Gas guzzlers: when great galaxies gobble gobs of matter, new stars are born

Young galaxies grow by making lots of new stars. Those stars begin to form when diffuse gas collects into clouds; eventually the clouds collapse under their own weight, and their cores become hot and dense enough to ignite nuclear fusion. So if you spot a galaxy whose mass is mostly interstellar gas, it's probably still young and still growing rapidly; in billions of years, as the galaxy passes through adolescence and into maturity, it will consume nearby gas clouds while its star population proliferates.

By that benchmark, our own galaxy is an old-timer. The Milky Way is known, on independent grounds, to be more than 10 billion years old, and the mass of its interstellar gas is only about a hundredth that of its stars. It could still grow by eating up its neighbors--the Sagittarius dwarf galaxy, for instance [see "Warp Factor," by Charles Liu, April 2003]--thus adding ready-made bunches of stars to its own stellar population. Or, if the galaxy being eaten also contains a large amount of gas, a new burst of star formation could take place.

Such episodes have happened in the past and may happen again soon: a vast river of gas is streaming toward the Milky Way from two smaller, gas-rich galaxies called the Magellanic Clouds, perhaps foreshadowing their eventual consumption by the Milky Way.

Astronomers have long wondered, however, whether our galaxy is also still forming stars the old-fashioned way, by gathering clouds from diffuse gas. In the past several decades we've detected hundreds of big, cold gas clouds moving rapidly all over the sky. Some astronomers have theorized that many of those clouds are made of interstellar gas left over from the Milky Way's youth--gas now in orbit about our galactic environs and slowly being sucked in. But it's been impossible to tell whether the clouds are orbiting the Milky Way, or simply occupying interstellar space at large, random distances from the galaxy--and thus not likely ever to fall in and form stars.

Now a team of astronomers led by David A. Thilker of the Johns Hopkins University in Baltimore has reported an important new result that addresses that question, albeit indirectly. The team has found a system of large, starless interstellar gas clouds orbiting a nearby galaxy: Andromeda.