Bubbly Burp Tracks Giant Black Hole's Last Meal

Observations with a number of telescopes, including the Hubble, have now dated a kind of burp in the eating habits of the giant black hole at the center of our Milky Way Galaxy. It appears that about six million years ago, the central black hole "ate" a large cluster of stars and the neighborhood around it was energized by the process of the meal.

Black holes are regions where material (starting with dead stars) has collapsed so much, that nothing, not even light, can get out. The black hole at the center of our Milky Way now includes enough material to make more than 4 million stars like the Sun! It's what we call a "supermassive black hole" and lurks in the middle of our Galaxy like a giant speed trap for unwary stars or star groups that get too close.

As material is in the process of being "swallowed" by the black hole, it glows with desperate radiation, just before it falls in and disappears from view. A great bubble produced by the black hole as it ate its last serious meal has now been tracked by astronomers with much greater precision.

Just like the sound of the burp your uncle makes (after a heavy Thanksgiving meal) can travel through the dining room, so the bubble from this last meal can be seen traveling through the Galaxy like a giant expanding shell. The shell was probe by the Hubble as the light of distant objects raced through it and astronomers were able to measure the speed of the bubble's motion.

Six million or so years ago, it appears that a large clump of stars or gas (the raw materials of stars) was consumed by the black hole, making two "Fermi bubbles" in the Galaxy. Since then, the black hole has only been "snacking" -- tearing apart and eating an occasional star or random bit of gas. But no serious meal has made a big bubble from the mouth of the black hole since then.

Our diagram shows how in the six million years since that meal, the bubble has expanded at speeds of two million miles per hour and made a giant bubbles north and south of the black holes that extend for tens of thousands of light years. That kind of puts your uncle's last burp into perspective!

Two Giant Black Holes Whirl at the Core of an Active Galaxy

Using the Hubble Space Telescope, a team of astronomers has found that a superbright (active) galaxy is powered at its core by two supermassive black holes whirling around each other in only a bit more than one year. This remarkable galaxy is called Markarian 231, after the Armenian astronomer who made a catalog of such active galaxies (with unusually bright centers.)

The galaxy is almost 600 million lightyears away, so we cannot see the tiny area in the center which contains the black holes directly. But a detailed study of the ultraviolet light from the core of the galaxy strongly implies that a black hole containing enough material to make 4 million Suns is whirling around a much larger black hole (with mass inside for 150 million Suns.)

Think about those numbers! You would not want to live near such overweight black holes, but luckily they are confined to the central regions of galaxies (including our own Milky Way) and are not a feature of the galactic outskirts where our solar system resides in comparative peace.

The idea that enormous black holes like this can share the same environment is not news to astronomers, but it's nice to have such a clear example. We now know that the giant islands of stars called galaxies probably all started much smaller and have been growing through "mergers" (if you'll pardon our appropriating a word from the world of business.)

Smaller galaxies are "eaten" by larger ones, or two galaxies of equal size are attracted together by their mutual gravity. If the smaller galaxies each contain a big black hole, both black holes will wind up near the center of the merged object. If the original galaxies had orbiting motion around each other, their inner black holes will have some of that motion, and can circle each other until -- later -- their gravity also pulls them together.

The fact that the two black holes in Markarian 231 take only about one year to go around means they will collide in a few hundred thousand years (a long time compared to the presidential nomination season, but short for galaxies.)

When two black holes collide you get -- surprise, surprise -- a bigger black hole. But we have caught Markarian 231 in the act of a small galaxy having been swallowed, but before the two black holes had time to merge. There are stars and huge clouds of gas and dust being pulled in by the pair of black holes and as they are torn apart and whirled around, they give off a lot of energy.

It's that energy of doomed material (before it falls into one or the other black hole) that makes Markarian 231's center so unusually bright.

