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

Marcin Glowacki Inter-University Institute for Data Intensive Astronomy (IDIA)
Department of Physics and Astronomy, University of the Western Cape, Robert
Sobukwe Road, Bellville 7535, South Africa  International Centre for Radio
Astronomy Research (ICRAR), Curtin University, Bentley, WA 6102, Australia
Jordan D. Collier Inter-University Institute for Data Intensive Astronomy
(IDIA)  Department of Astronomy, University of Cape Town, Private Bag X3,
Rondebosch 7700, South Africa  School of Science, Western Sydney University,
Locked Bag 1797, Penrith, NSW 2751, Australia  CSIRO Astronomy and Space
Science, PO Box 1130, Bentley, WA, 6102, Australia  Amir Kazemi-Moridani
Department of Physics and Astronomy, Rutgers, the State University of New
Jersey, 136 Frelinghuysen Road, Piscataway, NJ 08854-8019, USA  Bradley Frank
South African Radio Astronomy Observatory, 2 Fir Street, Black River Park,
Observatory, 7925, South Africa  Inter-University Institute for Data Intensive
Astronomy (IDIA)  Department of Astronomy, University of Cape Town, Private
Bag X3, Rondebosch 7700, South Africa  Hayley Roberts Center for Astrophysics
and Space Astronomy, Department of Astrophysical and Planetary Sciences,
University of Colorado, 389 UCB, Boulder, CO 80309-0389, USA  Jeremy Darling
Center for Astrophysics and Space Astronomy, Department of Astrophysical and
Planetary Sciences, University of Colorado, 389 UCB, Boulder, CO 80309-0389,
USA  Hans-Rainer Klöckner Max-Planck-Institut für Radioastronomie, Auf dem
Hügel 69, 53121, Bonn, Germany  Nathan Adams Astrophysics, University of
Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK  Jodrell Bank
Centre for Astrophysics, School of Physics and Astronomy, University of
Manchester, Manchester M13 9PL, UK  Andrew J. Baker Department of Physics and
Astronomy, Rutgers, the State University of New Jersey, 136 Frelinghuysen
Road, Piscataway, NJ 08854-8019, USA  Department of Physics and Astronomy,
University of the Western Cape, Robert Sobukwe Road, Bellville 7535, South
Africa  Matthew Bershady Department of Astronomy, University of Wisconsin-
Madison, 475 N. Charter Street, Madison, WI 53706, USA  South African
Astronomical Observatory, P.O. Box 9, Observatory 7935, South Africa
Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch
7700, South Africa  Tariq Blecher Department of Physics and Electronics,
Rhodes University, PO Box 94, Makhanda (Grahamstown), 6140, South Africa
Sarah-Louise Blyth Department of Astronomy, University of Cape Town, Private
Bag X3, Rondebosch 7700, South Africa  Rebecca Bowler Astrophysics, University
of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK  Jodrell
Bank Centre for Astrophysics, School of Physics and Astronomy, University of
Manchester, Manchester M13 9PL, UK  Barbara Catinella International Centre for
Radio Astronomy Research (ICRAR), The University of Western Australia, 35
Stirling Highway, Perth, WA 6009, Australia  ARC Centre of Excellence for All
Sky Astrophysics in 3 Dimensions (ASTRO 3D)  Laurent Chemin Centro de
Astronomia – CITEVA, Universidad de Antofagasta, Avenida Angamos 601,
Antofagasta 1270300, Chile  Steven M. Crawford unaffiliated  Catherine Cress
Department of Physics and Astronomy, University of the Western Cape, Robert
Sobukwe Road, Bellville 7535, South Africa  Romeel Davé Institute for
Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill,
Edinburgh, EH9 3HJ, UK  Department of Physics and Astronomy, University of the
Western Cape, Robert Sobukwe Road, Bellville 7535, South Africa  Roger Deane
Wits Centre for Astrophysics, School of Physics, University of the
Witwatersrand, 1 Jan Smuts Avenue 2000, South Africa  Department of Physics,
University of Pretoria, Private Bag X20, Pretoria 0028, South Africa  Erwin de
Blok ASTRON, the Netherlands Institute for Radio Astronomy, Oude
Hoogeveensedijk 4, 7991 PD, Dwingeloo, The Netherlands  Department of
Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7700, South
Africa  Kapteyn Astronomical Institute, University of Groningen, P.O. Box 800,
9700 AV Groningen, The Netherlands  Jacinta Delhaize Department of Astronomy,
University of Cape Town, Private Bag X3, Rondebosch 7700, South Africa
Kenneth Duncan Institute for Astronomy, University of Edinburgh, Royal
Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK  Ed Elson Department of
Physics and Astronomy, University of the Western Cape, Robert Sobukwe Road,
Bellville 7535, South Africa  Sean February  South African Radio Astronomy
Observatory, 2 Fir Street, Black River Park, Observatory, 7925, South Africa
Eric Gawiser Department of Physics and Astronomy, Rutgers, the State
University of New Jersey, 136 Frelinghuysen Road, Piscataway, NJ 08854-8019,
USA  Peter Hatfield Astrophysics, University of Oxford, Denys Wilkinson
Building, Keble Road, Oxford OX1 3RH, UK  Julia Healy ASTRON, the Netherlands
Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD, Dwingeloo, The
Netherlands  Patricia Henning National Radio Astronomy Observatory, Pete V.
Domenici Science Operations Center, P.O. Box O, 1003 Lopezville Road, Socorro,
NM 87801-0387, USA  Department of Physics and Astronomy, University of New
Mexico, 210 Yale Boulevard NE, Albuquerque, NM 87106, USA  Kelley M. Hess
Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía s/n,
18008 Granada, Spain  ASTRON, the Netherlands Institute for Radio Astronomy,
Oude Hoogeveensedijk 4, 7991 PD, Dwingeloo, The Netherlands  Kapteyn
Astronomical Institute, University of Groningen, P.O. Box 800, 9700 AV
Groningen, The Netherlands  Ian Heywood Astrophysics, University of Oxford,
Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK  Department of
Physics and Electronics, Rhodes University, PO Box 94, Makhanda (Grahamstown),
6140, South Africa  South African Radio Astronomy Observatory, 2 Fir Street,
