Science News

Star making gas fast depleting

Subhra Priyadarshini

doi:10.1038/nindia.2012.49 Published online 11 April 2012

The gas that makes stars in galaxies could be fast depleting due to supernova explosions and jets of monstrous black holes, according to a new study that has caught a galaxy in the act of destroying its own gaseous fuel1. Ultraviolet observations from NASA's Hubble Space Telescope and Galaxy Evolution Explorer have put together these findings that fill an important gap in the current understanding of galactic evolution.

When two star-forming spiral-galaxies such as the Milky Way collide, they form an elliptical galaxy. However, supernova explosions and jet outflows from supermassive black holes result in massive gas-losses. Though how exactly this happens has never been observed by telescopes.

A team of scientists led by Ananda Hota, has now detected this fast outflow of gas in a recently merged galaxy — the NGC 3801 — and determined the time since when no significant number of new stars have been forming in the galaxy. They predict that in just about ten million years, powerful shock waves from a black hole's jets would blow away the remaining hydrogen gas to dump the galaxy into a 'death' like state.

Big, round galaxies form very little stars. The reddish glow of aging stars dominates the complexion of elliptical galaxies and so astronomers refer to them as "red and dead."

Lead author Ananda Hota.

© Ananda Hota

"We have caught a galaxy in the act of destroying its gaseous fuel needed for new stars and slowly heading towards becoming a red-and-dead galaxy," Hota says. An astronomer in Pune, India, he conducted the study as a post-doctoral research fellow at the Institute of Astronomy and Astrophysics at Academia Sinica in Taipei, Taiwan.

The researchers used the Galaxy Evolution Explorer to determine the age of the galaxy's stars and decipher its evolutionary history. The ultraviolet observations show that NGC 3801's star formation has petered out over the last 100 to 500 million years. The lack of big, new, blue stars makes NGC 3801 look yellowish and reddish in visible light.

Hota and his co-workers say that the short-lived blue stars that formed right after it merged with another galaxy have already blown up as supernovae. Hubble Space Telescope data revealed that those stellar explosions have triggered a fast outflow of heated gas from NGC 3801's central regions. That outflow has begun to banish the reserves of cold gas, and thus cut into NGC 3801's overall star making.

The little star formation still happening in NGC 3801 will be over in just about a billion years — a small time frame by astronomical standards — due to colossal shock waves from the black hole's jets, as seen in X-ray light by NASA's Chandra X-ray Observatory.

The authors of this work are from: Chungnam National University, Daejeon, Republic of Korea; Johns Hopkins University, Baltimore, Md. and Academia Sinica Institute of Astronomy and Astrophysics, Taipei, Taiwan.


  1. Hota, A. et al. NGC 3801 caught in the act: a post-merger star-forming early-type galaxy with AGN–jet feedback. Mon. Not. R. Astron. Soc. doi: 10.1111/j.1745-3933.2012.01231.x (2012)