Free Astronomy Magazine January-February 2015

SPACE CHRONICLES contain as much mass as our Milky Way galaxy, but packed into a much smaller area. The smallest galaxies are about 650 light-years across. In such small regions of space, these galaxies are forming a few hundred suns a year. (By comparison, the Milky Way makes only about one sun a year.) This makes for a rowdy party that wears itself out quick- ly, in only a few tens of mil- lions of years. One reason for the stellar shutdown is that the gas rapidly heats up, becoming too hot to contract under gravity to form new stars. Another possibility is that the star- birthing frenzy blasts out most of the star-making gas via powerful stellar winds. "The biggest surprise from Hubble was the realiza- tion that the newly formed stars were born so close to- gether," said team member Aleks Diamond-Stanic of the University of Wisconsin- Madison, who first suggest- ed the possibility of star- burst-driven outflows from these galaxies in a 2012 science paper. "The extreme physical conditions at the centers of these galaxies explain how they can expel gas at millions of miles per hour." To identify the mechanism trigger- ing the high-velocity outflows, Sell and his team used the Chandra X-ray Observatory and other telescopes to determine whether the galaxies' su- permassive black holes (weighing up to a billion suns) were the power- houses driving them. After analyz- ing all of the observations, the team concluded that the black holes were not the source of the outflows. Rather, it was the powerful stellar T he 12 galaxies in these Hubble Space Telescope images are undergoing a firestorm of star birth, as shown by their bright white cores. Hubble reveals that the galaxies' star- making frenzy was ignited by mergers with other galaxies. The odd shapes of many of the galaxies are telltale evidence of those close encounters. The new Hubble Wide Field Camera 3 observations suggest that energy from the star-birthing frenzy created powerful winds that are blowing out the gas, squelching future generations of stars. This activity occurred when the universe was half its current age of 13.7 billion years. The gas-poor galaxies may eventually become so-called "red and dead" galaxies, composed only of aging stars. The galaxies are the most compact yet found. They contain as much mass as our Milky Way gal- axy, but packed into a much smaller area. The smallest galaxies are about 650 light-years across. The Hubble false-color images were processed to bring out important details in the galaxies. The images were taken in 2010. [NASA, ESA, and P. Sell (Texas Tech University)] n winds from the most massive and short-lived stars at the end of their lives, combined with their explosive deaths as supernovae. Based on their analysis of the Hub- ble and Chandra data, team mem- bers suggest that the "party begins" when two gas-rich galaxies collide, funneling a torrent of cold gas into the merging galaxies' compact cen- ter. The large amount of gas com- pressed into the small space ignites the birth of numerous stars. The energy from the stellar fire- storm then blows out the leftover gas, quenching further star forma- tion. "If you stop the flow of cold gas to form stars, that's it," explain- ed Sell, who conducted the research while a graduate student at the Uni- versity of Wisconsin-Madison. "The stars stop forming, and the galaxy rapidly evolves and may eventually become a red, dead elliptical galaxy. These extreme starbursts are quite rare, however, so they may not grow into the typical giant elliptical gal- axies seen in our nearby galactic neighborhood. They may, instead, be more compact."