Blogs / Bad Astronomy

I forgot to bring my camera cable with me to the press room yesterday, so I wasn’t able to download pictures from camera until today (&^#%$&$ proprietary cables!). But I grabbed a few and uploaded them to my Flickr account, where they are in the AAS 2008 set. As of right now there are only a few in there, but I’ll add more as they come in.

Here’s a preview:

That’s me wearing the astronaut glove John Grunsfeld brought to his press conference on Tuesday. I was actually miming picking my nose, but maybe this is better for delicate appetites.

That’s me, Pamela, Fraser, and Rebecca (Astronomy Casters) at the blogger meetup, which was a HUGE success. I think upwards of a hundred folks were there at different times. We met lots of people from the area, including BAUT moderator Tinaa (Hi Tina!), BAUTer Neverfly, Cross Country, and Captain K, and lots of other great people. Thanks especially to George for sponsoring the event!

Are there massive black holes screaming around just outside our Galaxy?

A new study says: Maybe.

Globular clusters are roughly spherical clusters of stars, some with populations numbering in the millions. They’re old, as old as the Galaxy, which means they’ve been around billions of years. There are no young stars in globular clusters, which means there are no massive stars, either: massive stars blow up after only a few million years.

But when these stars die, they can form black holes, which are abundant in globulars… and stars are so packed together in these clusters that they can interact gravitationally. These interactions can happen over great distances; enough that the star doesn’t get eaten, but enough that the path of the star can be affected by the hole; and vice-versa too.

When that happens, the more massive of the objects sinks to the center of the cluster, while the less massive object moves outwards. Over billions of years, this means the black holes tend to hang out in the center of the cluster, while normal stars tend to populate the suburbs. Eventually, the black holes crowded into the center will interact with each other. Most of these black holes will start out with masses close to that of the Sun (called stellar mass black holes) but as they merge they can grow in size to hundreds of times the Sun’s mass (called intermediate mass black hole).

But, it so happens, when they collide and eat each other, bad things can happen.

When black holes merge, they can actually distort the fabric of space/time, like a rock thrown into a pond ripples the surface. They emit a huge amount of energy in the form of these ripples (called gravitational radiation). Usually, this radiation is emitted in all directions, but sometimes it can be sent out more to one side than the other. When that happens, it acts like a rocket, pushing the black hole in the other direction. It is possible, if conditions are right, to accelerate a black hole to the incredible speed of 4000 kilometers per second (2500 miles per second)!

Stop for a moment, and think on the forces that can move an object that outweighs the Sun to a speed of well over ten million kilometers an hour. It makes the hairs on the back of my neck stand up.

Anything moving at that kind of speed will blow right out of the cluster; there isn’t nearly enough gravity in the cluster to hold on to such a stellar bullet.

This new study on how globular cluster black holes merge and what happens when they do shows that of the 150 known globulars surrounding or Galaxy, well over 100 may have blasted black holes from their cores, shooting them out into the Universe at large. There is a thin halo of stars (and dark matter, but that’s neither here nor there for this work) surrounding the Milky Way, called its halo. And now we might be able to add another citizen to its census: a few dozen or even as many as one hundred black holes, moving at phenomenal speed, silently cruising the intergalactic depths. We’re in no real danger from these Great Black Sharks, since they are so few in number and space is so mind-bogglingly vast. But to me, there’s something appealing in knowing they’re out there… especially since it makes it easier to write the black hole chapter of my book.

Centaurus A is a nearby galaxy which is bit of mess. Actually, it’s an incredible mess. 12 million light years away, it looks at first like an elliptical galaxy, an unassuming football of billions of stars. A closer look reveals a dark dust lane across the middle, which is our first hint things are amiss: ellipticals tend not to have very much dust, but Cen A has it in bucketloads (as you can see in the picture on the left, courtesy Chandra and AURA/NOAO/NSF).

Cen A has been known to harbor a central supermassive black holes — all big galaxies, including our own, have one. But Cen A’s is active: it is currently feeding off gas and dust, gobbling down huge amounts of matter. This material forms a gigantic flattened disk as it swirls around the hole. This disk is under the influence of a witch’s brew of forces magnetic, gravitational, and even mechanical: friction heats the disk fiercely, and it gets so hot it glows in visible, ultraviolet, and even X-rays.

It’s not clear exactly how these forces interact, but they can combine to focus twin beams of matter and energy, titanic jets that scream outwards from the disk. The jets come from the part of the disk just before the matter takes the Final Plunge, before it falls into the black hole.

The Chandra X-ray Observatory has taken deep images of Cen A, revealing new details about the jets emanating from its central black hole. The jet to the upper left is aimed more or less at us (never fear, it’s way too far away to hurt us), and you can see the counterjet as a shorter stubbier line of light to the lower right. The jets may be equal in size and intensity, but weird effects (due to Einsteinian relativity) make the one aimed away from us appear to be dimmer.

A closer-in view of the jet shows that it is plagued with knots, clumps of emission. These are probably due to shock waves, when faster material is ejected from the vicinity of the black hole and slams into slower moving material. This dumps vast amounts of energy into the material, reheating and re-accelerating it outwards. The magnetic fields of the material keeps it focused, and the shock wave re-energizing keeps it moving up, up, and away from the black hole. These jets can be many thousands of light years long, so the energies involved are fantastic, almost incomprehensible.

