LADUMA: Discovery of a luminous OH megamaser at $z>0.5$

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, LADUMA J033046.20 − 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 z OH = 0 . 5225 ± 0 . 0001 rather than H i at lower redshift. The detected spectral line has 18.4 σ peak significance, a width of 459 ± 59 km s − 1 , and an integrated luminosity of (6 . 31 ± 0 . 18 [statistical] ± 0 . 31 [systematic]) × 10 3 L (cid:12) , placing it among the most luminous OHMs known. The galaxy’s far-infrared luminosity L FIR = (1 . 576 ± 0 . 013) × 10 12 L (cid:12) marks it as an ultra-luminous 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 towards a systematic exploitation of OHMs as a tracer of galaxy growth at high redshifts.