A galactic vampire - The Milky Way is not as young as it looks (The Economist, 21st Feb)

AS EVERY horror fan knows, the secret of eternal youth is to suck the lifeblood of others. If you are a galaxy, that lifeblood is hydrogen gas, from which stars form. And it seems that some galaxies are indeed able to maintain a youthful appearance by sucking great clouds of the stuff in from intergalactic space.
According to Felix Lockman of America’s National Radio Astronomy Observatory, the Milky Way, humanity’s home galaxy, is one such vampire. Dr Lockman is part of a team that has been using the Green Bank radio telescope in West Virginia, which boasts the world’s largest steerable dish, to study the process.

The mystery to be solved is why, of the visible matter in the Milky Way, around 15% is still in the form of gas and dust. The galaxy is over 13 billion years of age, almost as old as the universe itself. In theory, that is plenty of time for all of its gas to have condensed into stars, as has happened in a class of galaxies known as ellipticals. But the Milky Way is a spiral galaxy, in which active star-formation continues at the rate of about one newborn a year. This must mean that the galactic supply of hydrogen is somehow being replenished. And Dr Lockman and his colleagues think they know how.

In the 1960s a strange intergalactic gas cloud was discovered near the Milky Way by an astronomer called Gail Smith. Smith’s cloud, as it is known, is an elongated structure almost 10,000 light-years from end to end. At roughly a tenth of the diameter of the Milky Way’s disc, that is big even by galactic standards. Smith’s cloud was a puzzle. No other such object had ever been seen. It was therefore put aside by astronomers, for it is hard to do useful science when you have only one example of something. Dr Lockman and his colleagues have now rectified that neglect. Their measurements show the cloud is on a collision course with the Milky Way. It will hit one of the arms of the spiral in about 30m years, and then be absorbed into the galaxy, probably triggering a burst of star formation in the process.
This suggested to the team that they had found the mechanism of eternal galactic youth: galaxies eat gas clouds. If that were true it would imply that such clouds must be quite common.
And that is what theory predicts. Recent calculations about how matter is distributed in the universe suggest that a good number of such clouds should, indeed, be out there. These calculations concern not the hydrogen itself, but the real fabric of the universe: a still-mysterious substance known as dark matter that interacts with the familiar, atomic, sort only through the force of gravity. The calculations suggest that there should be dark-matter globs of the right mass to attract interstellar gas clouds as big as Smith’s. And when Dr Lockman used Green Bank to search for such clouds, he found ten in just one small volume of nearby space.
The mystery of the ever-youthful Milky Way thus seems to be solved. Spiral galaxies are constantly rejuvenated by collisions with things like Smith’s cloud. Eventually, the supply of these clouds will run out, and with it the elixir of galactic life. But that will not be for many billions of years. Meanwhile, the universe’s spiral galaxies will keep on keeping young.