Black River Park, Observatory, 7925, South Africa  Benne W. Holwerda
Department of Physics and Astronomy, 102 Natural Science Building, University
of Louisville, Louisville, KY 40292, USA  Munira Hoosain Department of
Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7700, South
Africa  John P. Hughes Department of Physics and Astronomy, Rutgers, the State
University of New Jersey, 136 Frelinghuysen Road, Piscataway, NJ 08854-8019,
USA  Zackary L. Hutchens Department of Physics & Astronomy, CB3255, University
of North Carolina, Chapel Hill, NC 27516, USA  Matt Jarvis Astrophysics,
University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
Department of Physics and Astronomy, University of the Western Cape, Robert
Sobukwe Road, Bellville 7535, South Africa  Sheila Kannappan Department of
Physics & Astronomy, CB3255, University of North Carolina, Chapel Hill, NC
27516, USA  Neal Katz Department of Astronomy, University of Massachusetts,
Amherst, MA 01003, USA  Dušan Kereš  Center for Astrophysics and Space
Sciences, University of California San Diego, 9500 Gilman Dr, La Jolla, CA
92093, USA  Marie Korsaga Université de Strasbourg, CNRS, Observatoire
astronomique de Strasbourg, UMR 7550, F-67000 Strasbourg, France  Laboratoire
de Physique et de Chimie de l’Environnement, Observatoire d’Astrophysique de
l’Université Joseph Ki-Zerbo (ODAUO), 03 BP 7021,Ouaga 03, Burkina Faso  Renée
C. Kraan-Korteweg Department of Astronomy, University of Cape Town, Private
Bag X3, Rondebosch 7700, South Africa  Philip Lah Research School of Astronomy
and Astrophysics, Australian National University, Canberra, ACT 2611,
Australia  Michelle Lochner Department of Physics and Astronomy, University of
the Western Cape, Robert Sobukwe Road, Bellville 7535, South Africa  South
African Radio Astronomy Observatory, 2 Fir Street, Black River Park,
Observatory, 7925, South Africa  Natasha Maddox University Observatory,
Faculty of Physics, Ludwig-Maximilians-Universität, Scheinerstr. 1, 81679
Munich, Germany  Sphesihle Makhathini Wits Centre for Astrophysics, School of
Physics, University of the Witwatersrand, 1 Jan Smuts Avenue 2000, South
Africa  Gerhardt R. Meurer International Centre for Radio Astronomy Research
(ICRAR), The University of Western Australia, 35 Stirling Highway, Perth, WA
6009, Australia  Martin Meyer International Centre for Radio Astronomy
Research (ICRAR), The University of Western Australia, 35 Stirling Highway,
Perth, WA 6009, Australia  Danail Obreschkow International Centre for Radio
Astronomy Research (ICRAR), The University of Western Australia, 35 Stirling
Highway, Perth, WA 6009, Australia  Se-Heon Oh Department of Physics and
Astronomy, Sejong University, 209 Neungdong-ro, Gwangjin-gu, Seoul, Republic
of Korea  Tom Oosterloo ASTRON, the Netherlands Institute for Radio Astronomy,
Oude Hoogeveensedijk 4, 7991 PD, Dwingeloo, The Netherlands  Kapteyn
Astronomical Institute, University of Groningen, P.O. Box 800, 9700 AV
Groningen, The Netherlands  Joshua Oppor Department of Astronomy, University
of Wisconsin-Madison, 475 N. Charter Street, Madison, WI 53706, USA  Hengxing
Pan Department of Physics and Astronomy, University of the Western Cape,
Robert Sobukwe Road, Bellville 7535, South Africa  Astrophysics, University of
Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK  D. J. Pisano
Department of Physics & Astronomy, West Virginia University, Morgantown, WV
26506, USA  Gravitational Wave and Cosmology Center, Chestnut Ridge Research
Building, Morgantown, WV 26505, USA  Nandrianina Randriamiarinarivo
Department of Physics and Astronomy, University of the Western Cape, Robert
Sobukwe Road, Bellville 7535, South Africa  Swara Ravindranath Space Telescope
Science Institute, 3700 San Martin Drive, Baltimore, MD 21218-2410, USA  Anja
C. Schröder Max-Planck-Institut für extraterrestrische Physik,
Giessenbachstraße 1, D-85748 Garching bei München, Germany  Rosalind Skelton
South African Astronomical Observatory, P.O. Box 9, Observatory 7935, South
Africa  Oleg Smirnov Department of Physics and Electronics, Rhodes University,
PO Box 94, Makhanda (Grahamstown), 6140, South Africa  South African Radio
Astronomy Observatory, 2 Fir Street, Black River Park, Observatory, 7925,
South Africa  Mathew Smith School of Physics and Astronomy, University of
Southampton, Southampton, SO17 1BJ, UK  Univ Lyon, Univ Claude Bernard Lyon 1,
CNRS, IP2I Lyon / IN2P3, IMR 5822, F-69622, Villeurbanne, France  Rachel S.
Somerville  Center for Computational Astrophysics, Flatiron Institute, New
York, NY 10010, USA  Raghunathan Srianand IUCAA, Postbag 4, Ganeshkhind, Pune
411007, India  Lister Staveley-Smith International Centre for Radio Astronomy
Research (ICRAR), The University of Western Australia, 35 Stirling Highway,
Perth, WA 6009, Australia  ARC Centre of Excellence for All Sky Astrophysics
in 3 Dimensions (ASTRO 3D)  Masayuki Tanaka Department of Astronomical
Science, The Graduate University for Advanced Studies, SOKENDAI, 2-21-1 Osawa,
Mitaka, Tokyo, 181-8588, Japan  National Astronomical Observatory of Japan,
2-21-1 Osawa, Mitaka, Tokyo, 181-8588, Japan  Mattia Vaccari Inter-University
Institute for Data Intensive Astronomy (IDIA)  Department of Physics and
Astronomy, University of the Western Cape, Robert Sobukwe Road, Bellville
7535, South Africa  INAF - Istituto di Radioastronomia, via Gobetti 101, 40129
Bologna, Italy  Wim van Driel GEPI, Observatoire de Paris, PSL Université,
CNRS UMR 8111, 5 place Jules Janssen, 92190 Meudon, France  Marc Verheijen
Kapteyn Astronomical Institute, University of Groningen, Landleven 12, 9747
AD, Groningen, The Netherlands  Fabian Walter Max-Planck-Institut für
Astronomie (MPIA), Königstuhl 16, 69117 Heidelberg, Germany  John F. Wu Space
Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218-2410,
USA  Martin A. Zwaan European Southern Observatory, Karl-Schwarzschild-Strasse
2, D-85748 Garching, Germany