The method used to find the waltzing, whirling black holes in this galaxy holds promise for finding other pairs of giant black holes in other distant galaxies. And the existence of such pairs of hungry black holes is good evidence that our merger theory of how galaxies "bulk up" is correct.

(By the way, our image, above, is a painting, based on the Hubble data.  As we said, we can't take a picture of the inner part of the galaxy.  But below is a Hubble image of the entire disturbed galaxy with its bright center.)

A Monster Misses a Meal

There is a monster at the heart of our Milky Way Galaxy.  It's a super-massive black hole that has already eaten enough material to make 4 million stars like our Sun!  And, like all black holes, it is still hungry.  

For material to be "eaten" by a black hole, it must come quite close to the black hole "mouth" which astronomers call its event horizon.  Because black holes are the most compressed (squozen) objects in the universe, even overfed monster black holes have relatively small event horizons. Astronomers estimate that the one at the center of our Galaxy is 80 to 100 million miles across.  That's roughly the distance between the Earth and the Sun and is a tiny space in which to put 4 million(!) Suns.

So material near our black holes must come close to that tiny region to serve as food for the monster.  Things further away, like stars, can orbit around the black hole and not get swallowed.  (It is from the movement of such close, but not doomed, stars that we can estimate the gravity of the black hole.)

For the last few years, astronomers who monitor the center of our Galaxy have been predicting that a snack is on its way to the black hole.  A dusty cloud of material which they have nicknamed G2 was going to have a close encounter of the worst kind with the black hole in May 2014.  It was going to be torn apart by the enormous gravity of the monster and some of its material was then going to provide a meal for the black hole.  

When gas clouds (or other food) fall into a black hole, they are whirled around with unbelievable speed just before they fall in, and tend to glow briefly with x-rays and other forms of radiation before they disappear in the event horizon. However, no such flare-up of radiation was seen, even when the world's largest telescopes (like the Keck in Hawaii and the European Very Large Telescope in Chile). 

It appears G2 was not torn apart and consumed, because it wasn't a loose cloud of raw material, but a star with some of its birth material still around it.   The star managed to hold on to its "stuff" and make it away from the black hole, depriving it of a meal at this time.  Sorry, monster.   Better luck next time.

(In the picture, you see G2 in different colors going around the black hole (which is invisible, but whose position is marked by the plus sign.)  The blobs are shown at different times, and are red when G2 was moving away from us, and blue now that it was flung around the black hole and is coming toward us.)

A Baby Galaxy with a Grown-up Black Hole Inside

Using the Hubble and Gemini telescopes, astronomers have found a mystery --  a tiny galaxy that has a huge black hole in its center. That hungry black hole has eaten enough material to make 20 millions Suns!

The baby galaxy is really small -- its diameter is only 300 lightyears.  It's crowded in that little space; it contains about 140 million stars.  (Compare it with our Milky Way Galaxy, which stretches over 100,000 lightyears, and contains at least 200 billion stars.  Yet our central black hole has eaten only about 4 millions sun's worth of material.)  How could such a baby galaxy have such a big black hole?

Astronomers are no longer surprised to discover giant black holes at the centers of most galaxies.  Where a galaxy is most crowded (in its middle) is where a black hole (a star corpse with enormous gravity) has the most "food" to eat and can therefore grow.  But, in general, we have found that the larger a galaxy, the larger the monster black hole at its center.  So finding a baby galaxy sporting such a big black hole comes as a huge surprise.

A clue to this mystery comes from the name of the baby galaxy -- its awkward designation is "M60-UCD1."  UCD stands for ultra-compact dwarf (galaxy), which makes sense.   But M60 refers to the 60th entry in Charles Messier's catalog of fuzzy sky objects published in the 1780's.  That Messier catalog features some of the brightest and easiest to see galaxies and nebulae.   There is no way a tiny faint baby galaxy would have made his list!

