Looking at the distant universe with the MeerKAT array : discovery of a luminous OH megamaser at z>0.5

Abstract

In the local universe, OH megamasers (OHMs) are detected almost exclusively in infrared-luminous galaxies, with a prevalence that increases with IR luminosity, suggesting that they trace gas-rich galaxy mergers. Given the proximity of the rest frequencies of OH and the hyperfine transition of neutral atomic hydrogen (H I), radio surveys to probe the cosmic evolution of H I in galaxies also offer exciting prospects for exploiting OHMs to probe the cosmic history of gas-rich mergers. Using observations for the Looking At the Distant Universe with the MeerKAT Array (LADUMA) deep H I survey, we report the first untargeted detection of an OHM at z>0.5, LADUMAJ033046.20−275518.1 (nicknamed “Nkalakatha”). The host system, WISEA J033046.26−275518.3, is an infrared-luminous radio galaxy whose optical redshift z≈0.52 confirms the MeerKAT emission-line detection as OH at a redshift zOH=0.5225±0.0001 rather than H I at lower redshift. The detected spectral line has 18.4σ peak significance, a width of 459±59 km s−1, and an integrated luminosity of (6.31±0.18 [statistical] ±0.31 [systematic])×103 Le, placing it among the most luminous OHMs known. The galaxy’s far-infrared luminosity LFIR=(1.576± 0.013)×1012 Le marks it as an ultraluminous infrared galaxy; its ratio of OH and infrared luminosities is similar to those for lower-redshift OHMs. A comparison between optical and OH redshifts offers a slight indication of an OH outflow. This detection represents the first step toward a systematic exploitation of OHMs as a tracer of galaxy growth at high redshifts.