstuffed office, so I caught the gist of what has turned out to be a very cool story.

First, some background. For years, astronomers thought our galaxy probably had four spiral arms. But, it’s hard to tell because we can’t exactly SEE all of our galaxy in visible wavelengths. This is due to all the dust in the galactic plane, and since WE’RE in the galactic plane, it’s like trying to see through a dense fog when it comes to looking in various directions. Isaac Asimov put it best when he wrote, “In a sense, we are on a low roof on the outskirts of the city on a foggy day.”

Well, as it turns out, telescopes with infrared capability can look through that fog and see farther and sharper than visible-light counterparts. So, it makes sense to use such facilities when you want to see what’s out there.

Groups of astronomers from JPL/CalTech and the University of Wisconsin used the infrared-enabled Spitzer Space Telescope to look at the galaxy. Their survey, called the Galactic Legacy Infrared Midplane Extraordinaire (GLIMPSE), spanned 130 degrees in longitude (65 degrees on either side of the center of the galaxy), and 2-4 degrees in latitude. Since Earth is located about halfway out along the plane of a flattened spiral, this survey actually encompasses a large fraction of the volume of the Milky Way. The infrared views (shown below) show where the stars are, as well as the dust clouds in the plane, allowing a more complete star census to be done.

Here’s one of two views of the Milky Way from GLIMPSE, emphasizing wavelengths (3.6 - 8 microns) in the familiar blue-green-red that our eyes see, with the shortest wavelengths displayed in blue and the longest in red. For more technical details, check out the GLIMPSE page at the University of Wisconsin’s team site. They also have downloadable poster-size files of this and another view of the same data.

Now, if you could turn that image so we could see it from the “top down” this “artist’s concept” might be what our Milky Way really looks like. The survey showed, for the first time, that arms we thought were there, really aren’t. The Milky Way’s elegant spiral structure is dominated by just two arms wrapping off the ends of a central bar of stars.

Spitzer’s imagery of the galaxy comprises 800,000 snapshots, and when you piece them all together, they make the clearest view of the Milky Way ever produced. The team has created a poster that they showed at AAS that is 180 feet (289 meters) long and 3.5 feet (2.1 meters) wide. The poster is going on tour soon, apparently showing up at Griffith Observatory (where they have a similar large-scale image of a small part of the Virgo Cluster called “The Big Picture” on display), and then on to various other cities. You, however, can explore it from the comfort and privacy of your home or office by going to http://mipsgal.ipac.caltech.edu/p_map.html where they have a GoogleMaps zoomable map. Check it out!

The Milky Way Loses Mass the Easy Way

So, let’s say you’re an average spiral galaxy doing your thing in the Local Group. You have a hundred billion stars (give or take), some nebulae, a big core (with a black hole or two hidden in the center), and some planets (but you’re not sure how many). But, according to astronomers who have measured you, it seems that you’re much more massive than you thought. Could it be all that dark matter you’ve been carrying around with you since you were a young galaxy? What’s the deal here?

So, you do like everybody else who’s overweight does — you think about going on a diet. Galactically speaking, the only way you’re going to do that is lose mass, which is tough to do. In fact, short of having a near-miss with another galaxy (which would tear away some of your stars and mass), it’s not easy for a middle-aged galaxy to lose weight.

But wait! There’s another way. Maybe, you think, they didn’t weight you correctly. Yeah, that’s the ticket. The scale is off a little…

As a matter of fact, that’s exactly the issue that scientists set out to solve, using the Sloan Digital Sky Survey to measure stars in the galaxy and recalculate its mass based. Basically, they did a more efficient survey which allowed them to come up with a better estimate of the true mass of our galaxy. The difference between the older, fatter Milky Way and the new, improved, skinnier one is a pile of mass equivalent to a trillion Suns.

“The galaxy is slimmer than we thought,” said Xiangxiang Xue of the National Astronomical Observatories of China, who led an international team of researchers on the project. “That means it has less dark matter than previously believed, but also that it was more efficient in converting its original supply of hydrogen and helium into stars.”

How They Did It

The discovery that the Milky Way is slimmer than we thought is based on data from SEGUE, an enormous survey of stars in the Milky Way — one of the three programs that comprise SDSS-II. Using SEGUE measurements of stellar velocities in the outer Milky Way, a region known as the stellar halo, the researchers determined the mass of the galaxy by inferring the amount of gravity required to keep the stars in orbit. Some of that gravity comes from the Milky Way stars themselves, but most of it comes from an extended distribution of invisible dark matter, whose nature is still not fully understood.