It turns out that our baby galaxy is orbiting the much larger and brighter galaxy called M60.  In our picture, you see M60, a huge, blimp-shaped "grown-up" galaxy, which has its own super-massive black hole at the center.  You can see our baby galaxy in the inset of the photo. (You may need to click on the image to see it well.)

The fact that our baby galaxy is a "satellite" of the big galaxy may explain the mystery of its small size and big black hole.  The discoverers suggest that in the distant past, our baby was actually a big galaxy, with many more stars (explaining how it got its big black hole.)  But it had a "close encounter" with M60.  The gravity of the big galaxy stripped away its outer stars, leaving the "victim" of this encounter much smaller. 

If M60 took away and absorbed the outer layers of its neighbor, that would make M60 a cosmic cannibal.  That sounds awful, but in recent years astronomers have begun to realize that just about every big galaxy has grown to its present size by cannibalizing some of its smaller neighbors.  

It's a dog-eat-dog world out there among the galaxies, and the big bullies really get to throw their weight around.  Our little galaxy was once a more regular member of the galaxy club, but it lived in a rough neighborhood and got really beaten up by the local gravity bully.  Now it's a mere shadow of its former self.  

A Cluster of Stars Thrown Out of Its Galaxy at 2 Million MPH

A team of astronomers has discovered the first cluster of stars that has been thrown out of another galaxy. Whatever threw this rich grouping of stars out of the galaxy known by its catalog number as M87, was able to accelerate it to a speed of some 2 million miles per hour. Kids, don't try this at home without adult supervision!

It just so happens that the star cluster is being thrown roughly in our direction, but since M87 is 54 million light years away, no one is worried. The cluster will just wind up in the space between galaxies, wandering like the lost ships of legend, never finding its home port again.

M87 is a huge galaxy of stars and clusters, almost a million light years in diameter (our Milky Way Galaxy is about 100,000 light years across, by comparison.) In addition to thousands of billions of stars, M87 contains an estimated 12,000 "globular clusters" -- tightly bound groups of roughly 100,000 stars each. Our home galaxy only has about 150 of these globular clusters, and we are a pretty good-sized galaxy as far as cosmic requirements are concerned. So M87 makes us look like a 90-lb weakling in comparison.

So how did a cluster with many thousands of stars get loose from the considerable gravity of a giant galaxy like M87? No one knows for sure, but here is the clever idea that discoverers of the high-speed cluster are suggesting. Giant galaxies like M87 get bigger by eating smaller neighbor galaxies for lunch. Occasionally, they even merge with a big galaxy, gently pulling in the other galaxy's stars and other "inhabitants." What if M87, in the distant past, swallowed a big galaxy with a giant black hole at the center?

Astronomer have recently found that all big galaxies have big black holes in their crowded cores. The bigger the galaxy, the bigger the central black hole, in general. So M87 probably had a big black hole and the other galaxy would have had a big black hole too. As the two galaxies merged, their two black holes, like boxers circling each other in the ring, could have begun to orbit around each other. (We have seen pairs of big black holes in other such systems, so this is not a wild idea at all.)

Now, along comes our victim globular cluster, which had some orbit around the center of M87 that might well have brought it a bit too close to the pair of black holes. When the black holes interacted with the cluster, their gravity might have played a "game of pool" with it. In pool (or billiards, for some of you), you often see one ball interacting with another and then causing a third ball to go shooting off into a distant pocket. When the two black holes and the cluster had their moment of gravitational interaction, the cluster could have been thrown out of the galaxy by the tremendous gravitational energy of the giant black holes.

Astronomers have seen superfast stars thrown out of galaxies, but this is our first instance of seeing a whole cluster of stars shooting out from its home galaxy. M87 is one of my favorite galaxies anyway, with many other signs of violence and inner turmoil. This makes it even more interesting. The accompanying image is an artist's attempt to show the cluster coming out of M87, which is correctly shown as a fuzzy rounded blob.

