arXiv:2101.05818v1 [astro-ph.GA] 14 Jan 2021 egr,adterdhf nomto htcmsautomatic comes that dens information of gala with the regions OHMs and and of mergers, correlation 1997), strong associ- The al. 2007). et (Darling are Skinner gas 1991; they ac formation al. and star et nuclear (Henkel intense 2014), mergers, infra al. the es- with et ultra-luminous ated Zhang are for or ([U]LIRG; OHMs luminous signpost in 1999): a found well Baan is always (e.g. are wassentially which mechanism (OHMs) It pumping megamasers megamasers. FIR OH name with Galac- that the than correlated on them luminous early earning more recognized times , million OH ob a rare tic are relatively still majority are The e masers (Baan OH ago decades extragalactic four 1982), nearly discovery first their Despite Introduction 1. .M a e Hulst der van M. J. aur 8 2021 18, January Astronomy ⋆ elyM Hess M. Kelley [email protected] rgts Hmgmsr(H)kon emauealwrlimit lower a measure We known. (OHM) megamaser OH brightest epeettesrniiosdtcino h w anO mas OH main two the of detection serendipitous the present We z H aelt iei h aesuc.Wt oa Hluminos OH total a With source. same the in line satellite MHz aesaemre.Bsdo ulse nrrdadfrultr pu far maser log( and OH with of (ULIRG) to published models on line detailed Based satellite merger. compare OH late-stage to MHz which 1612 by the constraint for measured ratio limiting nec fteidvda anadstlieO 8c lines. cm 18 words. OH Key satellite AWES, and in main found individual be may the that of masers each OH in of number predicted conditi the physical on the with consistent are properties galaxy aais interactions Galaxies: .Mulder H. 12 11 10 ⊙ 9 8 6 7 5 4 3 2 1 = ikuiesti rnne etrfrIfrainTechn Information for Center Groningen Rijksuniversiteit no anke nttt,Uiest fAsedm Postb Amsterdam, of University Institute, Pannekoek Anton et fEetia niern,Camr nvriyo Te of University Chalmers Engineering, Electrical of Dept. ynyIsiuefrAtooy colo hsc,Univers Physics, of School Astronomy, for Institute Sydney SR srnm n pc cec,AsrlaTlsoeNat Telescope Australia Science, Space and Astronomy CSIRO sr pc etro eee hsclIsiue Profsoyu Institute, Physical Lebedev of Center Space Astro eateto hsc,Vrii oyehi nttt and Institute Polytechnic Virginia Physics, of Department et fAtooy nv fCp on rvt a 3 Ronde X3, Bag Private Town, Cape of Univ. Astronomy, A of Astronomy, Dept. and Physics Germany of Bochum, Faculty Bochum, University Ruhr USA 80309, of CO Department Boulder, Astronomy, UCB, Space and Astrophysics for Center aty srnmclIsiue nvriyo Groningen, of University Institute, Astronomical Kapteyn SRN h ehrad nttt o ai srnm,Pos Astronomy, Radio for Institute Netherlands the ASTRON, .Kutkin A. 0 . 91 nteutree pri ieae xrglci Su Extragalactic Wide-area Apertif untargeted the in 19612 & pri iwo h HMgmsrIA 10597+5926: IRAS Megamaser OH the of view Apertif Astrophysics 1 aes–Glxe:idvda:IA 10597 IRAS individual: Galaxies: – Masers .J Norden J. M. , H1 mstlielnsi ieae H wide-area in lines satellite cm 18 OH 1 2 1 , .H .M Coolen M. W. H. A. , 2 , L 6 ⋆ IR T .M Lucero M. D. , .Roberts H. , / L aucitn.ms_ohmaser_accepted no. manuscript ⊙ ) = 12 1 .C Oostrum C. L. , . 