(Received February 11, 2022; Revised March 28, 2022; Accepted March 29, 2022)

###### 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 $z>0.5$, 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 H i 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.

megamasers — hydroxyl masers — radio loud quasars — galaxy mergers — starburst
galaxies — ULIRG

††facilities: MeerKAT

##  1 Introduction

OH megamasers (OHMs) are luminous 18 cm wavelength masers, produced in the
centers of luminous and ultra-luminous infrared galaxies (LIRGs and ULIRGs)
that have undergone merger-induced starburst activity (Lo, 2005), are rich in
dense gas (Darling, 2007), and in some cases host luminous active galactic
nuclei (AGN; Klöckner et al., 2003). Four 18 cm lines connect the four
hyperfine levels within the ${}^{2}\Pi_{3/2}(J=3/2)$ ground state of the OH
molecule: two main lines at 1665 and 1667 MHz, and two satellite lines at 1612
and 1720 MHz. In contrast to Galactic OH masers, extragalactic OHMs have large
line widths and main line flux ratios $F_{1667}/F_{1665}>1$. These attributes,
along with the weakness of their satellite lines (McBride et al., 2013), can
be naturally explained by a model in which OHMs are powered by radiative
pumping through $53\,{\rm\mu m}$ lines that overlap in velocity, and in which
different projected distributions of masing clumps can account for
observations of both diffuse and compact emitting structures (Lockett &
Elitzur, 2008).

The most extensive OHM survey in the local Universe has been conducted with
Arecibo, with a focus on the detection of $z>0.1$ systems (Darling &
Giovanelli, 2002a). In total, 53 OHMs were detected, spanning the redshift
range $0.10\leq z\leq 0.27$. Although Combes et al. (2021) report a tentative
$3\sigma$ detection of the 1720 MHz satellite line in emission in a targeted
MeerKAT observation of the $z=0.89$ quasar PKS 1830$-$211, the main lines of
OH have not been detected in emission at $z>0.27$ up to now.

The local demographics of OHMs have been used to predict their occurrence at
higher redshifts, where they are likely to represent a significant source of
contamination for H i surveys (Briggs, 1998; Darling & Giovanelli, 2002b;
Suess et al., 2016; Roberts et al., 2021). Such surveys will be conducted by
the interferometers that include the Square Kilometre Array (SKA) and its
pathfinder facilities, such as MeerKAT (Jonas & MeerKAT Team, 2016) and the
Australian SKA Pathfinder (ASKAP; DeBoer et al., 2009). Although ULIRGs are
more prevalent at higher redshifts (Takeuchi et al., 2005) in part because
normal star-forming galaxies at earlier epochs are likely to have $L_{\rm
FIR}>10^{12}\,L_{\odot}$ even in the absence of recent merging (e.g., Reddy et
al., 2008), OHMs have the potential to provide new constraints on the cosmic
history of gas-rich mergers.

We report the first untargeted detection of an OHM at $z>0.27$ from early
observations for the Looking At the Distant Universe with the MeerKAT Array
(LADUMA; Blyth et al., 2016) deep H i survey. Section 2 describes the
acquisition and processing of the MeerKAT data in which the OH line was
detected, and Section 3 presents our measurements and interpretation of the
line parameters. In Section 4, we discuss the implications of this OHM in the
context of previous knowledge of OHM hosts at lower redshifts and future
observations probing to higher redshifts; §5 summarizes our conclusions. The
paper assumes a flat $\Lambda$CDM cosmology with $\Omega_{m}=0.3$,
$\Omega_{\Lambda}=0.7$, and $H_{0}=73.3\,{\rm km\,s^{-1}\,Mpc^{-1}}$ (Wong et
al., 2020).

