MNRAS 529, 3484–3494 (2024) https://doi.org/10.1093/mnras/stad3821 Advance Access publication 2023 December 14 The disco v ery of a z = 0.7092 OH megamaser with the MIGHTEE sur v ey Matt J. Jarvis , 1 , 2 ‹ Ian Heywood, 1 , 3 , 4 Sophie M. Jewell, 1 , 5 Roger P. Deane , 6 , 7 H.-R. Kl ̈ockner, 8 Anastasia A. Ponomare v a , 1 Natasha Maddox , 9 Andrew J. Baker, 2 , 10 Alessandro Bianchetti, 11 , 12 Kelley M. Hess, 13 Hayley Roberts, 14 , 15 Giulia Rodighiero , 11 , 12 Ilaria Ruffa , 16 Francesco Sinigaglia, 11 , 12 , 17 Rohan G. Varadaraj , 1 Imogen H. Whittam , 1 Elizabeth A. K. Adams, 18 , 19 Maarten Baes , 20 Eric J. Murphy, 21 Hengxing Pan 1 and Mattia Vaccari 22 , 23 , 24 Affiliations are listed at the end of the paper Accepted 2023 December 6. Received 2023 December 6; in original form 2023 November 5 A B S T R A C T We present the disco v ery of the most distant OH megamaser (OHM) to be observed in the main lines, using data from the MeerKAT International Giga-Hertz Tiered Extragalactic Exploration (MIGHTEE) surv e y. At a newly measured redshift of z = 0.7092, the system has strong emission in both the 1665 MHz ( L ≈ 2500 L �) and 1667 MHz ( L ≈ 4.5 × 10 4 L �) transitions, with both narrow and broad components. We interpret the broad line as a high-velocity-dispersion component of the 1667 MHz transition, with velocity v ∼ 330 km s −1 with respect to the systemic velocity. The host galaxy has a stellar mass of M � = 2.95 × 10 10 M � and a star formation rate of SFR = 371 M � yr −1 , placing it ∼1.5 dex above the main sequence for star-forming galaxies at this redshift, and can be classified as an ultraluminous infrared galaxy. Alongside the optical imaging data, which exhibit evidence for a tidal tail, this suggests that the OHM arises from a system that is currently undergoing a merger, which is stimulating star formation and providing the necessary conditions for pumping the OH molecule to saturation. The OHM is likely to be lensed, with a magnification factor of ∼2.5, and perhaps more if the maser emitting region is compact and suitably of fset relati ve to the centroid of its host galaxy’s optical light. This disco v ery demonstrates that spectral line mapping with the new generation of radio interferometers may provide important information on the cosmic merger history of galaxies. Key words: masers – ISM: molecules – galaxies: ISM – galaxies: starburst. 1 H m o ( w p W s H c l s w 1 r m ( o � h 1 r o e e O s t e O ( i f t t t e o S w D ow nloaded from https://academ ic.oup.com /m nras/article/529/4/3484/7473713 by U niversity of W itw atersrand user on 12 Septem ber 2024 I N T RO D U C T I O N ydroxyl masers were disco v ered o v er fiv e decades ago, with the ajority of early detections coming from compact H II regions in ur own Galaxy (Weaver et al. 1965 ). Perkins, Gold & Salpeter 1966 ) were the first to interpret these lines as maser emission, hich provided an explanation for the high brightness temperature, olarization properties, and the line ratios. The following year, ilson & Barrett ( 1968 ) disco v ered OH emission from four infrared tars, although many more did not exhibit detectable OH emission. eiles ( 1968 ) also detected OH emission from interstellar dust louds, suggesting that OH emission arose from regions with a arge preponderance of infrared emission. As observations of the ky at radio wavelengths became more widespread, OH emission as disco v ered in e xternal galaxies (e.g. Baan, Wood & Haschick 982 ). These masers tended to be extremely luminous, and were eferred to as megamasers due to them being over a million times ore luminous than typical Galactic interstellar OH maser sources see Lo 2005 , for a re vie w). The OH molecule has four hyperfine transitions due to the coupling f the spin of the unpaired electron with the nuclear spin of the E-mail: matt.jarvis@physics.ox.ac.uk 4 g Published by Oxford University Press on behalf of Royal Astronomical Socie Commons Attribution License ( https:// creativecommons.org/ licenses/ by/ 4.0/ ), whi ydrogen atom. These transitions occur at 1612, 1665, 1667, and 720 MHz, with line ratios of 1:5:9:1 in local thermodynamic equilib- ium. The conditions necessary for maser emission include a source f energy that ensures that there are more molecules in the upper nergy level than in the lower, in order to produce the stimulated mission. Indeed, it has become clear o v er the past few decades that H megamasers (OHMs) are closely associated with galaxies with ignificant infrared emission, exhibiting a tight correlation between he far-infrared luminosity and the OHM luminosity (e.g. Baan t al. 2008 ; Wang et al. 2023 ). Theoretical models have shown that HM emission is efficiently produced at relatively high temperatures ∼80–140 K), with a minimum temperature of ∼45 K needed for nversion (Lockett & Elitzur 2008 ). Ho we ver, the dust around star- orming regions in external galaxies tends to be much cooler than his (30–50 K; e.g. Hwang et al. 2010 ; Smith et al. 2013 , 2014 ). Thus, o produce maser emission in these cooler environments means that he OH gas is likely co-located with the heating source. Willett t al. ( 2011a , b ) investigate the dependence of the OHM luminosity n the mid-infrared emission using data from the Spitzer Infrared pectrograph to argue that a large smoothly distributed dust reservoir ith temperatures from ∼50 to 100 K and high opacity ( τ ∼ 100– 00) is required for OHM emission. Ho we ver, there are key differences between using an infrared alaxy surv e y as a parent sample to look for masers (e.g. Norris et al. © The Author(s) 2023. ty. This is an Open Access article distributed under the terms of the Creative ch permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. http://orcid.org/0000-0001-7039-9078 http://orcid.org/0000-0003-1027-5043 http://orcid.org/0000-0003-4100-0173 http://orcid.org/0000-0001-8312-5260 http://orcid.org/0000-0002-9415-2296 http://orcid.org/0000-0002-4578-1205 http://orcid.org/0009-0006-9953-6471 http://orcid.org/0000-0003-2265-5983 http://orcid.org/0000-0002-3930-2757 http://orcid.org/0000-0002-9160-391X http://orcid.org/0000-0002-6748-0577 mailto:matt.jarvis@physics.ox.ac.uk https://creativecommons.org/licenses/by/4.0/ A z = 0.7092 OH megamaser in the MIGHTEE survey 3485 1 a o o s r n ( o f t e p h a G l l c r a T w b r p ( d P ( A f M H 2 d s O d 0 2 T b i o h b T t b H e S f p 1 m t o p r L D M f 1 b t t o e t p W m M c p t c m C 1 i I a a a i a t s s o H d i 3 3 W 0 1 c s b 1 https://archive.sarao.ac.za 2 https:// github.com/ ska-sa/ katdal 3 https:// github.com/ Mulan-94/ smops 4 https:// github.com/ ska-sa/ katbeam 5 http:// montage.ipac.caltech.edu/ D ow nloaded from https://academ ic.oup.com /m nras/article/529/4/3484/7473713 by U niversity of W itw atersrand user on 12 Septem ber 2024 989 ; Baan, Haschick & Henkel 1992 ; Stav ele y-Smith et al. 1992 ), nd a purely spectral line surv e y that can detect OHMs irrespective f the host galaxy properties (see e.g. Townsend et al. 2001 ). It is nly the latter that allow us to understand the full range of conditions ufficient