By Ron Cowen
Darth Vaders of astronomy, step aside. Purveyors of theories of dark matter — the invisible, as-yet-undetected material supposedly needed for galaxy formation — have a seat. Astronomers say they have found evidence that the gravitational collapse of visible, swirling gas may suffice to make some dwarf galaxies.
Astronomers base the surprising claim, reported in the Feb. 19 Nature, on new ultraviolet observations of the Leo ring — a vast cloud of hydrogen and helium gas that orbits two massive galaxies in the constellation Leo. The cloud may be a pristine leftover from the formation of these galaxies, essentially unchanged since the early universe. Indeed, since the ring was discovered some 25 years ago, astronomers have scrutinized it with state-of-the art radio and visible-light telescopes and found no evidence of stars, nothing except the gas.
But when David Thilker of Johns Hopkins University in Baltimore and his colleagues trained an orbiting ultraviolet observatory, NASA’s Galaxy Evolution Explorer, on the ring, they found emissions indicating recent, massive star formation within several clumps of gas. The ultraviolet emissions from the clumps, Thilker and his colleagues assert, indicate tiny galaxies forming for the first time within the cloud.