Casting shadows on spiral galaxies.

Casting Shadows on Spiral Galaxies 

Dust spoils the view no matter which way astronomers look. This pervasive cosmic smoke blocks light emanating from stars at the Milky Way's core, dims nearby galaxies and distant quasars alike, and cuts off much of the universe from direct view. To make matters worse, a new analysis reveals that spiral galaxies -- long thought to contain only a modest amount of light-absorbing dust -- may themselves be much cloudier than astronomers had assumed.

Reporting in the July 12 NATURE, Edwin A. Valentijn of the Kapteyn Astronomical Institute in Groningen, the Netherlands, suggests that major parts of most spiral galaxies are heavily clogged with light-absorbing dust. In many cases, the dust enshrouding the inner parts of a galaxy appears so thick that astronomers actually see only the outer crust of stars.

The new findings may force astronomers to rethink their explanations for a number of important astronomical effects, including their reasons for invoking exotic forms of "dark matter" to explain why spiral galaxies seem to emit less light than their apparent masses would suggest. "If spiral galazies are opaque, astronomers -- and cosmologists in particular -- will need to revise some of their ideas," says Michael Disney of University College in Cardiff, Wales.

The old view that spiral galaxies are largely transparent arose from studies of how the average surface brightness of galaxies differs depending on whether an observer sees a galaxy face-on or tipped at an angle. Dust-free, transparent galaxies would have comparable luminosities when viewed from any direction, whereas dust-clogged galaxies would look brightest observed face-on.

Valentijn originally set out to determine what factors may have biased the original studies of spiral-galaxy transparency. His precise measurements of the brightness of more than 12,000 carefully selected spiral galaxies revealed that these galaxies appear much more opaque than earlier studies had indicated. "I was very surprised because the results contradicted many famous papers in this area," Valentijn says.

The brightness data also point to large collections of cool, compact molecular clouds as the prime candidates for the main light-absorbing components in typical spiral galaxies. To confirm this prediction, astronomers now face the difficult task of mapping infrared and submillimeter-wavelength emissions from spiral galaxies in sufficient detail to pick out the obscuring molecular clouds.

Moreover, Valentijn's analysis suggests that either the Milky Way is unusually transparent compared with other spiral galaxies, or its molecular clouds cast deeper shadows than most astronomers assume. Our own galaxy may be more opaque than we think, he says. "That is a possibility that at least does not contradict any present observations."

Because spiral galaxies make up a significant proportion of all galaxies, Valentijn's discovery that they are remarkably opaque could have considerable impact on astronomy. "If it's true, there are all kinds of implications," says Jeremiah P. Ostriker of Princeton (N.J.) University. "It makes obscuration of distant objects more likely. It lessens the amount of ultraviolet light that gets out of galaxies, and that's important in the intergalactic medium."

The results also call into question the need to assume the presence of extra, nonluminous mass in the form of halos of exotic dark matter around the disks of spiral galaxies to account for the way these galaxies rotate. Indeed, the hidden matter, at least in part, may be nothing more than stars buried in dust.