to produce OHM emission. To carry out such surv e ys, one needs large spectral bandwidths at adio wavelengths, where the OH emission is detected ‘blindly’, with o pre-selection of samples such as (ultra)luminous infrared galaxies, U)LIRGS, and/or mergers (e.g. Darling & Giovanelli 2006 ). Many f the most successful untargeted surv e ys for OHMs are nominally ocused on detecting and understanding neutral atomic hydrogen via he 21 cm line (e.g. Suess et al. 2016 ; Haynes et al. 2018 ; Hess t al. 2021 ; Roberts, Darling & Baker 2021 ), due to its relative roximity to the OH maser lines. The vast majority of these surv e ys ave concentrated their efforts on surveying relatively large areas t low redshift in order to co v er a large cosmic volume (Darling & iov anelli 2000 , 2001 , 2002 ). Ho we ver, in order to reach beyond the ocal Uni verse, high sensiti vity is required, coupled with a relati vely arge bandwidth that allows a surv e y to co v er a significant amount of osmic volume by virtue of a deeper sampling in the radial direction, ather than broader areal co v erage. One of the key facilities that is ble to carry out this type of surv e y is MeerKAT (Jonas & MeerKAT eam 2016 ; Camilo et al. 2018 ), which couples very high sensitivity ith a wide bandwidth at both the L - and ultra-high-frequency (UHF) ands. Indeed, Glowacki et al. ( 2022 ) have already disco v ered a high- edshift ( z = 0.52) OHM using data from the MeerKAT telescope, as art of the Looking At the Distant Universe with the MeerKAT Array LADUMA) surv e y (Blyth et al. 2016 ), and Combes et al. ( 2021 ) etected satellite-line absorption against the distant radio source KS 1830-211 at z = 0.89 with known OH main-line absorption Chengalur, de Bruyn & Narasimha 1999 ), as part of the MeerKAT bsorption Line Surv e y (MALS; Gupta et al. 2016 ). In this paper, we report the disco v ery of the most distant OHM ound to date from an untargeted surv e y, using data from the eerKAT radio telescope as part of the MeerKAT International Gigz- ertz Tiered Extragalactic Exploration (MIGHTEE; Jarvis et al. 016 ) surv e y. In Section 2 , we pro vide details of the MIGHTEE ata and the calibration and imaging procedure used for creating the pectral line cubes. In Section 3 , we determine the properties of the HM and the host galaxy in which it resides and in Section 4 we iscuss our results and summarize our conclusions. Throughout the paper, we assume H ◦ = 67.7 km s −1 Mpc −1 , �m = .31, and �� = 0.69 (Planck Collaboration VI 2020 ). M I G H T E E OBSERVATIONS he MIGHTEE surv e y is one of the Large Surv e y Projects currently eing conducted by the MeerKAT radio telescope in South Africa. It s surv e ying approximately 20 de g 2 o v er four of the most widely bserv ed deep e xtragalactic fields accessible from the Southern emisphere. It is conducting the bulk of the surv e y using the L - and receiver, which covers the frequency range 856–1711 MHz. he Early Science data were taken with 4096 channels spanning he L band, which has enabled a broad range of science topics to e addressed using the radio continuum (e.g. Whittam et al. 2022 ; ale et al. 2023 ), spectral line (e.g. Maddox et al. 2021 ; Ponomare v a t al. 2021 , 2023 ), and polarization (e.g. B ̈ockmann et al. 2023 ) data. ubsequent observations for the MIGHTEE surv e y were taken at the ull resolution offered by MeerKAT after its correlator was upgraded, roviding 32 768 channels with a velocity resolution of 5.5 km s −1 at 420 MHz. The COSMOS field was observed by MeerKAT in 32k channel ode for a total of 15 × 8 h tracks in a tightly dithered mosaic hat spans around 2 deg 2 at ∼1.4 GHz, resulting in 94.2 h of n-field integration. The target-only visibilities for each of these ointings were retrieved from the SARAO archive 1 at full spectral esolution using the KAT Data Access Library, 2 and with the evel-1 calibrations applied, as derived by the SARAO Science ata Processor. The MIGHTEE spectral line processing divides eerKAT’s L band into three regions that are relatively free of radio requency interference (RFI), namely 960–1150, 1290–1520, and 610–1650 MHz. Each set of visibilities is split into these three sub- ands that are processed independently following Doppler correction o a barycentric reference frame. For the lowest frequency sub-band, he frequency domain is averaged by a factor of 4 to a resolution f 104.5 kHz, as we do not expect to detect low-velocity dispersion mission line sources in the low-frequency band, while the upper wo sub-bands retain the full resolution. Flagging of the visibilities is performed using the TRICOLOUR ackage (Hugo et al. 2022 ). Each sub-band is imaged using the SCLEAN software (Offringa et al. 2014 ), with a pointing-specific ask for the continuum sources derived from the existing deep IGHTEE continuum images (Heywood et al. 2022 ). The spectral lean component model is interpolated using the SMOPS 3 tool to rovide smoothness in the spectral domain. F ollowing inv ersion of his model into the visibility domain, a round of (phase + delay) self- alibration and simultaneous subtraction of the smoothed continuum odel is performed using the CUBICAL (Kenyon et al. 2018 ) package. Each pointing is then imaged on a per-channel basis using WS- LEAN using three robustness (0.0, 0.5, and 1.0) parameters (Briggs 995 ), and deconvolution masks are constructed from the resulting mage using a custom PYTHON tool (Heywood et al., in preparation). maging is repeated with deconvolution within the masked regions, nd the resulting per-pointing cubes are homogenized to a common ngular resolution per channel, using custom PYTHON code as well s the PYPHER package (Boucaud et al. 2016 ). These homogenized mages are primary beam corrected using the KATBEAM 4 library, nd then linearly mosaicked assuming variance weighting using he MONTAGE 5 toolkit. A final process of image–plane continuum ubtraction is performed using custom PYTHON code along every ightline through the resulting cubes. Full details of the procedure utlined abo v e and the custom methodology involv ed is pro vided in eywood et al. (in preparation). We also note that the MIGHTEE ata are taken in full polarization mode, details of which can be found n Taylor et al. (in preparation). T H E O H MEGAMASER J 0 9 5 9 0 3 . 2 2 + 0 2 5 3 5 6 . 1 .1 OH megamaser emission lines e visually inspected the low-frequency (960–1150 MHz) robust- .0 spectral-line cube, which has a spatial resolution of 0 × 15 arcsec 2 and a median rms sensitivity of ∼75 μJy per hannel (channel width of 104.5 kHz), o v er the COSMOS field to earch for high-redshift emission line galaxies. We disco v ered a right, unresolved source at RA = 09 h 59 m 03 . s 22 Dec. = 02 d 53 m 56 . s 1 MNRAS 529, 3484–3494 (2024) https://archive.sarao.ac.za https://github.com/ska-sa/katdal https://github.com/Mulan-94/smops https://github.com/ska-sa/katbeam http://montage.ipac.caltech.edu/ 3486 M. J. Jarvis et al. M Figure 1. The observed-frame spectrum of the OHM J095903.22 + 025356.1 from 960 to 1020 MHz. The rms noise across the spectral range spans 70– 80 μJy per 104.5 kHz channel across this range (see Heywood et al., in preparation for further details). Figure 2. The observed-frame spectrum of the OHM J095903.22 + 025356.1 (black solid line). The best-fitting ( χ2 red = 0 . 