But, in fact, there are comprehensible. Jets like these are incredible, but we are beginning to understand them. Observations like this one from Chandra are aiding astronomers’ understanding of how matter behaves when it’s On The Brink.

Look out in the sky today, and you’ll see only a few kinds of galaxies. Most are elliptical, fuzzy round blobs containing billions of stars. Others are spirals, of course, flattened disks with magnificent spiral arms. Others are irregular in shape, and the fourth kind are peculiar; they have a definite shape (rings, for example) which are due to collisions with other galaxies.

When two galaxies merge — physically collide like two trucks on the highway — they might start out as nice pretty spirals, then distort into peculiars midway through the merger, then finally, after billions of years, they settle into elliptical shapes as gravity smooths out the distribution of stars. Detailed calculations of how galaxies interact gravitational have supported this idea, as well as observations of countless galaxies.

But we don’t know enough about how all this works. We need more data, and more detailed data. Happily, when astronomers desire, Hubble and Spitzer provides. The Galaxy Evolution and Morphology (GEMS, which is easier to remember) survey was undertaken a few years ago using the Hubble Advanced Camera for Surveys and the Spitzer (infrared) Space Telescope. GEMS was an international effort to look at the galactic merger history over the last 7 billion years.

They looked at 5000 galaxies that appear to be merging systems, examining the overall shapes of the galaxies, as well as the infrared light that points out where new stars are forming — colliding galaxies slam their gas and dust clouds together, which can cause a burst of star formation. Bright young stars heat up the dust around them, causing these regions to shine in infrared.

The survey found that when the Universe was young — a couple of billion years old — 40% of the massive galaxies were actively merging. However, in more recent times — in the last 7 billion years — that drops to only 10%. That’s not too surprising, since the Universe has been expanding, and the chances of collision drop as the space between them increases. Also, as more galaxies collide, there are fewer left to collide, so the numbers of interactions drops.

The big surprise, however, is that the star formation in these interacting galaxies isn’t as vigorous as thought. In other words, in recent times, collisions don’t seem to overly enhance bursts of star formation. There is an enhancement, it’s just not as strong as it should be — at least, as strong as we thought it should be. Only 20% of cosmic star formation is coming from interacting galaxies — the rest is coming from normal galaxies like the Milky Way.

There have been other surprising results too. Most spirals have a central bulge, a big round component in the middle of the galaxy — a downtown region if you will. Disk galaxies with no bulges should be rare according to prediction. However, 20% of modern galaxies have no bulge, and the same is true even a long time ago in the Universe. This means we don’t understand everything that goes into how galaxies form, how they interact, how the stars and gas in galaxies "talk" to each as the galaxies merge. According to theory, bulges should be a natural result of how spirals form, depending on the amounts of gas and dust are ion the galaxies before they merge, and how this all comes together. But since bulges don’t actually crop up as much as we predicted, it means we need to observe more and figure out why the Universe isn’t behaving the way we expect.

These surveys will continue to investigate the Universe around us, and tell us more about the neighborhood in which we live. Sometimes this may seem pretty far removed from our everyday lives, but don’t forget: it’s because of all this that you have an everyday life to begin with.

If you want to understand what’s going on in the sky, there are lots of ways to do it. You can, for example, look around for specific examples of objects — supernovae, black holes, spiral galaxies — and examine them. Or, you can take a survey of the sky, looking everywhere, and take a census of objects. That gives you a pretty good idea of how many objects are out there, and how many of each kind.

Surveys of the sky tend to revolutionize our thinking about astronomy. When you get large samples of vast stretches of sky, you get a feel for what’s going on, and sometimes that overall view can be very powerful.

The UK Infrared Telescope just released a new survey, the deepest and largest infrared survey of the sky ever made. IR light is tremendously useful in astronomy; it can pas through gas and dust pretty well, revealing objects otherwise hidden under a thick blanket of obscuring material.

The UKIRT Infrared Deep Sky Survey was started back in 2005, and looks at the near infrared, the light just outside the eye’s sensitivity. It does so with exquisite sensitivity and depth; producing a wealth of data as well as beautiful imagery. They have found the coolest (literally!) brown dwarf yet — a star that is too small to sustain fusion in its core, dooming it to slowly cool with time as its internal heat is released.

Besides revealing previously unknown objects, surveys can give us details of familiar ones as well, like in the Ring Nebula picture at the top of this post. That’s one of the most famous objects in all the sky, a dying star shedding layers of gas which light up as they collide and are heated by the central star. It’s visible in small telescopes and has been photographed countless times. But a deep, wide survey unveils the far outer halo of the nebula, gas flung out long ago in the earliest stages of the star’s death. This will tell astronomers details about what happens just at the point when stars like the Sun begin to shuffle off their mortal coil.

And the UKIRT survey isn’t even done! When it’s complete, in 2012, they will have detected 100 million galaxies, which is phenomenal (imagine having to catalog all of them…), and revealed many hidden treasures in our own. Surveys like this are extremely powerful tools in an astronomer’s kit, and will be mined for data for decades to come.