4 negigasa us iha siae trformation star estimated an with burst star a undergoing 24, 3 .Dénes H. , 7 Ra , 1 .J Wijnholds J. S. ff .Damstra S. , el Morganti aella 1 .Adebahr B. , 1 , 11 n epnil o eybih H msin ial,w pr we Finally, emission. OHM bright very for responsible ons .Ruiter M. , + voe ue,w n htteglx sa lr uiosinf luminous ultra an is galaxy the that find we fluxes, aviolet tivity jects. 96—Glxe:IM—Rdolns aais—Glxe:star Galaxies: — galaxies lines: Radio — ISM Galaxies: — 5926 ABSTRACT 1 al. t ally lg,PO o 14,90 AGoign h Netherlands the Groningen, CA 9700 11044, Box P.O. ology, red .Ivashina M. , tt nvriy 0Ws apsDie lcsug A2406 VA Blacksburg, Drive, Campus West 50 University, State ..Bx80 70A rnne,TeNetherlands The Groningen, AV 9700 800, Box P.O. xy hooy ohnug Sweden Gothenburg, chnology, ny t.84 Str. znaya t fSde,Sde,NwSuhWls20,Australia 2006, Wales South New Sydney, Sydney, of ity 1 s929 00G mtra,TeNetherlands The Amsterdam, GE 1090 94249, us e rlnsa 67ad16 H soitdwt RS10597 IRAS with associated MHz 1665 and 1667 at lines er ae Hstlieln esrmnspoieapotentially a provide measurements line satellite OH date. 1 n .Ziemke J. and , , t flog( of ity bs2 90A,Diglo h Netherlands The Dwingeloo, AA, 7990 2, tbus srpyia n lntr cecs nvriyo Color of University Sciences, Planetary and Astrophysical pn ehnss pia mgn hw h aayi likel is galaxy the shows imaging Optical mechanisms. mping 2 4 oa aiiy OBx7,Epn S 70 Australia 1710, NSW Epping 76, Box PO Facility, ional vy(WS,adtesbeun esrmn fteO 1612 OH the of measurement subsequent the and (AWES), rvey .A Moss A. V. , n ore therein). may regions sources emission compact and the in of found 30% is to emission Up nucleus. di the withi of be confined region is pc emission resoluti 100 maser arcsecond the a than that show greater have at that Those mega- studied OH Gree been dozen a have 2010; order masers of al. target- Only et 2012). Observatory, Fernandez Willett telescopes Telescope, by 2002, Parkes Bank Obse Giovanelli radio Arecibo 1992a; line & 1989; Darling al. dish al. et tory, et MHz Norris Baan single 1992; al. FT, 1667 et with Staveley-Smith 300 the sources NRAO IRAS (e.g. of bright detection ing the through e 2009). galaxy Roy hierarchical & McKean of 2005; test Lo a (e.g. hi thus models merger and lution the Universe, of the tracer valuable of a tory them make detection, a from n siaetettlnme fO aesta ilb detect be will that masers OH of number total the estimate and .Darling J. , oc 71 ot Africa South 7701, bosch tooia nttt ARB,Uiesttsrse150, Universitätsstrasse (AIRUB), Institute stronomical 1 o h 1667 the for or a Leeuwen van Joeri , h aoiyo xrglci Hmsr aebe found been have masers OH extragalactic of majority The ff s ihnte10p ein hl h aoiyo the of majority the while region, pc 100 the within use / L 2 197Mso,Russia Moscow, 117997 32, 10 / L .Mre Loose Marcel G. , ⊙ ) 3 / 8 .A .Adams K. A. E. , 1 62rtoof ratio 1612 = , , 9 12 , 3 1 . 90 .A Oosterloo A. T. , ± 0 aeo 179 of rate . 1 3 RS10597 IRAS 03, , 11 R i 1612 .J Vermaas J. N. , surveys 1 oehMaan Yogesh , 1 > , 2 ± 45 .J .d Blok de G. J. W. , 1 40 . , hc stehighest the is which 9 ∼ 2 ril ubr ae1o 9 of 1 page number, Article .Schulz R. , + M 0p nsz L 2005, (Lo size in pc 10 96i h fourth the is 5926 ⊙ yr vd nupdate an ovide 1 − .Vohl D. , 1 ae galaxy rared hs host These . 1 , 11 + d,389 ado, © valuable ,USA 1, us — burst 96at 5926 1 .Mika Á. , 44780 , S 2021 ESO 1 a y , ed 1 5 , , 2 , rva- vo- on. 1 s- n n , A&A proofs: manuscript no. ms_ohmaser_accepted