6) three-Gaussian model (blue dashed line) includes broad emission from a redshifted 1667 MHz line component (orange dotted line), along with narrow 1667 MHz (green dotted line) and 1665 MHz (red dotted line) emission lines. ( r l p s b a e t m p a a 6 s t Table 1. ( top ) Best-fitting parameters for the three-Gaussian fit to the emission lines, where the broad component is assumed to be the 1665 MHz line at the systemic redshift. ( bottom ) Best-fitting parameters for the three- Gaussian fit to the emission lines, where the broad component is assumed to be the 1667 MHz line that is redshifted with respect to the systemic velocity of the narrow lines. OHM properties with broad 1665 line z spec 0.7092 ± 0.0001 log 10 ( L 1667 / L �) 4.20 ± 0.02 log 10 ( L 1665n / L �) 3.53 ± 0.06 log 10 ( L 1665b / L �) 4.46 ± 0.03 FWHM 1667 /km s −1 172 ± 6 FWHM 1665n /km s −1 72 ± 8 FWHM 1665b /km s −1 917 ± 48 χ2 red 1.88 OHM properties with broad redshifted 1667 line z spec 0.7092 ± 0.0001 log 10 ( L 1667n / L �) 4.02 ± 0.02 log 10 ( L 1667b / L �) 4.54 ± 0.02 log 10 ( L 1665 / L �) 3.4 ± 0.05 FWHM 1667n /km s −1 118 ± 5 FWHM 1667b /km s −1 832 ± 27 FWHM 1665n /km s −1 60 ± 6 Velocity of inflow/km s −1 333 ± 20 χ2 red 0.60 t a t ( t a c p p f 2 a c w o T m E 1 i r fl fl 3 T a O D ow nloaded from https://academ ic.oup.com /m nras/article/529/4/3484/7473713 by U niversity of W itw atersrand user on 12 Septem ber 2024 J2000), and a frequency of ν = 975.31 MHz with a signal-to-noise atio (SNR) of ∼80 at the peak of the line. A second bright emission ine centred at ν = 974.1 MHz was also observed at the same osition on the sky (SNR = 51 at the peak of the line). The full pectrum, e xtracted o v er the restoring clean beam and normalized y the beam area, is shown in Fig. 1 and a zoom in on the 1665 nd 1667 MHz region is shown in Fig. 2 . These two lines correspond xactly to the rest-frame 1667 and 1665 MHz main emission lines of he OH molecule at z = 0.7092. Thus, this is the highest redshift OH ain-line megamaser in emission disco v ered to date, 6 eclipsing the revious record holder ( z = 0.52; Glowacki et al. 2022 ), which was lso disco v ered with MeerKAT. As can be seen in Fig. 2 , the 1667 MHz emission line is bright nd relatively narrow, and the 1665 MHz emission line also appears NRAS 529, 3484–3494 (2024) We note that the highest-redshift OHM in emission remains the tentative atellite-line detection of PKS1830-211 by Combes et al. ( 2021 ), which was argeted as part of the MeerKAT Absorption Line Surv e y. t d a b S o have a narrow component. However, there also appears to be broader underlying component around the 1665 MHz line. We herefore initially fit the emission lines with three Gaussian profiles two components for the 1665 MHz line and a single component for he 1667 MHz line), with the normalization, width, and redshift left s free parameters (assuming the redshift is the same for all three omponents). The resulting parameters are listed in Table 1 (top anel). The ratio of the luminosities of the 1667 and 1665 MHz lines rovides information on the physical conditions within the gas clouds rom which the OH emission arises (e.g. Darling & Giovanelli 002 ). Under the assumption of local thermodynamic equilibrium nd optically thin lines, we would expect a line ratio of 1.8, and onsidering only the narrow 1665 MHz component (3.4 × 10 3 L �), e measure a line ratio of 4.7, which is similar to the ratio observed in ther OHMs (e.g. McBride, Heiles & Elitzur 2013 ; Hess et al. 2021 ). he observed line ratio is compatible with the models in which the ain OH lines have non-negligible optical depths (e.g. Lockett & litzur 2008 ). Unfortunately, we have no constraints on the satellite line at 612 MHz as it falls below the lower end of the spectral co v erage n our L -band data, and we find no evidence for emission at the edshifted 1720 MHz line at 1006.4 MHz. We estimate the limiting ux for the 1720 MHz line of ∼150 μJy per beam (2 σ ) for the peak ux, and a corresponding integrated luminosity of log 10 ( L / L �) = .08 assuming a Gaussian line profile with FWHM = 120 km s −1 . his leads to a lower limit on the 1667/1720 ratio of > 3, which is gain within the range of line ratios exhibited in other lower redshift HM systems (McBride, Heiles & Elitzur 2013 ). Giv en the v ery different line widths for the broad component and he 1667 MHz line it is likely that the emission is coming from ifferent regions, one with high velocity dispersion and a region t much lower velocity dispersion. Large gaseous outflows have een observed in infrared luminous galaxies (e.g. Rupke, Veilleux & anders 2005 ; Spoon et al. 2013 ; Gowardhan et al. 2018 ). Therefore, A z = 0.7092 OH megamaser in the MIGHTEE survey 3487 Figure 3. Postage stamps of the OHM host galaxy in B -, g -, V -, r -, i -, and z-band filters from Subaru. The galaxy ∼2.6 arcsec to the west has a photometric redshift of z phot = 0.35 or 0.43 (see text) and is not associated with the OHM host galaxy. Note that the different filters have different depths and the colour scale is chosen to bring out the key features. The flux density of the OHM host galaxy for each filter is given in Table 2 . a d t a t T t a 1 b c 1 o χ w h b i p d a t f w p s l L s m m a e η w i t 1 O 2 w a 5 m t a a Figure 4. Three-colour ( B , V , and i ) image of the OHM host galaxy with the OHM emission denoted by the contours (40, 70, 100, and 140 Jy Hz). The OHM is unresolved at the spatial resolution of our data and as such the contours represent the restoring beam. s w m 4 l h e S o w 3 T m d W u w f t c S e D ow nloaded from https://academ ic.oup.com /m nras/article/529/4/3484/7473713 by U niversity of W itw atersrand user on 12 Septem ber 2024 possible second explanation for the broad component is that it is ue to a high velocity outflow (or inflow) in the 1667 MHz line. We herefore refit the emission line with an extra free parameter that llows a broad 1667 MHz component to have a different redshift o the systemic redshift. The best-fitting parameters are given in able 1 (bottom panel) and the fit is shown in Fig. 2 . In this case, he inflo w/outflo w velocity of the 1667 MHz line is ∼330 km s −1 nd provides a marginally better fit than the assumption of a broad 665 MHz line (reduced χ2 red = 0 . 6 compared to χ2 red = 1 . 88 for the road 1665 MHz component model). We note that more complex ombinations of broad and narrow line components for the 1665 and 667 MHz lines could replicate the observed spectrum. Ho we ver, ur current data do not support more complex models as the reduced 2 red = 0 . 