##  2 Observations

###  2.1 MeerKAT

MeerKAT is a fixed-configuration array of 64 antennas equipped with receivers
spanning the L (900–1670 MHz) and UHF (580–1015 MHz) bands. These overlapping
frequency ranges access redshift ranges of $0\leq z_{\rm H\,I}\leq 0.58$ and
$0.40~{}\leq~{}z_{\rm H\,I}~{}\leq 1.45$ for the H i line, and
$0~{}\leq~{}z_{\rm OH}~{}\leq~{}0.85$ and $0.64~{}\leq~{}z_{\rm
OH}~{}\leq~{}1.87$, adopting the stronger of the two main OH lines (at a rest
frequency of 1667.359 MHz) to define $z_{\rm OH}$. The LADUMA deep H i survey
is using both L and UHF bands to probe the evolution of gas in galaxies over
cosmic time. LADUMA is targeting a single pointing on the sky (03:32:30.4
$-$28:07:57 J2000) that encompasses the extended Chandra Deep Field South
(ECDFS) and lies roughly at the center of near-IR imaging coverage from the
VISTA Deep Extragalactic Observations (VIDEO) survey (Jarvis et al., 2013).
Because the solid angle of MeerKAT’s primary beam is inversely proportional to
the square of the observing frequency, the cosmic volume LADUMA probes for any
single spectral line expands with redshift like a trumpet (e.g., a South
African vuvuzela). Across the L band in particular, MeerKAT’s circular field
of view increases (at the half-power level) from $0.7\,{\rm deg}^{2}$ at 1667
MHz to $0.9\,{\rm deg}^{2}$ at 1420.4 MHz to $2.3\,{\rm deg}^{2}$ at 900 MHz.

The first official L-band survey observation for LADUMA that used the 32k mode
of the MeerKAT correlator (featuring 32,768 channels, each of width 26.1 kHz)
was taken on 2019 December 12 with 58 of the 64 antennas in operation. The
bright radio galaxy PKS 1934–63 was used as a flux and bandpass calibrator
(observed for 10 minutes); the nearby quasar PKS 0237–233 was observed as a
gain calibrator for 3.5 minutes after each 20-minute observation of the LADUMA
field. The total on-source integration time was 7.3 hours.

We reduced the data across the full L band, excluding regions affected by
radio frequency interference (RFI), after splitting into 25 MHz spectral
windows (SPWs). For the source discussed in this paper, the relevant SPW spans
1086–1111 MHz. Bandpass, flux, and phase calibration were performed using the
processMeerKAT pipeline111https://idia-
pipelines.github.io/docs/processMeerKAT, which is written in Python, uses a
purpose-built CASA (McMullin et al., 2007) Singularity container, and employs
MPICASA (a parallelized form of CASA). Flux calibration used the Reynolds
(1994) model for the spectrum of PKS 1934$-$63, which is ultimately tied to
northern hemisphere calibrators with flux uncertainties of $\sim 5\%$ at $\sim
1\,{\rm GHz}$ (e.g., Perley & Butler, 2017). The CASA task tclean was used
with robust = 0 to create an initial continuum model as a basis for phase and
amplitude self-calibration, after which model continuum visibility data were
subtracted from the corrected visibility data using the CASA task uvsub. A
third-order polynomial fit to the continuum was then calculated and subtracted
using the CASA task uvcontsub for all channels in each SPW to remove residual
continuum emission from the spectral line data. Finally, spectral line cubes
were created using tclean with robust = 0.5 and no cleaning; all channels in
the resulting (dirty) cubes were convolved to a common synthesized beam of
$14.2^{\prime\prime}\times 12.2^{\prime\prime}$ at a position angle of
$-$15.8∘. The RMS per 26.1 kHz channel was found to increase with frequency
across the 1086–1111 MHz SPW, with a value of 0.40 mJy beam-1 near 1095 MHz.
All data were reduced on the ilifu cloud computing
facility222https://docs.ilifu.ac.za/#/about/what_is.

Visual inspection of the continuum-subtracted 26.1 kHz channel data cube with
the Cube Analysis and Rendering Tool for Astronomy (CARTA; Comrie et al.,
2020) revealed a bright spectral line at an observed frequency of 1095 MHz,
which we designate as LADUMA J033046.20$-$275518.1 and describe in detail in
§3.1.

Table 1: Selected photometry for LADUMA J033046.20$-$275518.1.

| Observed | Flux |
---|---|---|---
Source | wavelength | density | Reference
WISE | $3.4\,{\rm\mu m}$ | $0.122\pm 0.005\,{\rm mJy}$ | Wright et al. (2010)
WISE | $4.6\,{\rm\mu m}$ | $0.084\pm 0.007\,{\rm mJy}$ | Wright et al. (2010)
WISE | $12\,{\rm\mu m}$ | $0.928\pm 0.085\,{\rm mJy}$ | Wright et al. (2010)
Spitzer/MIPS | $70\,{\rm\mu m}$ | $141.11\pm 0.66\,{\rm mJy}^{a}$ | Hanish et al. (2015)
Spitzer/MIPS | $160\,{\rm\mu m}$ | $208.34\pm 2.13\,{\rm mJy}^{a}$ | Hanish et al. (2015)
Herschel/SPIRE | $250\,{\rm\mu m}$ | $90.93\pm 0.65\,{\rm mJy}$ | Shirley et al. (2021)
Herschel/SPIRE | $350\,{\rm\mu m}$ | $42.36\pm 1.53\,{\rm mJy}$ | Shirley et al. (2021)
Herschel/SPIRE | $500\,{\rm\mu m}$ | $17.6\pm 3.7\,{\rm mJy}$ | Shirley et al. (2021)
ATCA | 20 cm | $0.42\pm 0.029\,{\rm mJy}$ | Franzen et al. (2015)

