During the past decade astronomers looking deep into space with supersensitive electronic detectors have found millions of faint blue galaxies at distances exceeding 4 billion light-years. These galaxies are about five times more numerous for a given volume of space than galaxies in our own cosmic neighborhood. That’s strange: Why should there be more faraway galaxies than nearby ones? Astronomers have concocted a number of unwieldy theories to explain the discrepancy. Lately, though, it has begun to seem that the solution may be quite simple. There appear to be more far- off galaxies only because astronomers have been missing most of the nearby ones--the ones that contain so few stars that they are intrinsically dim.
Low-surface-brightness galaxies, as astronomers call them (dim being too simple a word), are not new. The first ones were discovered 30 years ago. But it is only recently that they have begun popping up on photographic plates in large numbers, and only recently that they have been revealed as a whole new class of galaxy whose members actually outnumber the more familiar galaxies we see in books and posters. These galaxies are hard enough just to take an image of, let alone try to get the kind of detailed information you want, says University of Arizona astronomer Chris Impey.
In a recent photographic search of two small strips of sky, Impey and Greg Bothun of the University of Oregon turned up 516 LSB galaxies. That would project to 25,000 over the whole sky if we did the same kind of survey, says Impey. Most of the galaxies they discovered are within 300 million light-years of Earth; most LSB galaxies farther away would be too dim to show up in their photographs. The number of dim galaxies they observed is roughly equivalent to the number of normal galaxies in the same region of space. If you think of our Milky Way galaxy as an isolated farmhouse, then Impey and Bothun have found evidence of 25,000 new farms within a 20-mile radius of our own.
Like normal galaxies, LSB galaxies come in a wide variety of shapes. Some are spirals like the Milky Way; others are elliptical or irregular in shape. But the ones Impey and Bothun observed are all much dimmer than the Milky Way, and many are also larger. The two things are connected. LSBs are essentially enormous disks of hydrogen gas that are massive enough to outweigh normal galaxies but too diffuse to form stars in large numbers. They form stars only in a few isolated pockets, where the gas is dense enough to collapse under its own gravity.
With so few stars, LSB galaxies are typically 5 to 20 times dimmer than normal galaxies of comparable mass. The LSB galaxy called Malin 1 is a good example. Discovered by Impey and his colleagues in 1986, it is the most massive spiral galaxy known, about 20 times more massive than the Milky Way. Yet it is also 10 times greater in diameter than the Milky Way and 100 times more diffuse. Thus, at a distance of 700 million light-years- -not very far on a cosmic scale--it is barely observable through the background glow of stars in our own galaxy.
Why are Malin 1 and all the other LSBs so poor in stars? Some astronomers have suggested that the dim galaxies may be the faded, burnt- out remains of normal galaxies that were once bright. But Impey and Bothun don’t buy that idea. They point out that the few stars that are found in LSBs are mostly hot and blue, indicating that they are young and suggesting that these dim galaxies were probably even dimmer in the past and are just now getting around to forming stars.
Before small gas clouds can collapse to form stars inside a galaxy, the galaxy itself has to collapse from a giant, roughly spherical gas cloud into a spinning disk. Impey and Bothun think LSB galaxies like Malin 1 were born of giant gas clouds that just happened to be much more diffuse than the clouds that gave rise to normal galaxies. Whereas normal galaxies like the Milky Way collapsed soon after the Big Bang, between 10 and 15 billion years ago, it would have taken a lot longer for gravity to collapse the diffuse progenitors of LSBs--which would explain why they are still dim and relatively starless today. They might have collapsed only a few billion years ago, so they’d be relatively unevolved, says Bothun.
The same scenario may hold for the distant faint galaxies that astronomers discovered before they saw LSBs in large numbers. The distant galaxies have the same mass as nearby LSBs, the same color--they are blue, presumably because they contain young stars--and the same intrinsic brightness. The two classes of celestial object seem to be related, if not one and the same.
If so, then why were the faraway ones discovered first, at least in large numbers? The answer lies in the different detectors astronomers use for different purposes. The distant blue galaxies were discovered in the 1980s with telescopes connected to electronic detectors called charge- coupled devices, or CCDs. CCDs capture light about 70 times more efficiently than the best film, but they have a very small field of view-- they can image only about a millionth of the sky at a shot. That piece of sky is like a piece of pie pointed at the telescope: it includes a much bigger volume of space--and many more galaxies--at a distance of 4 billion light-years than at 100 million light-years. It’s easier to conduct a galaxy survey for more distant galaxies, explains Bothun. A deep CCD exposure may contain hundreds of distant galaxies but only one nearby galaxy.
For their local survey, Impey and Bothun used regular photographic film, albeit of a particularly sensitive kind, in a camera with a much wider field of view than a CCD can muster. That allowed them to see a lot of dim nearby galaxies--but only large dim nearby galaxies. Until we have a CCD survey of the whole sky, which is not going to happen anytime soon, we really can’t be confident that we have a good sample of nearby galaxies, says Bothun.
Indeed, astronomers are almost certainly still missing a lot of the action in their own neighborhood. Not all LSBs are large: there are also dwarf LSBs, similar in mass to the Magellanic Clouds (two satellites of the Milky Way) but so dim that even a CCD can detect them only out to about 100 million light-years. Very little is known about these dim dwarfs. Yet in recent years astronomers have found ten of them close to the Milky Way. If that sample is representative of the whole universe, then the majority of all galaxies anywhere are dwarf LSBs--and astronomers have only just begun to catalog them.