Using the Hubble Space Telescope, astronomers have pieced together the star-formation history of a strange-looking spiral galaxy, finding that star birth is drawing to a close.

Star formation in the outskirts of galaxy NGC 2976 shut down millions of years ago as gas was stripped away from its outskirts, forcing the rest to collapse towards the centre where it subsequently fueled a central hub of activity. This action likely disrupted any organised spiral arm structure in the galaxy, explaining its oddball shape. What caused this intriguing behaviour? The likely explanation is a boisterous interaction with neighbouring group of galaxies, M81.

“Astronomers thought that grazing encounters between galaxies can cause the funneling of gas into a galaxy’s core, but these Hubble observations provide the clearest view of this phenomenon,” says Benjamin Williams of the University of Washington who directed the Hubble study, which is part of the ACS Nearby Galaxy Survey Treasury (ANGST) program. “We are catching this galaxy at a very interesting time. Another 500 million years and the party will be over.”

The funnel of gas piped into the central region has fueled rapid star birth for at least the past 500 million years in the relatively armless disc, say the astronomers, while star birth ceased in the galaxy’s outer regions because the gas supply was exhausted. Now, the inner disc is also running out of gas as new stars burst to life, shrinking the birthing zone to a 5,000 light-year-wide area around the galaxy's core.

“At one point during this process, the density of gas in the inner regions of this galaxy was very high, about five times higher than it is today,” says ANGST team leader Julianne Dalcanton. “The gas vanished incredibly fast, and the galaxy now appears to be settling down.”

The findings were enabled thanks to the sharp vision of Hubble's ACS, which could pick out the colour and brightness of individual stars, which in turn provided information about when the stars formed. Combining the Hubble results with a radio map obtained as part of The HI Nearby Galaxy Survey conducted with the National Radio Astronomy Observatory’s Very Large Array revealed the current distribution of hydrogen across the galaxy. The ANGST team used this information to reconstruct the star-making history for large areas of the galaxy.

“This type of observation is unique to Hubble,” says Williams. “If we had not been able to pick out individual stars, we would have known that the galaxy is weird, but we would not have dug up evidence for a significant gas rearrangement in the galaxy, which caused the stellar birth zone to shrink toward the galaxy’s centre.”

Simulations also predict the phenomenon of gas-funneling as a source of frenzied star birth in dwarf galaxies that have interacted with other, larger galaxies.

The results of the study will appear in the 20 January edition of The Astrophysical Journal.

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