Note. — a flux density extracted from a point spread function (PSF) fit

###  2.2 Archival data

LADUMA J033046.20$-$275518.1 has a clear galaxy counterpart in WISE imaging
(WISEA J033046.26–275518.3) and in previous radio continuum mapping by the
Australia Telescope Large Area Survey (Mao et al., 2012; Franzen et al.,
2015). The galaxy has a bent-tail radio morphology (Dehghan et al., 2014), and
has been classified as a narrow-angle (or head-tail) radio galaxy (that is,
the bent radio structure lies on one side of the optical host galaxy). The
source has also been detected in dust emission by the Multiband Imaging
Photometer for Spitzer (MIPS; Rieke et al., 2004) on the Spitzer Space
Telescope (Hanish et al., 2015) and by the Spectral and Photometric Imaging
Receiver (SPIRE; Griffin et al., 2010) on the Herschel Space Observatory
(Oliver et al., 2012; Shirley et al., 2021). Table 1 lists selected photometry
for the source in the infrared and radio. Optical spectroscopy has yielded two
independent redshift measurements of $z=0.5245$ (Eales et al., 2009) and
$z=0.5247$ (Mao et al., 2012), neither with a quoted uncertainty; in this
paper, we average these two measurements and adopt the $\sim 100\,{\rm
km\,s^{-1}}$ uncertainty that is typical for AAT redshifts of similar vintage
(see, e.g., §1 of Baldry et al., 2014), yielding $z_{\rm opt}=0.5246\pm
0.0005$.

We can determine the host galaxy’s infrared luminosity using the source
redshift and far-infrared photometry. By fitting emissivity-weighted blackbody
functions to 1000 realizations of the Spitzer/MIPS and Herschel/SPIRE flux
densities between 70–350 $\mu$m (sampling their respective uncertainties), we
derive a rest-frame dust temperature $T_{d}=41.9\pm 0.6\,{\rm K}$, a dust
emissivity index $\beta=1.67\pm 0.06$ (for $\tau_{\nu}\propto\nu^{\beta}$),
and a far-IR (rest-frame 42.5–122.5 ${\rm\mu m}$) flux $(6.30\pm 0.05)\times
10^{-12}$ erg s-1 cm-2. Using the cosmology adopted for this paper and the
redshift measured in §3.1 below, we then arrive at $L_{\rm FIR}=(1.576\pm
0.013)\times 10^{12}\,L_{\odot}$, confirming the source as a ULIRG as
previously noted by Moncelsi et al. (2011).

Figure 1: _Left:_ RGB composite image made from the $grz$ filters of archival
Hyper Suprime-Cam (HSC) data. White contours are from the zeroth moment map of
our MeerKAT observation with no masking applied, integrated across $\sim
600\,{\rm km\,s^{-1}}$, at multiples of 5$\sigma$ (1$\sigma$ = $0.0295\,{\rm
Jy\,beam^{-1}\,km\,s^{-1}}$). The cyan cross indicates the centroid position
and uncertainties from the Franzen et al. (2015) observation with ATCA. The
$14.2^{\prime\prime}\times 12.2^{\prime\prime}$ MeerKAT synthesized beam is
plotted as a dashed yellow ellipse. _Right:_ Spectrum of the detected OHM,
with frequency Doppler-corrected to a heliocentric reference frame and
rebinned from its native 26.1 kHz resolution by a factor of 2. The rest-frame
velocity range included in the moment map is shaded grey; the best Gaussian
fit to the OH emission profile is overlaid in blue. The red dashed line
indicates the frequency (1093.7 MHz)

where the 1667 MHz line would have appeared for $z_{\rm OH}$ = $z_{\rm opt}$;
that $z_{\rm OH}<z_{\rm opt}$ suggests a possible OH outflow.

Figure 2: Independent velocity channel maps of a 8$\times$ rebinned 26.1 kHz
data cube, with velocities indicated relative to our measured systemic
redshift of $z_{\rm OH}=0.5225$. Contours are multiples of 2$\sigma$
(1$\sigma$ = $0.283\,{\rm mJy\,beam^{-1}}$). The $14.2^{\prime\prime}\times
12.2^{\prime\prime}$ synthesized beam is plotted in the top left panel.

##  3 Results

###  3.1 OH emission properties

The spatial centroid of LADUMA J033046.20–275518.1 lies at right ascension
03:30:46.20 and declination $-$27:55:18.17 (J2000) with uncertainties of $\pm
0.7^{\prime\prime}$ (calculated as beam size divided by S/N) — consistent with
the centroids determined for its counterparts at other wavelengths, within
their own uncertainties of $\sim 0.5^{\prime\prime}$ or greater. At this
position and the observed line frequency, the primary beam correction was a
factor $\sim(0.72)^{-1}$, which we applied using CASA before measuring line
parameters.