6 for the broad velocity outflo w/inflo w model, suggests e are already o v erfitting the observations. To explore this further, igher spatial resolution would be required, which requires longer aselines than currently available to MeerKAT. Thus, we are limited n terms of what we can say about the underlying emission line rofiles, other than we find evidence for at least two distinct velocity ispersion components, and that we marginally prefer a model with broad component that is either inflowing or outflowing with respect o the systemic velocity. We cannot differentiate inflow from outflow rom the spectrum alone, due to the fact that we cannot determine hether the high-velocity component is in front of, or behind, the osition of the narrow-line components, which we assume to trace the ystemic redshift. Ho we ver, the large width of the broad component is ikely due to the large-scale motions of individual maser clouds (e.g. ockett & Elitzur 2008 ) and suggests a system which is undergoing ome level of disruption, possibly due to a major merger. This second odel for the emission-line components reaffirm it as the brightest ain-line OHM disco v ered thus far (see Table 1 for a summary of ll the derived properties). We are able to constrain the pumping efficiency, ηOH of the maser mission using the following ratio: OH = L OH × νIR L IR × νOH , (1) here L OH is the luminosity of the 1667 MHz OH line and L IR is the nfrared luminosity across the 53 μm line, which (supplemented by he 35, 80, and 120 μm lines) can pump the OH molecule (Elitzur 982 ; Lockett & Elitzur 2008 ). νOH and νIR are the widths of the H and IR lines. Using Arp 220 as a local analogue (e.g. He & Chen 004 ), we assume pumping line widths ∼200 km s −1 and equi v alent idths ∼ 0 . 04 μm, and determine the rest-frame continuum emission t 53 μm from the spectral energy distribution (SED) fit in Fig. (we note that it is very close to the observed frame 100 μm easurement from Herschel so is well constrained). We find that he 53 μm line has an estimated luminosity of 5.2 L �, leading to n OH pumping efficiency of �100 per cent for both the narrow nd redshifted broad components of the 1667 MHz line. This is not urprising given the extreme luminosity of the OHM, although as e discuss in Section 3.3 , differential lensing may also lead to a uch higher OHM luminosity relative to the host galaxy. Given the 5 K dust temperature inferred in Section 3.2 , this estimate is in ine with the finding of Kl ̈ockner ( 2004 ), suggesting that those OHM ost galaxies with cooler ( < 50 K) infrared emission exhibit greater fficiency ( �1 per cent), and that the maser emission is saturated. uch saturated maser emission is thought to arise from high-density, ptically thick regions ( > 10 5 cm −3 ) (Elitzur 1982 ; Darling 2007 ), hich are also some of the strongest maser sources. .2 Host galaxy properties he COSMOS field has been widely observed across the full electro- agnetic spectrum, we therefore have a wealth of multiwavelength ata with which to measure the properties of the OHM host galaxy. e have identified an optical source at the position of the OHM sing the COSMOS2020 catalogue (Weaver et al. 2022 ). In Fig. 3 , e show the B -, g -, V -, r -, i -, and z-band imaging around the OHM rom the Subaru telescope, and in Fig. 4 we show the emission from he OHM o v erlaid on a three-colour optical image. The host galaxy is lear alongside a galaxy 2.6 arcsec away (but at a lower redshift – see ection 3.3 ). The OHM host galaxy itself appears to have a stream of mission across all visible bands to the north, which may be indicative MNRAS 529, 3484–3494 (2024) 3488 M. J. Jarvis et al. M Table 2. Measured photometry for the OHM host galaxy from the COS- MOS2020 catalogue complemented with mid- and far-infrared data from WISE , Spitzer , and Herschel using products from the Herschel Extragalactic Le gac y Project (HELP; Shirley et al. 2021 ) and radio data from MIGHTEE and the VLA 3GHz surv e y. WISE filters with a † superscipt are not used in the SED fitting due to better similar wavelength data from Spitzer . Filter Ef fecti v e wav elength S ν σS ν ( μm) ( μJy) ( μJy) u 0.346 1.65 0.02 u ∗ 0.350 2.03 0.03 g 0.460 3.11 0.02 r 0.538 6.68 0.03 i 0.652 12.16 0.03 z 0.866 15.36 0.05 y 0.906 19.68 0.08 IRAC1 3.56 71.1 0.2 IRAC2 4.51 82.8 0.2 IRAC3 5.76 304 7 IRAC4 8.00 2076 80 W1 † 3.37 72.0 14.4 W2 † 4.62 99.2 19.8 W3 12.08 3003 600 W4 22.19 2813 560 MIPS 24 2785 347 PACS100 100 164 496 435 PACS160 160 151 030 540 SPIRE250 250 78 641 834 SPIRE350 350 36 268 1396 SPIRE500 500 8146 2807 Radio band Ef fecti v e frequenc y S ν σS ν ( μJy) ( μJy) L band 1.19 GHz 250 9 S band 3.0 GHz 118 7 o g e t o l a p W T u ( fl o u 2 M S i T i 7 8 h a m fi t d f o a a T d s a c d a i g i t G o i 0 t O d i r d m t a a a a b S s i i c t t M r ( t i t p b a fi D ow nloaded from https://academ ic.oup.com /m nras/article/529/4/3484/7473713 by U niversity of W itw atersrand user on 12 Septem ber 2024 f an interaction, i.e. a tidal tail from a merger e vent. Ho we ver, the round-based data are not high enough resolution to disentangle this mission and confirm whether it is due to a merger. Unfortunately, he OHM falls outside the HST co v erage of the COSMOS field. The ther possibility is that this elongated emission is due to gravitational ensing by the nearby galaxy in projection and lies at a lower redshift, nd we return to this possibility in Section 3.3 . The available u band through to Spitzer /IRAC photometry is resented in Table 2 , supplemented with mid-infrared data from the ide-field Infrared Survey Explorer ( WISE ) and the Spitzer Space elescope and far-infrared data from the Herschel Space Observatory sing the Herschel Extragalactic Le gac y Project (HELP) data base 7 Shirley et al. 2021 ). We are also able to measure the radio continuum ux from both the MIGHTEE data itself, with an ef fecti v e frequenc y f 1.2GHz at the position of the OHM (Heywood et al. 2022 ), and sing the VLA 3GHz surv e y of the COSMOS field (Smol ̌ci ́c et al. 017 ). 8 In order to determine the properties of the host galaxy, we use AGPHYS (da Cunha, Charlot & Elbaz 2008 ) to perform a fit to the ED, in which the energy absorbed in the UV part of the spectrum s balanced with the energy re-emitted at far-infrared wavelengths. he best-fitting SED along with the measured photometry is shown n Fig. 5 and the best-fitting parameters are presented in Table 3 NRAS 529, 3484–3494 (2024) https://hedam.lam.fr/HELP/ We note that the VLA 3GHz data may resolve out some emission from this ost galaxy, see e.g. Hale et al. ( 2023 ). A i a i d fter fixing the redshift to z = 0.7092, assuming a Chabrier initial ass function (Chabrier 2003 ). We are able to obtain a very good t to the vast majority of the photometric data (the exception being he measurement at 8 μm, which we return to below). The key erived properties suggest a rapidly star-forming galaxy with a star ormation rate (SFR) of 371 ± 20 M � yr −1 and a total stellar mass f log 10 ( M � /M �) = 10.5 ± 0.2, which together means that it lies pproximately 1.5 dex above the star-forming galaxy main sequence t this redshift (e.g. Whitaker et al. 2014 ; Johnston et al. 2015 ). he stellar mass derived from MAGPHYS is also consistent with that etermined in the COSMOS2020 catalogue, which reports a total tellar mass of log 10 ( M � /M �) = 10.31 ± 0.04. Ho we ver, we find much higher SFR compared to the value in the COSMOS2020 atalogue ( ≈50 M � yr −1 ), which is unsurprising given the level of ust extinction from the model fit ( A V = 2.94) and the significant mount of obscured star formation evidenced from the high far- nfrared luminosity of L IR = 3.6 × 10 12 L �, which confirms the host alaxy as an ULIRG. Such galaxy properties are similar to the host galaxies of OHMs n the local Universe, where there is a strong correlation between he OHM luminosity and the far-infrared luminosity (e.g. Darling & iovanelli 2002 , 2006 ; Zhang et al. 2014 ). To highlight the properties f this OHM, in Fig. 6 we show the OH luminosity against far- nfrared luminosity for a complete sample of OHM galaxies at z < .5 from the compilation Roberts & Darling (in preparation) and he OHM at z = 0.52 from Glowacki et al. ( 2022 ). The z = 0.7092 HM discussed in this paper is clearly the brightest OHM thus far isco v ered, b ut the far -infrared luminosity is also very high, such that t lies along the known correlation. Thus, although very luminous, its atio between far-infrared and OH luminosities is not significantly ifferent from the low-redshift populations. As mentioned earlier, one interesting detail in the SED is that the easured flux in IRAC Channel 4 at 8 μm is around 1.5 dex higher han the best-fitting SED. This is the only measurement that does not gree within the uncertainties with the best-fitting SED, and is such n extreme outlier it warrants discussion. We note that the galaxy is lso very bright in the W 3 filter, with a very red colour between the W 2 nd W 3 bands, thus confirming that the 8 μm emission is unlikely to e a spurious measurement. One possible explanation is that the total ED could be a combination of multiple galaxies within the point pread function (PSF) of the 8 μm data. Ho we ver, the only galaxy that s nearby is ∼2.6 arcsec away and does not appear to show a strong ncrease in flux from the 3.6 to 8 μm bands in the COSMOS2020 atalogue, where S 3.6 = 9.86 μJy and S 8 = 22.5 μJy (compared o the flux from the maser host galaxy of S 8 = 2076 μJy). We herefore rule out contamination from the projected nearby galaxy. oreo v er, as can be seen in Fig. 5 , the other mid-infrared bands all equire significant emission from polycyclic aromatic hydrocarbons PAHs); in particular the rest-frame 3.3 μm feature is needed to fit he 5.8 μm photometry. The presence of strong PAHs is not unusual n OHM host galaxies (e.g. Willett et al. 2011a ), again suggesting hat the OHM presented here is not anomalous and has very similar roperties to the vast majority of other OHM host galaxies. Another possible explanation for the excess emission in the 8 μm and could be hot dust emission from a torus around an accreting ctive galactic nucleus (AGN). We therefore use the CIGALE SED tting code (Boquien et al. 2019 ) to attempt to fit a composite GN + galaxy SED. We adopt parameters similar to those used n studies of radio-selected AGNs (Best et al. 2023 ; Zhu et al. 2023 ) nd more broadly selected galaxy samples (Zou et al. 2022 ), which ncorporate two models for AGN emission from both the accretion isc and the obscuring torus (Fritz, Franceschini & Hatziminaoglou A z = 0.7092 OH megamaser in the MIGHTEE survey 3489 Figure 5. SED of the OHM host galaxy with the best fit to the combined optical, near -infrared, and far -infrared SED, derived using MAGPHYS (black line). The blue line represents the intrinsic stellar spectrum that has been reddened with A V = 2.94 in order to fit both the optical and near-infrared data and the reprocessed dust emission at mid- to far-infrared wavelengths. The red solid circles are the observed photometry, and the open black circles denote the predicted photometry from the best-fitting model. The best-fitting parameters are listed in Table 3 . The residuals of the fit with respect to the data are presented in the lower panel.The point at 8 μm lies ∼1.6 dex above the best-fitting SED and is therefore not shown in the lower panel. Table 3. Best-fitting parameters for the SED from MAGPHYS . SFR in the radio is calculated assuming the measured spectral index between 1.2 and 3 GHz, and the flux at 3 GHz from VLA COSMOS. Host galaxy properties z phot 0 . 706 + 0 . 09 −0 . 08 log 10 ( M � /M �) 10.47 ± 0.2 SFR SED (M � yr −1 ) 371 ± 20 SFR Radio (M � yr −1 ) 342 ± 68 log 10 (sSFR SED / yr −1 ) −7.92 ± 0.6 log 10 ( L dust /L �) 12.56 ± 0.02 T dust / K 45 ± 2 log 10 ( M dust /M �) 8.27 ± 0.04 A V 2.94 ± 0.02 log 10 ( L 1 . 4GHz / W Hz −1 ) 23.90 ± 0.04 2 s i t m f t a d s S y Figure 6. The far-infrared–OHM luminosity relation for the sample of OHMs from Roberts & Darling (in preparation) (open circles), the OHM from Glowacki et al. ( 2022 ) (red star), and the OHM: J95903.22 + 025356.1, presented in this paper (blue star). The OHM luminosity shown is the combination of the broad- and narrow-1667 MHz emission line luminosities. The dashed line towards the light-blue diamond denotes where the OHM and host galaxy would lie if the OHM was magnified by a factor of 2.5 and the host by a factor of 2 (the two most likely lensing magnifications). u D S w D ow nloaded from https://academ ic.oup.com /m nras/article/529/4/3484/7473713 by U niversity of W itw atersrand user on 12 Septem ber 2024 006 ; Stalevski et al. 2012 , 2016 ), alongside stellar population ynthesis models from Bruzual & Charlot ( 2003 ). Ho we ver, there s no combination that can reproduce the high luminosity within he 8 μm band, while also still adequately fitting the range of other ultiwavelength data. We are likewise not aware of any spectral eature that could be boosting the 8 μm flux density at the redshift of he OHM system. In order to check whether there are any other signatures from n AGN component, we make use of the radio continuum imaging ata from both the MIGHTEE data and the VLA COSMOS 3 GHz urv e y (Smol ̌ci ́c et al. 2017 ). We measure a flux density at 1.2 GHz of 1 . 2GHz = 250 ± 9 μJy, which corresponds to a SFR = 342 ± 68 M � r −1 ; the uncertainty encompasses the systematic uncertainty of sing different conversions from 1.4 GHz to a SFR from Bell ( 2003 ), elhaize et al. ( 2017 ), and Delvecchio et al. ( 2021 ). Thus, the FR using the best-fitting MAGPHYS SED is completely consistent ith the SFR estimated from the radio continuum. Moreo v er, the MNRAS 529, 3484–3494 (2024) 3490 M. J. Jarvis et al. M Figure 7. Left: Data from the Subaru i -band image and o v erlaid white self-contours, with galaxy G1 labelled. Middle left: Median posterior model from the single lens model convolved with the PSF. The critical curve is overlaid in red and the median image–plane position of the source centroid is labelled ‘A’. A point source at this location has a magnification of μpt, A = 20.4, while the entire source has a magnification μtot = 1.8. Middle right: residual image (data – model) with data contours, critical curve, and point A indicated. Right: Source plane representation of the lensing with a circular Gaussian of radius R src = 0.25 arcsec. The caustic curve is shown