In fact, at radio , OH has two main lines at 1665 width of new and upgraded instruments undertaking these sur- and 1667 MHz and two satellite lines at 1612 and 1720 MHz veys will allow the simultaneous measurement of main and (Radford 1964). Extragalactic OHMs exhibit a broad range of satellite lines, while also detecting or putting limits on the H i 1667/1665 hyperfine ratios (from RH < 1 to RH > 10) with an emission and absorption strength (Gupta et al. 2020). Apertif average close to RH ∼ 5 (McBride et al. 2013). The variation is one of these upgraded survey machines: a new phased ar- observed in this property has been successfully modeled as a ray feed (PAF) instrument on the Westerbork Synthesis Radio combination of a clumpy maser medium of overlapping clouds Telescope undertaking commensurate , continuum, (Parra et al. 2005), and far-infrared line overlap with pumping and polarization surveys in the range 1130-1430 MHz primarily occurring through the 53 µm line (Lockett & Elitzur (Adams & van Leeuwen 2019; van Capellen et al., in prep; Hess 2008). The former explains how compact masers (with typically et al., in prep). In addition to directly detecting H i in galaxies high RH values) appear embeddedin a diffuse emission (with low out to z ∼ 0.07 (and through H i stacking or absorption out to the RH) in high resolution very long baseline interferometry (VLBI) bandwidth limit, z ∼ 0.2), the observations are capable of detect- observations (Lonsdale 2002). Together these models account ing extragalactic OH masers where the 1667 MHz line has been for how warmer dust is correlated with higher infrared luminosi- redshifted to 0.166 < z < 0.476. ties (Willett et al. 2011; Lockett & Elitzur 2008). In this paper, we present the untargeted, serendipitous detec- However, very little is known about the ratio between main tion of IRAS 10597+5926in both the 1667 and 1665 main lines, and satellite lines in OHMs. In local thermodynamic equilib- and a measurement of the 1612 MHz satellite line with a inte- rium (LTE), the emission line ratios between are expected to grated signal-to-noise ratio of 2.2σ which we treat as an upper be 1 : 5 : 9 : 1 for 1612 : 1665 : 1667 : 1720 MHz. Mea- limit. We use published infrared (IR) and far ultraviolet (FUV) suring the ratio of satellite to main lines provides a test for fluxes available in the NASA/IPAC Extragalactic Database1 to complex maser pumping models which predict the 1667/1612 calculate the host galaxy properties including rate (R1612) and 1667/1720 (R1720) ratios based on variation in the and stellar mass, and confirm that IRAS 10597+5926 is an ul- optical depth of the 1667 MHz line and the assumption that all tra luminous infared galaxy (ULIRG). The 1612 MHz emission lines have the same excitation temperature (Lockett & Elitzur measurement, is the highest 1667/1612 ratio lower limit mea- 2008; McBride et al. 2013, see also Willett et al. 2011). There sured to date, and the most stringent constraint on existing maser are only four extragalactic detections of 1612 MHz in emis- pumping models. This measurement demonstrates the power of sion which is a requirement to measure the 1667/1612 line ratio: upcoming untargeted, large-volume H i surveys, where the broad (Baan & Haschick 1987), Arp 299/IC694 and Mrk 231 bandwidth is valuable for simultaneously observing multiple OH (Baan et al. 1992a), and IRAS F15107+0724 (McBride et al. lines either directly or through the potential of OH stacking. The 2013). (A tentative detection of II Zw 96/IRAS 20550+1655 by detection of IRAS 10597+5926 also showcases the automated Baan et al. 1989 was refuted by McBride et al. 2013). Their cor- source finding/inspection tools under development