Free Astronomy Magazine September-October 2016

SPACE CHRONICLES graduate student and the study’s lead au- thor. “Close galaxies are much brighter, and we have a very good method of determining their amount of oxygen,” Sanders added. In faint, distant gal- axies, the task is dra- matically more dif- ficult, but COSMOS- 1908 was one case for which Sanders was able to apply the “ro- bust” method com- monly applied to nearby galaxies. “We hope this will be the first of many,” he sta- ted. Shapley confir- med that prior to San- ders’ discovery, re- searchers didn’t know if they could measure how much oxygen there was in these distant galaxies. “Ryan’s discovery shows we can mea- sure the oxygen and compare these obser- vations with models of how galaxies form and what their his- tory of star forma- tion is,” Shapley said. The researchers used an extremely ad- vanced and sophis- ticated instrument called MOSFIRE (Multi-Object Spec- trometer for Infra-Red Exploration) installed on the Keck I telescope at the Keck Observatory. This five-ton instrument was de- signed to study the most distant, faintest galaxies, said UCLA physics and astronomy professor Ian Mc- Lean, co-project leader on MOSFIRE and director of UCLA’s Infrared Lab- oratory for Astrophysics. McLean and co-principal investigator Chuck Stei- del from the California Institute of Technology built the instrument with colleagues from UCLA, Cal- tech, UC Santa Cruz and industrial sub-contractors. The amount of oxygen in a galaxy is determined primarily by three factors: how much oxygen comes from large stars that end their lives vio- lently in supernova explosions — a ubiq- uitous phenomenon in the early Universe, when the rate of stel- lar births was dramat- ically higher than the rate in the Universe today; how much of that oxygen gets ejected from the galaxy by so-called “super winds,” which propel oxygen and other interstellar gases out of galaxies at hundreds of thou- sands of miles per hour; and how much pristine gas enters the galaxy from the intergalactic medi- um, which doesn’t contain much oxy- gen. “If we can mea- sure how much oxy- gen is in a galaxy, it will tell us about all these processes,” said Shapley, who, along with Sanders, is in- terested in learning how galaxies form and evolve, why gal- axies have different structures, and how galaxies exchange material with their intergalactic environ- ments. Shapley expects the measurements of oxygen will reveal that super winds are very important in how galaxies evolved. “Measuring the oxygen content of galaxies over cosmic time is one of the key meth- ods we have for understanding how galaxies grow, as well as how they spew out gas into the intergalactic medium,” she concluded. A ”grazing” image of the Keck telescope that hosts the MOSFIRE in- strument. [W.M. Keck Observatory, Rick Noyle] n