in white and the image stretch ranges from the zero to the peak of the source-plane Gaussian. As can be seen, the centroid ‘A’ lies very near the cusp caustic. The scale-bar corresponds to the source plane. r o d p S c i r d s U w c o c w l c W s t t w t t D p d 3 G s i l s O 9 s t t h r h m z ( f o s m u i n a s h c a a e 2 p B e p a t o m ( s c D ow nloaded from https://academ ic.oup.com /m nras/article/529/4/3484/7473713 by U niversity of W itw atersrand user on 12 Septem ber 2024 adio continuum emission is unresolved at the highest resolution f both the MIGHTEE data ( ∼5 arcsec) and the VLA COSMOS ata ( ∼0.7 arcsec). Taken together, the radio emission does not ro vide an y indication that there may be a contribution to the ED from an AGN. 9 Therefore, although the SED fit and the radio ontinuum measurements are consistent, the excess emission at mid- nfrared wavelengths remains a puzzle and will require much higher esolution mid-infrared data to resolve, e.g. from JWST . We can also use the measurements from WISE and Spitzer /IRAC to etermine where the host galaxy of the megamaser resides in colour pace compared to the broader classifications of galaxy populations. sing the colour–colour plots from Roberts, Darling & Baker ( 2021 ), ho investigate where the host galaxies of masers should reside ompared to those of H I galaxies, we find that the host galaxy f OHM J095903.22 + 025356.1 has WISE and IRAC colours fully onsistent with the local OHM host galaxy population. Ho we ver, e note it has a relatively red W 2 − W 3 = 6.15 mag, meaning it ies at the extreme red end of the ULIRG population in the WISE olour–colour diagram of Jarrett et al. ( 2011 ). The extreme W 2 − 3 colour is partly due to the fact that the W 2 and W 3 bands are ampling the rest-frame ∼2.7 and ∼7.1 μm emission, and that at hese rest wavelengths, the PAH emission contributes significantly o the measured photometry, particularly in the W 3 filter. Ho we ver, e note the W 1 − W 2 = 0.714 colour is very much in the middle of he expected range for ULIRGs and starburst galaxies and lies within he colour range expected for OHM host galaxies (see e.g. Roberts, arling & Baker 2021 ; Glowacki et al. 2022 ), with this emission redominantly arising from old stellar populations, rather than warm ust and PAH emission. .3 Lensing iven the very high luminosity of the OH emission line and the tream of emission to the north of the OHM host galaxy (Fig. 3 ), t is worth assessing whether the system could be gravitationally ensed. The host galaxy mass and SFR are not extreme, ho we ver, uggesting that the host is unlikely to be significantly lensed. If the HM originates from a very compact region within the host galaxy, NRAS 529, 3484–3494 (2024) We note that the position of the OHM is not co v ered by Chandra X-ray urv e y o v er the COSMOS field (Ci v ano et al. 2016 ). 1 hen differential lensing could still provide significant magnification o the emission lines. There is a galaxy 2.6 arcsec away from the OHM ost galaxy (the galaxy lying directly to the west in Figs 3 , 4 , and 7 , eferred to as G1), which is present in the COSMOS2020 catalogue. It as a best-fitting photometric redshift of z phot = 0 . 43 + 0 . 02 −0 . 03 and stellar ass of log 10 ( M � /M �) = 9.56 using LE PHARE (Ilbert et al. 2006 ) and phot = 0.35 ± 0.01 and log 10 ( M � /M �) = 9.75 derived using EAZY Brammer, van Dokkum & Coppi 2008 ). The Einstein radius for a lens of this mass and redshift (similar or both photometric redshifts) is ∼1.1 arcsec, assuming a halo mass f M halo = 3.2 × 10 11 M �, estimated using an empirically derived tellar-mass to halo-mass ratio (Behroozi, Conroy & Wechsler 2010 ). The primary aim of our lens modelling is to estimate a plausible agnification factor range of the OH emission, which is spatially nresolved with the MeerKAT 10 × 15 arcsec beam. Our objective s to robustly constrain a macro lens model, rather than precise on-parametric reconstruction of the stellar light distribution. This pproach is taken for the following reasons: (i) the ∼0.7 arcsec eeing of the ground-based data limits the inference possible for this igh ellipticity, small Einstein ring foreground lens; (ii) the lack of onstraints from unambiguously identified multiply lensed images; nd (iii) the expectation that the source has an intrinsically complex, symmetric morphology that is typical of ULIRGS (e.g. Clements t al. 1996 ; Veilleux, Kim & Sanders 2002 ; Yuan, K e wley & Sanders 010 ; Larson et al. 2016 ). We perform our lens modelling analysis with the LENSTRONOMY 10 ackage (Birrer & Amara 2018 ; Birrer et al. 2021 ), enabling a ayesian approach to the lens model parameter estimation. We mploy the Particle Swarm Optimization non-linear fitting routine to rovide the starting point for the Markov chain Monte Carlo sampler, s described in Birrer, Amara & Refregier ( 2015 ). We explore two lens models: the first is the ‘single lens model’ with he closest galaxy (G1) as the only lens, with a projected separation f 2.4 arcsec, the second model, the ‘two lens model’, includes a ore distant galaxy (G2) which is 9.1 arcsec away to the north-west and can be seen in Fig. 4 ), and may perturb both the convergence and hear of the lensing system. G2 is also listed in the COSOMOS2020 atalogues and has a photometric redshift of z phot = 0.35, with stellar 0 https://github.com/lenstronomy/lenstronomy A z = 0.7092 OH megamaser in the MIGHTEE survey 3491 Figure 8. Magnification, μ, as a function of source-plane radius for a circular Gaussian located at the median value of the centroid posterior probability density functions, ( x 0 , y 0 ). The blue and red curves correspond to the single lens model and two lens model, respectively, with the latter showing systematically higher magnification. The ‘bump’ seen at 0.5–1 arcsec is a result of the source size enveloping the entire caustic, resulting in higher total magnification. m E d z t m l a a m i b t f w i n s f a c o o a i a d t T m a s r t b w a i t h e ( 1 2 e d l c T n t h a a ( i c c a b m c I a l 1 i c a o t o t S w e r i i i o T d t p c r r s 4 W O o c D ow nloaded from https://academ ic.oup.com /m nras/article/529/4/3484/7473713 by U niversity of W itw atersrand user on 12 Septem ber 2024 ass M � = 1.9 × 10 10 M � ( M halo = 8.8 × 10 11 M �), which has an instein radius of θE = 1.8 arcsec. Both models assume Single Isothermal Ellipsoid (SIE) mass ensity profiles for the foreground lens(es) and lens redshift(s) of = 0.35. Both models also assume lens light follows mass (i.e. he lens light and mass distrib utions ha ve co-located centroids and atching ellipticity and position angle). We first fit the G1 galaxy ight profile without considering any lensing (i.e. θE = 0), deriving S ́ersic index of n s = 3.74. We find that MCMC convergence for ll subsequent lens modelling (i.e. θE > 0) requires that we fix the ain lens light profile S ́ersic index to this value. We note that if there s a counter-image blended with the lens light, this approach may ias the lens light profile, and, hence the derived lens model. We est the sensitivity