which make responding measurements of the 1667/1612 ratio span R1612 ∼ discoveries within these surveys, and the harvesting of data from 7 − 29 over a relatively narrow range in 1667 MHz optical spectral line sources possible. depth (Baan et al. 1992a; McBride et al. 2013). In addition,M82 Throughout this paper we assume a ΛCDM cosmology with −1 −1 is detected with a complex mix of 1612 MHz emission and H0 = 70km s Mpc , ΩM = 0.27, ΩΛ = 0.73. absorption (Seaquist et al. 1997) and three extragalactic OH masing sources have been detected in 1612 MHz absorption: NGC 253 (Frayer et al. 1998); PKS 1413+135 (Darling 2004); 2. Apertif data PMN J0134-0931(Kanekar et al. 2005). In all of the cases when 2.1. Data reduction, imaging, and source finding 1612 MHz is detected in absorption, the 1720 MHz line is found in conjugate emission. In contrast, while Arp 220 is detected The area of sky containing IRAS 10597+5926 was observed in in 1612 MHz emission, the 1720 MHz line is seen in absorp- October 2019 (ObsId 191003042) as part of the ongoing Aper- tion (Baan & Haschick 1987). These complex cases have not yet tif wide-area extragalactic imaging survey, AWES. The Apertif been captured in the discussion of the maser pumping models. spectral line, continuum, and polarization surveys will be de- In addition to probing the state of the masing gas, satel- scribed in detail in Hess et al (in prep). In short, Apertif is a lite lines of OH are used to measure the universality of fun- phased array feed instrument upgrade to the Westerbork Synthe- sis Radio Telescope, which increases the field-of-view to 6.4- damental constants at high redshift (Varshalovich & Potekhin 2 1995; Darling 2003; Chengalur & Kanekar 2003). The four OH 8 deg and the spectral bandwidth to 300 MHz from 1130- lines have different dependencies on the fine structure constant, 1430 MHz. Every AWES survey pointing is observed for 11.5 2 hours. The data are reduced through an automated data reduc- α = e /~c, the electron-proton mass ratio, µ = me/mp, and tion pipeline called Apercal (Schulz et al. 2020, Adebahr et al, the proton gyromagnetic ratio, gp (Kanekar et al. 2005). Early quasar studies claimed a possible changing α (e.g. Webb et al. in prep) which produces cleaned multi-frequency continuum 2001). While no evidence for changing fundamental constants (Stokes I) images; polarization (Stokes V) images and (Stokes has been found to date from cm lines, only two OH sources are Q- and U-) cubes; and continuum-subtracted spectral line dirty known which have been used for such studies (PKS 1413+135, image cubes with 12.2–36.6 kHz resolution. The raw, calibrated, z = 0.24671; PMN J0134-0931, z = 0.765; Darling 2004; and imaged data from the first 12 months of observing (1 July 2019 - 30 June 2020) are publicly available as part of the first Kanekar et al. 2012, 2018), and a small number of sources us- 2 ing other lines (e.g. ammonia, Flambaum & Kozlov 2007; or Apertif data release (Apertif DR1; Adams et al, in prep). , Jansen et al. 2011). As part of ongoing Apertif work towards developing a cata- i Large volume radio spectral line surveys at 1.4 GHz, typ- log of H spectral line sources, the data was run through a spec- ically aimed at targeting the 21 cm line of neutral , 1 http://ned.ipac.caltech.edu; The NASA/IPAC Extragalactic Database H i, provide an unprecedented opportunity to discover and study (NED) is funded by the National Aeronautics and Space Administration the rare, but cosmologically important extragalactic OH mega- and operated by the California Institute of Technology. masers (e.g. Briggs 1998; Suess et al. 2016). The large band- 2 PID: http://hdl.handle.net/21.12136/B014022C-978B-40F6-96C6-1A3B1F4A3DB0

