Windows on where a star is born.

Windows on where a star is born

Astronomers usually observe young stars in and about the spiral arms of galaxies, but they have argued whether these stellar infants are created by the spiral arms or form between arms and later collect there. Now, by pairing radio and optical observations of M51 -- a typical spiral galaxy -- a trio of astronomers has acquired "strong evidence" indicating a spiral galaxy's arms can directly trigger star formation.

Stars form as gravity forces clouds of molecular gas within a galaxy to collapse slowly upon themselves. In the Aug. 4 NATURE, Stuart N. Vogel of Rensselaer Polytechnic Institute in Troy, N.Y., and colleagues at the California Institute of Technology in Pasadena report the first wide-scale mapping of molecular gas in a classic spiral galaxy "with sufficient resolution to resolve the molecular cloud complexes and their streaming motions."

Although hydrogen is the most common molecule in these gas clouds, the scientists focused their radio observations on carbon monoxide. Except where stars are actually forming, the hydrogen in molecular clouds is too cold to be visible Vogel explains. But carbon monoxide, a trace constituent of hydrogen clouds, offers "an easily observable signal." To locate these clouds, the researchers superimposed radio maps of the carbon monoxide sources over visible images of M51 made at the 60-inch Mt. Palomar telescope in California.

The strongest carbon monoxide emissions came from lanes of dust centered within M51's spiral arms. The emissions also showed that the gas clouds slow in velocity as they enter and move through the spiral arms. Vogel says this indicates that gas clouds -- which ordinarily rotate around the galactic center faster than the spiral arms -- tend to "pile up in the dust lanes" as they attempt to flow through the slower arms. It's a bit like a slow truck on a two-lane highway causing faster vehicles to slow and cluster as they pass the truck, he notes.

Finally, emissions of ionized hydrogen showed up downstream from the dust lanes, signaling the formation there of massive, young, hot stars. That this happens downstream suggests that the compression necessary to initiate star formation occurs where the clouds collide, Vogel says -- in the spiral arms.

"We were able to show quantitatively that the gas in the spiral arm is much more efficient at making stars than the gas between the spiral arms," Vogel says -- about three times more efficient. However, he points out, stars do form in galaxies with no discernible "arms." This indicates arms are not a requirement, he adds, just a facilitator.