Galaxy mix: No dark matter required

Dark matter may not matter in forming dwarf galaxies

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.

LIGHT CLUMPS The circles show where within the Leo ring of gas clumps of star formation could be dwarf galaxies in the making — galaxies forming without the help of dark matter. IMAGE CREDIT: Credit: GALEX, Digitized Sky Survey, Arecibo Observatory, JPL/NASA

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.

Radio-wave studies of the Leo ring dating back to 1986 show that the ring has little if any dark matter, although theory says the stuff must account for about 85 percent of the mass of the cosmos.

The team suggests that the dwarf galaxies that appear to be forming in the Leo ring are doing so without the gravitational scaffolding of a dark matter halo. That’s in contrast to nearly all other galaxies, in which concentrations of dark matter act as seeds that gather together the visible components: stars, gas and dust.

“The basic implication is that there may be an auxiliary pathway to dwarf galaxy formation, which can proceed in the absence of a halo provided there is enough neutral gas,” Thilker says.

Clouds of neutral gas were more common in the early universe, before the radiation from the first stars and quasars ionized gas. Moreover, an analysis of the Leo emissions suggests that the newborn stars contain extremely low concentrations of elements heavier than helium, more evidence that until recently the composition of the ring resembled that of the early universe, when heavy elements had not yet been forged.

The newly discovered mode of galaxy formation, devoid of dark matter, may therefore have occurred with greater frequency when the universe was very young, Thilker and his colleagues suggest.

“This is big news indeed,” comments Jacqueline van Gorkom of Columbia University. Although the standard picture holds  that gas falls into dark-matter halos and then collapses into stars, “the new results suggest that stars might be able to form anywhere where sufficient gas is floating around,” she says. The study also begs the question of whether gas that resides far from any galaxy, like that in the Leo ring, can nonetheless form stars. “Intergalactic star formation may be much more common than we ever imagined,” she concludes.

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