  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 (HI), radio surveys to probe the cosmic evolution of HI 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 HI 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 $z \approx 0.52$ confirms
the MeerKAT emission line detection as OH at a redshift $z_{\rm OH} = 0.5225
\pm 0.0001$ rather than HI at lower redshift. The detected spectral line has
18.4$\sigma$ peak significance, a width of $459 \pm 59\,{\rm km\,s^{-1}}$, and
an integrated luminosity of $(6.31 \pm 0.18\,{\rm [statistical]}\,\pm
0.31\,{\rm [systematic]}) \times 10^3\,L_\odot$, placing it among the most
luminous OHMs known. The galaxy's far-infrared luminosity $L_{\rm FIR} = (1.576
\pm 0.013) \times 10^{12}\,L_\odot$ 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.