to this assumption by fixing the S ́ersic index at a e w v alues in the range 3 < n s < 4 and find consistent lens models ithin the statistical uncertainties. For the two lens model, the S ́ersic ndex of G2 is a free parameter, with a median posterior value of s = 2.31 ± 0.02. Both lens models assume a circular Gaussian ource, as we find large degeneracies in any model that deviates rom this simplistic assumption. The resultant residuals are large, s one would expect, but given our objective of macro lens model onstraints with the available data, we find this an appropriate level f model complexity. Using the single lens model we find a lens Einstein radius for G1 f θE = 1.32 ± 0.16 arcsec, while with the two lens model we find slightly lo wer v alue of θ = 1.16 ± 0.16 arcsec. The uncertainty s dominated by the systematics of lens model selection, and we ssign an indicative uncertainty as the difference between the two eri ved v alues. Both of the Einstein radius values are comparable o that derived from the estimated halo mass, θE, halo = 1.08 arcsec. his supports both the lensing hypothesis and the derived macro lens odels. In Fig. 7 , we show the results of the single lens model. In Fig. 8 , we show the lensing magnification expected for a source t the redshift of the OHM as a function of angular and physical ource size for both lens models. This shows that if the OHM is elatively compact ( � 1 arcsec), then the magnification could be of he order of μ ∼ 2.5–3.5 ± 1.0, where the uncertainty is assumed to e dominated by the systematic uncertainty between the two models, hich has an average of 36 per cent for radii between 0.05 and 3 rcsec. Even larger magnifications ( μ > 5) are possible if the OHM s very compact ( ≤0.1 arcsec) and the OHM happens to lie closer to he caustic than the centroid of the optical emission, as could easily appen in a complex merging system. The physical size of OHM mitting regions observed with Very Long Baseline Interferometry VLBI) show them to be compact (1–100 pc in size; Chapman et al. 990 ; Lonsdale et al. 1994 ; Diamond et al. 1999 ; Polatidis & Aalto 000 ; Kl ̈ockner, Baan & Garrett 2003 ; Rovilos et al. 2003 ; Pihlstr ̈om t al. 2005 ; Momjian et al. 2006 ; Baan et al. 2023 ), thus a higher egree of lensing is plausible. Ho we ver, e ven with this lensing magnification (and assuming no ensing of the more extended host galaxy) it would still remain onsistent with the L FIR –L OHM relation (see lower point in Fig. 6 ). his differential lensing of compact components near the host galaxy ucleus in a near cusp-cautic lensing configuration is very similar to hat seen in Deane et al. ( 2013a , b ), where the compact AGN core ad an order of magnitude higher magnification factor than the stellar nd cold molecular gas emission. We note that the lens model cannot ccount for all of the emission to the north of the OHM host galaxy see residual in Fig. 7 ). The most likely reason for this emission s due to an interaction, combined with the limitations of a simple ircular Gaussian source model. Given the likelihood of lensing of a compact OHM towards the entre of the host galaxy, it is worth revisiting the excess emission round 8 μm. A very compact region of hot dust could be both very right around the 8 μm region and, given the lensing probability, agnified in the same way as the OHM (or more, depending on how ompact the region is and where it reside with respect to the caustic). f we assume that the hottest dust is emitted from the inner parts of putative torus around a supermassive black hole, which is the most ikely source of such compact hot dust, then the typical radius is –10 pc (Tristram et al. 2009 ; Tristram & Schartmann 2011 ), similar n size to OHM emission regions. Thus, differential lensing of a very ompact hot dusty region could magnify the mid-infrared emission by similar amount to the OHM emission, and possibly more depending n the geometry and distribution of the dust. Again, it is very difficult o determine the actual amount of magnification with the resolution f the ground-based data, and this becomes even more difficult at he longer wavelengths, where we are currently reliant on WISE and pitzer data which have much poorer resolution than the visible- avelength data. Ho we ver, we note that such an additional source of mission at mid-infrared w avelengths w ould also likely remove the equirement to have such strong PAH emission as currently required n the SED fit shown in Fig. 5 and reduce the large residuals (shown n the lower panel of Fig. 5 ) at these mid-infrared wavelengths. This s because a significant fraction of the mid-infrared emission could riginate from the hot dusty torus, and would therefore be magnified. he degree of magnification would depend on both the temperature istribution and the spatial distribution of the dust. It is clear that here are many uncertainties around this system, both due to the ossible lensing and the likelihood of an ongoing merger, and the urrent data preclude us from investigating this further. Thus, higher esolution multiwavelength observations with JWST are needed to esolve the question of lensing in this object, and to understand the ystem more fully. C O N C L U S I O N S e have presented details of the most distant known OHM in the main H lines disco v ered thus far at a redshift of z = 0.7092. Analysis f the spectrum of the OHM suggests that there are two velocity omponents that produce the o v erall line profile, one in which the MNRAS 529, 3484–3494 (2024) 3492 M. J. Jarvis et al. M O s f b i O o r i w b c t n O l t a m d T a t I t h p O A T o f p t l m a w i t T o m p p p G a s s a o d a r a c U m s r ( O 2 l s U A W t b f D o s W t i f U v a ( F A t g a F t S o D f D N t A D ( o f ( o S R D 1 g u A m ( A w n 11 http://www .astropy .org D ow nloaded from https://academ ic.oup.com /m nras/article/529/4/3484/7473713 by U niversity of W itw atersrand user on 12 Septem ber 2024 HM gas has a relatively low velocity dispersion (FWHM ∼ 100 km −1 ) and from which we observe both the 1665 and 1667 MHz lines rom the OHM molecule. The second component is significantly roader, with a full width at half-maximum of 832 km s −1 and s redshifted with a velocity of 333 km s −1 relative to the narrow HM lines. This latter component could be either an inflow or utflow (depending on whether it lies in front of or behind the egion responsible for the narrow emission). However, the optical maging shows what appears to be a tidal-tail feature similar to hat is expected from a gas-rich merger, that cannot be explained y gravitational lensing. We therefore infer that the broad velocity omponent is dynamically separate from the galaxy from which he narrower OHM emission lines arise. The merger process also aturally produces sufficiently high infrared luminosity to pump the H molecules and produce the very high luminosity of the OHM ine ( L 1667 > 10 4 L �). We fit the host galaxy SED and find that the host is consistent with he properties of OHM galaxies in the local Universe, i.e. having high SFR (SFR ∼ 350 M � yr −1 ) and high dust