Sunday, September 28, 2014
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.