In the right panel of Figure 1, we show the spectrum integrated over a
$15^{\prime\prime}$ aperture, with frequency Doppler-corrected to the
heliocentric reference frame and rebinned by a factor of 2. At the observed
frequency, each rebinned (52.2 kHz) channel corresponds to $14.3\,{\rm
km\,s^{-1}}$ in rest-frame velocity. A single-Gaussian fit to the line
spectrum yields (with fit uncertainties noted as errors) a central frequency
of $\nu=1095.07\pm 0.09\,{\rm MHz}$, a rest-frame line width $\Delta v=459\pm
59\,{\rm km\,s^{-1}}$ (full-width at half-maximum, which corresponds to an
observed frequency width of 1.68 MHz), a peak flux density of $1.42\pm
0.16\,[\pm 0.07]$ mJy,333Errors in brackets represent 5% flux scale
uncertainties. and a total spectral line flux $F_{\rm line}=0.69\pm 0.02\,[\pm
0.03]\,{\rm Jy\,km\,s^{-1}}$ that agrees well with a direct integral of the
spectrum ($0.69\pm 0.02\,[\pm 0.03]\,{\rm Jy\,km\,s^{-1}}$).

We note that the 1665 MHz OH line may contribute to the measured line flux and
width, although this fainter feature is not yet detected in this single-track
observation, nor currently preferred over a single Gaussian fit. Given the
presence of a multiwavelength counterpart with a previously measured optical
redshift $z=0.5246$, it is clear that our detection is hydroxyl emission at
$z_{\rm OH}=0.5225\pm 0.0001$ (identified as the 1667.359 MHz transition)
rather than H i emission at $z_{\rm H\,I}=0.2970$. From the integrated line
flux, we use the general relation for spectral line luminosity

${\frac{L_{\rm line}}{L_{\odot}}}=1.0234\,\Big{(}{\frac{\nu}{\rm
MHz}}\Big{)}\,\Big{(}{\frac{F_{\rm line}}{\rm
Jy\,km\,s^{-1}}}\Big{)}\,\Big{(}{\frac{D_{L}}{\rm Gpc}}\Big{)}^{2}$ (1)

in terms of observed frequency $\nu=1095.1\pm 0.1\,{\rm MHz}$ and luminosity
distance $D_{L}=2.85\pm 0.02\,{\rm Gpc}$ for our adopted cosmology, obtaining
an equivalent isotropic luminosity of $L_{\rm OH}=(6.31\pm 0.18\,[\pm
0.31])\times 10^{3}\,L_{\odot}$.

The left panel of Figure 1 shows contours from the zeroth moment map for the
source, integrated over $600\,{\rm km\,s^{-1}}$ and overlaid on a composite
image made from the $grz$ filters of archival Hyper Suprime-Cam (HSC; Miyazaki
et al., 2018) data. The cyan cross indicates the ATCA position and its
uncertainties from Franzen et al. (2015), and the yellow ellipse shows the
MeerKAT synthesized beam. The OH emission is spatially unresolved, with S/N
$\sim 18.4$ relative to the RMS in the zeroth moment map away from the source.
Figure 2 shows the velocity channel maps of the source after rebinning by an
additional factor of 4 to a rest-frame velocity resolution of $57.2\,{\rm
km\,s^{-1}}$.

###  3.2 Confirmation of the 1667 MHz identification

Figure 3: Predicted distributions of gas-rich mergers traced by OH at $z_{\rm
OH}\sim 0.52$ (blue), and H i-rich disk galaxies at $z_{\rm H\,I}\sim 0.30$
(red), in WISE color-magnitude and color-color space, generated using
algorithms presented in Roberts et al. (2021). Contours enclose the upper 99%
(dashed), 90%, 80%,… 20%, and 10% of the Gaussian kernel density estimates for
the respective galaxy distributions. In the left panel, both OH and H i
identifications are supported for LADUMA J033046.20$-$275518.1, while in the
right panel, OH is clearly preferred.

Notwithstanding the good agreement with a previously measured optical
redshift, we have considered whether the detected emission could correspond to
a weaker OH line (e.g., the 1612 or 1720 MHz satellite line, which can exhibit
conjugate behavior: Darling, 2004) rather than the main 1667 MHz transition.
In this scenario, we would expect an additional detection of the (brighter)
1667 MHz main line at a different frequency (1132.5 or 1061.2 MHz) in our
spectrum. No such emission feature is seen at the radio position. We also
looked for emission in the satellite lines assuming our detection is indeed
the 1667 MHz main line at 1058.9 or 1130.0 MHz, with no such features seen.
This result is not surprising given the weakness of satellite lines in OHMs
(McBride et al., 2013), although as the LADUMA survey proceeds and reaches
greater depths, we may be able to detect them in some systems. We also found
no evidence of Hi emission or absorption at this redshift, although such
features may become evident from the upcoming deeper LADUMA observations.

In Figure 3, we consider the implications of WISE magnitudes and colors for
our identification of the detected emission line. We employ the algorithms
presented in Roberts et al. (2021), who use machine learning to determine the
redshift evolution in WISE magnitude and color space for an OHM host and a
typical Hi source. The predicted distributions in WISE properties expected for
gas-rich disk galaxies emitting H i at redshift $z\sim 0.30$ (red) and a
galaxy merger traced through OH at redshift $z\sim 0.52$ (blue) show that an
OH identification for our source is plausible for all diagnostics, while an H
i identification is not always supported (see right panel). Therefore, we are
confident that the emission detected corresponds to the main OH transition at
1667 MHz.