oscuration. The easured SFR and stellar mass of the host suggest that it lies 1.5 e x abo v e the star-forming galaxy main sequence at this redshift. he close agreement between SFR estimated from the SED fitting nd the radio continuum measured at both 1.2 and 3 GHz suggests hat there is no evidence for AGN-related emission from the host. ndeed, we do not find a combined galaxy + AGN SED model hat could reproduce the measured photometry. Ho we ver, the OHM ost galaxy lies in close proximity to a galaxy 2.6 arcsec away in rojection (at z ∼ 0.35) which may be gravitationally lensing the HM emission (and possibly compact hot dust emission from an GN torus for example, around observed wavelengths of ∼8 μm). he lensing magnification depends on the exact size and location f the OHM emission region, with magnification factors μ ∼ 2.5 or emission regions on the ∼1 arcsec scale of the host galaxy, but otentially higher ( μ > 5) if the emitting regions are similar in size o OHM emission regions in local galaxies and displaced towards the ensing caustic. Ho we ver, the ground-based data preclude us from aking stronger statements on the level of magnification of the OHM nd the hot dust emission. We note that the gravitational lensing ould produce similar magnification factors to both the optical and nfrared data that trace the star formation and the near-infrared data hat traces the stellar mass, if they arise on similar spatial scales. hus the interpretation of a starburst galaxy still holds in the event f significant gravitational lensing. The disco v ery of this OHM, along with another high-redshift egamaser recently discovered by MeerKAT (Glowacki et al. 2022 ), oints towards a plausible new window on the obscured galaxy opulation at high redshift, providing an observational realization of revious proposals (Briggs 1998 ; Townsend et al. 2001 ; Darling & iovanelli 2002 ). The fact that two new megamasers have been found t z > 0.5 within just two of the MeerKAT fields thus far analysed uggests that many more will be disco v ered with the full MIGHTEE urv e y, which has a factor of ∼10 more area than analysed to date, lbeit at slightly lower sensitivity. Indeed, assuming approximately ne OHM per MeerKAT primary beam at ∼1 GHz, using the OHM iscussed in this paper and that found by Glowacki et al. ( 2022 ) as guide, then we should expect to find around 10–20 more high- edshift (0.45 � z � 0.8) OHMs o v er the full MIGHTEE surv e y rea (Jewell et al., in preparation). This effort should lead to new onstraints on the evolution of the most obscured systems in the niverse and possibly an independent measure of the gas-rich galaxy erger rate (e.g. Briggs 1998 ). These new disco v eries will come from pectral line surv e ys, meaning that the uncertainty in photometric NRAS 529, 3484–3494 (2024) edshifts, particularly for these v ery obscured systems, is ne gated although some uncertainty may remain around confusion between H and H I lines; Suess et al. 2016 ; Roberts, Darling & Baker 021 ). Therefore, as we mo v e to ev er deeper and wider spectral ine surv e ys spanning a large spectral bandwidth, three-dimensional pectroscopic information for some of the most dusty systems in the niverse will become available. C K N OW L E D G E M E N T S e would like to thank the anonymous referee for useful comments hat impro v ed the manuscript. The MeerKAT telescope is operated y the South African Radio Astronomy Observatory, which is a acility of the National Research Foundation, an agency of the epartment of Science and Innovation. We acknowledge the use f the ilifu cloud computing facility – www.ilifu.ac.za , a partner- hip between the University of Cape Town, the University of the estern Cape, Stellenbosch University, Sol Plaatje University, and he Cape Peninsula University of Technology. The ilifu facility s supported by contributions from the Inter-University Institute or Data Intensive Astronomy (IDIA – a partnership between the niversity of Cape Town, the University of Pretoria and the Uni- ersity of the Western Cape), the Computational Biology division t UCT, and the Data Intensive Research Initiative of South Africa DIRISA). MJJ acknowledges generous support from the Hintze amily Charitable Foundation through the Oxford Hintze Centre for strophysical Surv e ys. MJJ, IH, and AAP acknowledge support of he Science and Technology Facilities Council (STFC) consolidated rants [ST/S000488/1] and [ST/W000903/1] and MJJ, IH, SMJ, nd HP from a United Kingdom Research and Innovation (UKRI) rontiers Research Grant [EP/X026639/1], which was selected by he European Research Council. IH acknowledges support from the outh African Radio Astronomy Observatory which is a facility f the National Research Foundation (NRF), an agency of the epartment of Science and Innovation (DSI). RPD acknowledges unding by the South African Research Chairs Initiative of the SI/NRF (Grant ID 77948). AJB acknowledges support from the ational Science Foundation through grant AST-2308161 and from he Radcliffe Institute for Advanced Study at Harv ard Uni versity. B, FS, and GR acknowledge support from the Instituto Nazionale i Astrofisica (INAF) under the Large Grant 2022 funding scheme project ̀ MeerKAT and LOFAR Team up: a Unique Radio Window n Galaxy/AGN co-Evolution’). MV acknowledges financial support rom the Inter-University Institute for Data Intensive Astronomy IDIA), a partnership of the University of Cape Town, the University f Pretoria, and the University of the Western Cape, and from the outh African Department of Science and Innovation’s National esearch Foundation under the ISARP RADIOSKY2020 and RA- IOMAP + Joint Research Schemes (DSI-NRF grant numbers 13121 and 150551) and the SRUG HIPPO Projects (DSI-NRF rant numbers 121291 and SRUG22031677). This research made se of ASTROPY , 11 a community-developed core PYTHON package for stronomy (Astropy Collaboration 2013 , 2018 ). This research has ade use of the Cube Analysis and Rendering Tool for Astronomy CARTA; Comrie et al. 2021 ). This research has made use of NASA’s strophysics Data System. This research made use of Montage, hich is funded by the National Science Foundation under grant umber ACI-1440620, and was previously funded by the National file:www.ilifu.ac.za http://www.astropy.org A z = 0.7092 OH megamaser in the MIGHTEE survey 3493 A o A I D T f a a r p R A A B B B B B B B B B B B B B B B B B B C C C C C C C C d D D D D D D D D D D E F G G G H H H H H H H H I J J J J K K K L L L L M M M N O P P P P P P R R R S S S S S S S S S D ow nloaded from https://academ ic.oup.com /m nras/article/529/4/3484/7473713 by U niversity of W itw atersrand user on 12 Septem ber 2024 eronautics and Space Administration’s Earth Science Technol- gy Office, Computation Technologies Project, under Cooperative greement Number NCC5-626 between NASA and the California nstitute of Technology. ATA AVAILABILITY he MeerKAT visibility data for the MIGHTEE project are available rom the SARAO archive under proposal IDs SCI-20180516-KH-01 nd SCI-20180516-KH-02. 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