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On a personal note, I would like to dedicate today's post to the memory of my good friend and colleague, Alan Friedman, the former director of the New York Hall of Science, who passed away this weekend.  Alan and I taught weekend courses on Einstein together, wrote papers on interdisciplinary approaches to teaching astronomy and physics, and bemoaned the state of science education in the U.S. over many dinners.  Alan also helped chart the course of the Lawrence Hall of Science in Berkeley, the Parc de la Villette science museum outside of Paris, and dozens of other museums and science centers where his advice was frequently sought.  He leaves an emptiness in the universe that will be impossible to fill.

Talk on the Work of and Crisis at Lick Observatory Now on YouTube

As you may have heard, the Lick Observatory (on whose Council I am now proud to serve) is being threatened by budget cuts at the University of California. Recently, Dr. Alex Filippenko (who was named the best professor at Berkeley a record nine times, and is a wonderful speaker) gave a very exciting talk on: "Exploding Stars, New Planets, Black Holes, and the Crisis at Lick Observatory". The video is now available on YouTube at
https://www.youtube.com/watch?v=JEY2pzxda1w

Lick Observatory, the first mountain-top telescope facility in the world, was founded in 1888, but continues to be a vibrant research facility and an important site for student and public education. Dr. Filippenko, who chairs the Lick Observatory Council, discusses some of the most exciting research being pursued at Lick, but also explains the funding crisis, what is being done by local citizens, and how you can help.

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Today's image is a snapshot I took at Lick, showing Dr. Filippenko (left) with noted venture capitalist Jim Katzman, standing in front of the Katzman Automated Imaging Telescope. It was with the use of this telescope that astronomers discovered a number of special exploding stars in other galaxies that could be used as distance markers for the universe. That work led to the discovery of the accelerating expansion of the cosmos and won the two groups involved the 2011 Nobel Prize in Physics. We can't let them shut this facility down, folks!

Black Hole at Center of the Milky Way is Found to be Cooking its Dinner

Astronomers working with the European Herschel Space Observatory have discovered really hot gas in the vicinity of the monster black hole at the center of our Galaxy. Over the years, many lines of evidence have shown us that there is a black hole with enough material to make 4 million Suns at the heart of the Milky Way. The new observations, made using infrared (or heat) rays, show that gases such as water vapor and carbon monoxide have been heated to about 1000 degrees Centigrade within a lightyear of the black hole.

While energy from nearby stars may also be heating this inner gas, the astronomers can't account for so much heat from stars alone. They think that great streamers of gas heading toward the black hole may be colliding and the shock waves from the collisions may be significant contributors to the heating. Some of the streamers of gas will someday be "eaten" by the black hole. In other words, like many a hungry diner, the black hole appears to be "cooking" its dinner in anticipation of eating it.

In fact, other observations have recently shown a cloud of gas weighing as much as several Earths, falling to its doom much closer to the black hole. This cloud may be consumed by the black hole as soon as the end of 2013. When such clouds actually spiral inward to their doom, they heat up a lot at the end. The last thing we observe from them before they fall into the black hole (and are no longer visible) is a "burp" of x-rays. Several x-ray telescopes in space are prepared to record such burps when they happen.

If you are cooking a barbecue this Memorial Day Weekend (a holiday in the U.S.), you can enjoy the idea that some serious cooking may also be going on at the center of our Galaxy. The center region is 26,000 lightyears away from us, so none of this poses the least danger to planet Earth and its cooks.

(By the way, to see one of the lines of evidence for the existence of the monster black hole, we recommend a great new movie made from observations by astronomer Andrea Ghez' group at UCLA. The movie shows the whirling orbits of stars very close to the black hole, being pulled around by the enormous gravity of the black hole. Check it out at:

http://www.astro.ucla.edu/~ghezgroup/gc/images/media/ghezGC_comp3-18_H264_864.mov

Note that each second of the movie shows two years of star motion. It's enough to make you dizzy.)