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Jan 7, 2009 07:38 PM | 10
LONG BEACH, CALIF.—One of the oddities of the universe revealed over the past decade is that galaxies and the giant black holes at their hubs fit together as if they were made for each other. This is one of those facts of life that sound obvious at first glance, but get stranger the more you think about them.
A giant black hole is a formidable beast, surely able to bend the surrounding swarm of stars to its will. Yet even a giant black hole is still fairly small by the standards of a galaxy, so the galaxy should pay little heed to the monster within. The monster, for its part, is in direct contact with only a fairly small neighborhood and should be oblivious to what happens in the galaxy at large.
Yet astronomers find that black holes consistently have about 0.1 percent of the mass of their galaxies—or, more precisely, of the portion of their galaxies that has an spheroidal shape (which, for an spheroidal galaxy, is the whole thing and, for a spiral galaxy such as our Milky Way, is only the innermost parts). Some astronomers argue that the black hole mass is related not to the mass of the galaxy per se, but to the velocity of the stars, but it amounts to much the same thing: black holes and their host galaxies are blood brothers.
Did the holes come first and guide the formation of their galaxies, did the galaxies come first and build up holes, or did some common factor sculpt both of them? Because galaxies are the building blocks of the universe, this question lies at the heart of many of the puzzles of cosmic evolution.
And the answer? The holes came first, Christopher Carilli of the National Radio Astronomy Observatory and his colleagues announced today at the American Astronomical Society meeting. They used radio telescopes to study four host galaxies bearing black holes from about 12 billion years ago, when the universe was only about a billion years old. By measuring the velocity of gas clouds, they estimated the mass of each galaxy, and by studying the spectral emission lines emitted by material taking a death plunge into the coresponding hole, they estimated the mass of the hole.
These ancient black holes proved to be proportionately much heavier than those in the present-day universe—about 3 percent of the galaxy’s mass. Because black holes never shrink, the galaxy must have beefed up in order to account for the present mass ratio of 0.1 percent. “Black holes came first and somehow—we don’t know how—grew the galaxy around them,” Carilli says.
One caveat is that the study took in only four galaxies, and abnormally massive ones at that. It remains to be seen whether the trend holds for all galaxies at that early stage of cosmic history. Moreover, the study only ascertains which came first. It says nothing about how the black holes originally formed or how, in an apparent case of tail wags dog, they managed to control the formation of entire galaxies. “This doesn’t really address the origin of that relationship,” Carilli says. “What it addresses is when that relationship was established.”
Indeed, the result presents a brand new mystery. Black holes, being rather destructive, could surely stop a galaxy from forming—say, by blasting out radiation or jets of material. As Andrew Fabian of the University of Cambridge discussed in another presentation at the meeting, the formation of a giant black hole should release enough gravitational energy to blast the entire galaxy apart. Yet in the newly discovered cases, the holes appear to be helping their galaxies to form. Maybe black holes belie their monstrous reputation. They might be the great cosmic gardeners, tending the galaxies around them and making some, like ours, hospitable to life.
Update (January 8): Tod Lauer of the National Optical Astronomy Observatory says he worries that the study may have fallen into a statistical trap. Even if the average galaxy is 1,000 times the mass of its hole, there is still a spread around this average -- some galaxies are smaller or bigger. Yet smaller galaxies are inherently more common than bigger ones, so a black hole of given mass is more likely to be found in a smallish galaxy and hence appear oversized. For this reason, the galaxy-hole pairings seen by Carilli's team might not be representative of the ancient universe, in which case the question of which came first remains unanswered.
Radio telescope image of gas in a young galaxy courtesy of NRAO/AUI/NSF, SDSS
Tags:
big bang cosmology,
black holes,
astrophysics,
galaxy birth,
history of the universe
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10 Comments
Add CommentThis doesn't seem to me to be as big a mystery as it seems. I'm a solid state physicist, mind you, and not a cosmologist, but if we understand how stars attract planets into orbits to form solar systems then why is it so hard to say that black holes similarly attract matter in space and form galaxies? Seems like a case of stellar dust bunnies to me. Is there any reason to believe this is not how they came to be?
Reply | Report Abuse | Link to thisi agree, before i read the article this was the first though to come to mind, and i am a lowly cannoneer, so being that youre all educated its not much of a stretch of the imagination...(chicken or the egg?)
Reply | Report Abuse | Link to thisSorry to have failed you -- the last thing you should come away with is that this result is so obvious. Check out the longer article we had on this topic: www.sciam.com/article.cfm?id=the-galactic-odd-couple -- I'll see whether we can make it available online gratis.
Reply | Report Abuse | Link to thisIf there is a black hole at the galactic centre then are stars spiraling into it and thus forming the shape of our galaxy. Sorry if it appears an elementary question but I am a newbie.
Reply | Report Abuse | Link to thisNo, stars do not spiral into black holes. They orbit black holes like another mass concentration. For example if you replaced the sun with a black hole of its same mass, the planets would continue to orbit just as they have always done for billions of years.
Reply | Report Abuse | Link to thiswery easy, blackholes was first. BH made all materia in our universum. In a fact BH are spliting antimateria and materia and made a two universe. This what we see, and a antimateria universe. BH are in a middle every galaks. Stars are product of BH, materia on our side and antimateria in a next universe. The universe is bipolar. But where is universe of antimateria?
Reply | Report Abuse | Link to thiswery easy, blackholes was first. BH made all materia in our universum. In a fact BH are spliting antimateria and materia and made a two universe. This what we see, and a antimateria universe. BH are in a middle every galaks. Stars are product of BH, materia on our side and antimateria in a next universe. The universe is bipolar. But where is universe of antimateria?
Reply | Report Abuse | Link to thisblack holes are realy quite simple they are no more than an area of space that has accumlated enough gravity to be strong enough to hold every thing that comes into is power to capture... high school kids know that the more matter you have in one spot the stronger the pull of gravety will be ...at some point nothing we know can escape ...not even light... presto! we have what we call a black hole...there are many theroies about black holes that means little or nothing as the blackhole will always be just an accumilation of matter and increased gravity ...
Reply | Report Abuse | Link to thisblack holes are just a place in space that has accumilated enough matter and gravity and is dense enough and strong enough to hold light in its grasp ...any high school student knows that the more matter in one place the stroger the force of gravity will be...
Reply | Report Abuse | Link to thisAh, but when two Black holes collide, they touch at a "zero gravity" point the attractive force at the event horizons is offset by the same force from the second anomoly. This allows the matter at that contact point to take off due to its own inertia, spewing matter out into a skirt plane, (see Homunculus Nebula).
Reply | Report Abuse | Link to this.So, if two half galaxy sized black holes that have coursed through space for billions of years should attract to each other, an emission of matter in a flat rotating circular disc will result. In time, the matter will clump and form "arms". The spewing goes on in the center until one of the objects can no longer offer offsetting force.
.The same happens on a stellar scale. With the billions of stars Carl Sagan taught us about you just know that a fair (1-5%) are massive enough to collapse to a point where density causes a singularity to form. Don't fall into the BH is purely a gravity quirk. When the star that collapses into one creates no new mass, so the g well due to the mass alone should be the same. It is the density factor which increases the local g. As stated above, the Earth would continue to circle the sun if it were replaced by a same mass BH.