Free Astronomy Magazine November-December 2018

21 NOVEMBER-DECEMBER 2018 SPACE CHRONICLES T he monstrous galaxy COSMOS-AzTEC-1 observed with ALMA, which re- vealed the distribution of molecular gas (left) and dust particles (right). In addition to the dense cloud in the center, the research team detected two thick clouds several thousand light years from the center. They are dynami- cally unstable clouds, and are thought to harbor intense star formation activ- ity. [ALMA (ESO/NAOJ/NRAO), Tadaki et al.] the James Clerk Maxwell Telescope in Hawai`i, and later the Large Mil- limeter Telescope (LMT) in Mexico found an enormous amount of car- bon monoxide gas in the galaxy and revealed its hidden starburst. The LMT observations also measured the distance to the galaxy, and found that it is 12.4 billion light-years. Researchers have found that COS- MOS-AzTEC-1 is rich with the ingre- dients of stars, but it was still difficult to figure out the nature of the cosmic gas in the galaxy. The team utilized the high resolution and high sensitivity of ALMA to ob- serve this monster galaxy and obtain a detailed map of the distribution and the motion of the gas. Thanks to the most extended ALMA an- tenna configuration of 16 km, this is it the highest resolution molecular gas map of a distant monster galaxy. “We found that there are two dis- tinct large clouds several thousand light years away from the center,” explains Tadaki. “In most distant starburst galaxies, stars are actively formed in the center. So, it is surpris- ing to find off-center clouds.” The astronomers further investi- gated the nature of the gas in COS- MOS-AzTEC-1 and found that the clouds throughout the galaxy are very unstable, which is unusual. In a typical situation, the inward gravity and outward pressure are balanced in the clouds. Once gravity over- comes pressure, the gas cloud col- lapses and forms stars at a rapid pace. Then, stars and supernova ex- plosions at the end of the stellar life cycle blast out gases, which increases the outward pressure. As a result, gravity and pressure reach a bal- anced state and star formation con- tinues at a moderate pace. In this way star formation in galaxies is self- regulating. However, in COSMOS- AzTEC-1, the pressure is far weaker than the gravity and hard to bal- ance. Therefore, this galaxy shows runaway star formation and mor- phed into an unstoppable monster galaxy. The team estimated that the gas in COSMOS-AzTEC-1 will be completely consumed in 100 million years, which is ten times faster than in other star-forming galaxies. However, why is the gas in COS- MOS-AzTEC-1 so unstable? Re- searchers do not have a definitive answer yet, but galaxy merger is a possible cause. Galaxy collision may have efficiently transported the gas into a small area and ignited intense star formation. “At this moment, we have no evidence of merger in this galaxy. By observing other sim- ilar galaxies with ALMA, we want to unveil the relation between galaxy mergers and monster galaxies,” summarizes Tadaki. !