The most recent previous studies of the mass of the Milky Way used mixed samples of 50 to 500 objects. They gave implied masses up to two trillion times the mass of the Sun for the total mass of the Galaxy. By contrast, when the SDSS-II measurement within 180,000 light years is corrected to a total mass measurement, it yields a value slightly under one trilliion times the mass of the Sun.

And that’s how the Milky Way got a little slimmer, without the galactic equivalent of a diet. Wish it were so easy for humans!!

Milky Way, Spitzer-style

One of the most fascinating aspects of this year’s meeting (for me anyway) is the continued exploration of the center of the Milky Way. I’m interested because right now I’m working on some material for the Griffith Observatory exhibits that tells people about our home galaxy. Of particular interest is the center of the galaxy, where we know there’s a supermassive black hole. But, it also turns out there is a whole lot of other activity happening there, making the core of the Milky Way one of the great “rediscoveries” of current astronomy.

Today (January 10) Spitzer Space Telescope unveiled a beautiful image of the central 900 light-years of the Milky Way, and the view gives us a peek at throngs of old stars, hot young stars, and clouds of gas that are lit by the glow from the nearby stellar youngsters.

The new stars are a bit of a surprise. For a long time, astronomers assumed that no new stars would form at the galactic center because it’s not a place where you would think the clouds of gas that coalesce into stars could “get it together” to make stars. It turns out that these massive young stars probably formed elsewhere and are spiraling into the center of the galaxy, their orbits warped by the gravitational force of the black hole. And, the image also shows newborn stars and the heavy clouds that give birth to stars, all lying more distant from the black hole.

The beauty of the Spitzer image is that it lets us look through the clouds of dust that hide the core of the Milky Way from our optical telescopes. Infrared light just cuts right through the dust, lifting the veil on the action at the heart of the galaxy.

Click on the link above to read more about the center of our galaxy, and view a larger version of the image above. It’s really quite beautiful!

I’ve been cleaning out my filing cabinets lately, trying to rid myself of the paper and other stuff that accumulates through the mail, meetings, etc. I had two filing cabinets full of old press releases from science institutions. Most of them came my way at American Astronomical Society meetings. I usually register as press when I go (usually because I AM writing something for somebody), and thus these tidbits of research come my way.

One that I ran across that I decided to keep was a story about wide-field images of the center of the Milky Way taken using the Very Large Array radio telescopes. Nowadays it’s an accepted fact that there is a black hole in the center of the Milky Way. The past few years, astronomers have been perfecting techniques to zero in on the culprit and take the measure of its size and mass. You don’t actually see the black hole because — well… it’s black. Or, rather, it’s not allowing any light to escape from itself. But, you CAN see its effects on surrounding material — stars, gas, and dust. They give off emissions for various reasons related to the existence of the black hole — such as heating due to the strong radiation given off by material as it spirals into the hole. And some things here glow due to a whole raft of other causes. Unraveling what’s what in this area is a full-time job for a lot of astronomers!

However, much of this activity is still invisible to us at optical wavelengths (that which we see with our eyes) because the middle of the Milky Way is hidden behind a veil of gas and dust. So, astronomers use radio and infrared and other “probes” to peer behind that veil. All the features you see in the image radiate at a wavelength of 90 centimeters (in the radio portion of the electromagnetic spectrum), and these features are related to the black hole — which is the largest, brightest object, labeled Sgr A. The actual black hole is hidden deep within that blare of light, and it is called Sagittarius A*.

Sag A* map

As you can see, Sgr A is clearly not the only source of emissions in this image. Hot young stars form in this region, and as they do, they heat the gas around them. Eventually, the gas becomes hot enough that it glows, giving away the position of starforming regions. These are labeled Sgr B1 and B2 and part of Sgr D. When these same stars run out of fuel, they explode as supernovae, spreading debris in shock waves, and glowing in emissions given off as high-speed electrons spiral around magnetic fields. There are several supernova remnants in this image — look for the objects with the letters SNR in front of them. This radiation (called synchrotron radiation) may also be what’s causing a collection of sources known as the Galactic center arc, filaments, and threads to glow.

Since I received that press release (in 1999) more research into the goings-on at the center of the Milky Way is zeroing in on details in these structures. If you’re interested in learning more, you can visit the web page where this image is posted: The Galactic Center or there’s a good book out from a guy named Fulvio Melia called “The Black Hole at the Center of our Galaxy” (Princeton University Press, 2003) that takes the reader on an amazing journey to visit this cosmic beast that is the center of so much activity!