##  4 Discussion

###  4.1 OH and FIR luminosities

Figure 4: $L_{\rm OH}$ vs. $L_{\rm FIR}$ for known OHMs (Roberts et al. 2022,
in preparation). LADUMA J033046.20$-$275518.1 (green star) is one of the most
luminous OHMs on both axes.

LADUMA J033046.20$-$275518.1 is one of the most luminous OHMs known at any
redshift; only three sources within the sample of 53 OHMs presented in Darling
& Giovanelli (2002a) and Darling & Giovanelli (2002b) have higher $L_{\rm OH}$
values, and it is only marginally less luminous than the $z=0.1961$ OHM
recently detected by Apertif (Hess et al., 2021), which would have $L_{\rm
OH}=7.18\times 10^{3}\,L_{\odot}$ for our cosmology. In recognition of this
power and its unprecedentedly high redshift, we have given it the nickname
“Nkalakatha,” an isiZulu word that means “big boss.” It is not overly
surprising that a detection reached in a single LADUMA track will be among the
most luminous OHMs when compared to sources from the local Universe at
$z<0.27$. Using the relationship between OH and far-IR luminosities derived by
Darling & Giovanelli (2002a) for an Arecibo survey + literature sample of
OHMs, i.e., ${\rm log}\,(L_{\rm OH}/L_{\odot})=(1.57\pm 0.11)\,{\rm
log}\,(L_{\rm FIR}/L_{\odot})-(15.76\pm 1.22)$, we find that the predicted OH
luminosity for LADUMA J033046.20$-$275518.1 is $L_{\rm OHpred}\approx
2.4\times 10^{3}\,L_{\odot}$. This is roughly a factor of 2.6 smaller than
what we observe, but well within the large scatter observed for the local
relation (Fig. 4). The dust temperature recovered from the fit to the system’s
FIR photometry ($T_{d}\approx 42\,{\rm K}$) is roughly consistent with
theoretical expectations and observational results that $T_{d}\geq 45\,{\rm
K}$ is required for OH masing to occur (Lockett & Elitzur, 2008; Willett et
al., 2011), particularly if cooler dust outside the masing region contributes
to $L_{\rm FIR}$, and is at the lower end of the distribution of global dust
temperatures for OHM hosts.

We have also considered the FIR–radio flux ratio parameter $q$, defined by
Helou et al. (1985) as

$q={\rm log}\,\Big{\\{}\Big{(}{\frac{F_{\rm FIR}}{\rm
W\,m^{-2}}}\Big{)}\,\Big{(}{\frac{10^{29}}{3.75\times 10^{12}\,{\rm
Hz}}}\Big{)}\,\Big{(}{\frac{S_{\rm 1.4\,GHz}}{\rm
mJy}}\Big{)}^{-1}\,\Big{\\}}$ (2)

For LADUMA J033046.20$-$275518.1, we correct the observed radio continuum flux
density444This value is comparable to the value measured from MeerKAT
continuum imaging of the LADUMA field, which will be published in a
forthcoming paper analogous to Heywood et al. (2022). (Table 1) to 1.4 GHz in
the rest frame assuming a spectrum $S_{\nu}\propto\nu^{-0.7}$, and arrive at
an estimated $q\approx 2.7$. This value is consistent with the infrared-radio
correlation for star-forming galaxies with no evidence of AGN (see, e.g., Fig.
16 of Delhaize et al., 2017), higher than the median $q=2.37$ recently
measured for 89 star-forming galaxies in the COSMOS field (An et al., 2021),
and much higher than the threshold $q\approx 1.8$ at which galaxies are three
times more radio-loud than the mean for star-forming systems (Condon et al.,
2002). Spitzer/IRAC photometry for this source (Lonsdale et al., 2003) also
disfavor an AGN identification according to well-established criteria (Stern
et al., 2005; Donley et al., 2012), as does a classification of its optical
spectrum (Mao et al., 2012). As a result, we view the far-IR emission from
LADUMA J033046.20$-$275518.1 as more likely due to starburst activity than to
a bolometrically significant AGN. Such a conclusion is also not surprising
given evidence from the local Universe that ULIRGs hosting OHMs are less
likely to show evidence of AGN at infrared wavelengths than non-masing ULIRGs
(Willett et al., 2011).

###  4.2 A possible molecular outflow

Large-scale outflows emanating from the central regions of galaxies and
powered by both starbursts and AGN have been known for decades to play an
important role in the evolution of galaxies and the intergalactic medium
(e.g., Heckman et al., 1990). Such outflows have been detected through OH
observations in a number of systems. Baan et al. (1989) detect three distinct
outflows in emission in a sample of five OHMs, with one galaxy showing a
maximum outflow velocity of 800 km s-1. González-Alfonso et al. (2014)
meanwhile find far-IR OH features blueshifted by over 1000 km s-1 in Mrk 231,
with the central AGN likely responsible for the high mass outflow rate and
outflow velocities detected.

The OH emission in LADUMA J033046.20$-$275518.1 is blueshifted by $407\pm
118\,{\rm km\,s^{-1}}$ relative to the redshift measured from optical
spectroscopy. The simplest explanation for a one-sided velocity offset is a
starburst-driven outflow, given the high value of $q$ noted above. However,
the situation could be more complex, involving multiple nuclei or a disk close
to an (obscured) AGN, possibly interacting with outflows. As our MeerKAT
observation of this galaxy and its OH emission is unresolved, higher spatial
resolution observations would be required to further characterize the
outflow’s extent and energetics. The study of Gowardhan et al. (2018),
investigating molecular gas outflows in two starburst ULIRGs in mid-$J$ CO and
18 cm OH lines, is instructive in this regard. CO outflow velocities are seen
to exceed 1600 km s-1, with corresponding mass outflow rates of 300–700
$M_{\odot}$ yr-1; meanwhile, OH outflow velocities are seen to extend to 1000
km s-1, with velocity wings for one source agreeing “remarkably well” with
previous detections of outflowing molecular gas. A higher-resolution study of
other molecular species (e.g., CO) in LADUMA J033046.20$-$275518.1 may thus be
a productive strategy for confirming the existence of an outflow in OH.

##  5 Conclusions

We present the first detection of an OHM in the LADUMA field, LADUMA
J033046.20$-$275518.1 “Nkalakatha,” which is also the highest redshift
detection of such a system to date in the main 1667 MHz OH emission line. OH
emission is found to be redshifted to $z_{\rm OH}=0.5225$, which agrees well
with an optical redshift of $z_{\rm opt}=0.5246$ for a host galaxy already
known to be a ULIRG. The system’s total OH luminosity of $L_{\rm OH}=(6.31\pm
0.18\,[\pm 0.31])\times 10^{3}\,L_{\odot}$ makes it one of the most luminous
OHMs known (all other 1667 MHz detections have redshifts $z<0.27$), and is
consistent with its large far-IR luminosity. The $\sim$400 km s-1 offset
between the OH and optical redshifts is most simply explained by a starburst-
driven outflow.

This detection highlights the potential of upcoming spectral line surveys,
whose wide frequency coverage will enable further high-redshift measurements.
Roberts et al. (2021) predict that $83\pm 20$ OHMs will be detected in LADUMA
alone, which will nearly double the number of known OHMs. Greater numbers are
expected in lower redshift but wider area surveys, such as the Widefield ASKAP
L-band Legacy All-sky Blind surveY (WALLABY; Koribalski et al., 2020), the
Apertif Wide-area Extragalactic Survey (AWES; Hess et al., in prep), and the H
i component (MIGHTEE-H i; Maddox et al., 2021) of the MeerKAT International
GigaHertz Tiered Extragalactic Exploration (MIGHTEE) survey (Jarvis et al.,
2016). As LADUMA reaches greater depths, we expect to set further OHM redshift
records, which will enable studies of the cosmic rate of gas-rich galaxy
mergers and further constrain galaxy evolution models.

The authors sincerely thank Zolile Tibane for suggesting the nickname
“Nkalakatha” for LADUMA J033046.20$-$275518.1. We also thank the anonymous
referee for comments that have helped improve the paper. The MeerKAT telescope
is operated by the South African Radio Astronomy Observatory (SARAO;
www.sarao.ac.za), which is a facility of the National Research Foundation
(NRF), an agency of the Department of Science and Innovation. The authors
thank the members of the SARAO engineering, commissioning, and science data
processing teams for building and operating an absolutely superb facility. The
MeerKAT data presented in this paper were processed using the ilifu cloud
computing facility (www.ilifu.ac.za), which is operated by a consortium that
includes the University of Cape Town (UCT), the University of the Western
Cape, the University of Stellenbosch, Sol Plaatje University, the Cape
Peninsula University of Technology and the South African Radio Astronomy
Observatory. The ilifu facility is supported by contributions from the Inter-
University Institute for Data Intensive Astronomy (IDIA, which is a
partnership between the UCT, the University of Pretoria and the University of
the Western Cape), the Computational Biology division at UCT, and the Data
Intensive Research Initiative of South Africa (DIRISA). Data processing used
pipelines that were developed at IDIA and are available at https://idia-
pipelines.github.io. MG acknowledges support from IDIA and was partially
supported by the Australian Government through the Australian Research
Council’s Discovery Projects funding scheme (DP210102103). AKM and AJB
acknowledge support from NSF grant AST-1814421. AKM also thanks the LSSTC Data
Science Fellowship Program, which is funded by the LSST Corporation, NSF grant
OAC-1829740, the Brinson Foundation, and the Moore Foundation; his
participation in the program has been helpful for this work. HR and JeD
acknowledge support from NSF grant AST-1814648; MB and JO acknowledge support
from NSF grant AST-1814682; RB acknowledges support from an STFC Ernest
Rutherford Fellowship (grant number ST/T003596/1); JaD and HP acknowledge the
financial assistance of SARAO; KMH acknowledges funding from the State Agency
for Research of the Spanish Ministry of Science, Innovation and Universities
through the “Center of Excellence Severo Ochoa” awarded to the Instituto de
Astrofísica de Andalucía (SEV-2017-0709); from grant RTI2018-096228-B-C31
(Ministry of Science, Innovation and Universities / State Agency for Research
/ European Regional Development Funds, European Union); and from the
coordination of the participation in SKA-SPAIN, funded by the Ministry of
Science and innovation (MICIN); and ZLH and SK acknowledge support from NSF
grant AST-1814486. Parts of this research were supported by the Australian
Research Council Centre of Excellence for All Sky Astrophysics in 3 Dimensions
(ASTRO 3D), through project number CE170100013, and by the South African
Research Chairs Initiative of the Department of Science and Technology and the
NRF.

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