Article number, page 2 of 9 K. M. Hess et al.: Apertif view of an OH megamaser

1⊙⊙ 1⊙1 1⊙⊙

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Fig. 1. Top: spectrum of the 1665 and 1667 MHz main lines. Bottom: spectrum of the 1612 MHz satellite line. The 1667 MHz line center is provided by a fit to the line profile by SoFiA. The 1665 and 1612 MHz lines are plotted at the expected frequency based on the 1667 MHz line redshift. The orange/green/red shaded regions indicate the channel range used to calculate the intergrated flux of each line which correspond to widths of 433, 308, 138 km s−1, respectively. Grey shaded region in the bottom plot shows the ±1 sigma noise range of the smoothed data; Grey line indicates ±2 sigma. tral line source finding pipeline. The pipeline is still under de- tions. In this case the OH nature of the source was clear because velopment and will be described in a future paper. In short, we the 1665 MHz line was also clearly detected. A literature search use a well-tested software application, SoFiA3 (Serra et al. 2015) found that the 1667 MHz line was presented in an unrefereed to find candidates spectral line sources. SoFiA finds sources by IAU Conference Proceedings by Willett (2012), but the 1665 searching for emission at multiple angular and velocity resolu- MHz line was not discussed and may have been left unreported tions after smoothing the data cube with 3D kernels specified due to a poorly determined or perhaps over-subtracted baseline. by the user. At each resolution, voxels are detected if their ab- We note that unfortunately frequencies below ∼ 1300 MHz, solute value is above a threshold given by the user (in units of corresponding to the lower half of the Apertif band, are strongly the rms noise). The final 3D mask is the union of the 3D masks impacted by radio frequency interference from satellites. As a constructed at the various resolutions. Objects are cleaned to result, Apertif data to date are only processed between 1430- 0.5σ within masks defined by SoFiA encompassing the source. 1280 MHz, decreasing the effective available redshift range to The data cubes are compound beam-corrected using standard 0.16599 < z < 0.30262 for OH line detections. Table 1 lists the tasks in the Miriad software package (Sault et al. 1995, 2011), redshift range in which each of the OH maser lines are visible to and adjusted to the NVSS flux scale (see documentation in the Apertif. Apertif data release). Final data products which include mo- ment maps and spectrum plots are created using Astropy v2.0 (Astropy Collaboration et al. 2018). The final data products are 2.2. Post-processing then inspected by-eye and unusual objects are flagged for poten- In the field of IRAS 10597+5926 are two bright continuum tial science interest. sources at roughly 770 ± 15 mJy (20 arcseconds away) and In the context of OH masers, the 300 MHz frequency range 580 ± 10 mJy (30 arcseconds away) which are not perfectly covered by the Apertif surveys provide a window to observe the modeled by the calibration in the Apercal pipeline, leaving faint 1667 MHz line between of 0.16599 < z < 0.47554. concentric rings around the continuum sources. These rings vary This redshifted OH maser line can effectively masquerade as an slowly in frequency and are faintly evident even in the line cubes. H i source in the nearby Universe with similar flux and To remove the artefacts, we mask the OH maser source and re- line widths (e.g. Briggs 1998; Suess et al. 2016; Haynes et al. fit the baseline at every spatial pixel in the cubes with a 5-knot 2018). A spectral line source finding pipeline works effectively natural cubic spline4 along the frequency axis. The line cubes for both H i and OH lines because it cannot distinguish between consist of 1216 channels of approximately 7 km s−1, so a spline the two. In this way, we serendipitously detected the 1667 MHz segmentspans more than 1700 km s−1, ensuring that every spline line of IRAS 10597+5926 (J2000 11:02:46.9 +59:10:37), red- segment is dominated by emission free channels. The resulting shifted to z⊙ = 0.19612, while inspecting candidate H i detec- data cube has a noise of 1.27 mJy beam−1.

3 https://github.com/SoFiA-Admin/SoFiA 4 https://github.com/madrury/basis-expansions/

Article number, page 3 of 9 A&A proofs: manuscript no. ms_ohmaser_accepted

Table 1. Range of redshifts for OH maser lines visible to Apertif

OHLine 1430MHz 1280MHz 1130MHz 1612MHz 0.12743 0.25956 0.42675 1665MHz 0.16462 0.30110 0.47381 1667MHz 0.16599 0.30262 0.47554 1720MHz 0.20316 0.34416 0.52259

Notes. Apertif observes frequencies up to 1430 MHz which defines the low redshift limit of the Apertif bandwidth; 1280 MHz is the redshift upper limit for frequencies relatively unimpaired by interference; 1130 MHz is the low frequency (high redshift) limit for the Apertif band- width.

The source finding pipeline was then re-run on the new data cube, which identified the 1667 MHz and 1665 MHz emission as separate line sources. For the subsequent analysis we extracted a pixel spectrum at the location of the 1667 MHz peak in the total intensity map. The spectrum was then boxcar smoothed by 3 channels (top panel of Figure 1). For the redshift of the system, we take the line center calculated by SoFiA for the 1667 MHz line and use it to calculate the expected frequencies for the 1665 Fig. 2. Continuum contours 6, 12, 24×10−4 Jy beam−1 on a PanStarrs MHz and 1612 MHz lines. To calculate the line flux we integrate g-band image. The radio source is unresolved by the 13 × 17 arcsec − the pixel spectrum over ±27 channels (433 kms 1) of the 1667 beam of Apertif (hatched grey ellipse in the bottom left corner). The MHz line center, and ±19 channels (308 kms−1) of the 1665 continuum image has a noise of 33 µJy beam−1. The disturbed optical MHz line which correspond to the widths of the SoFiA masks appearance is consistent with the galaxy in the late stages of a merger. for each source. predicted location of the 1612 MHz satellite line. The peak of 2.3. 1612 MHz data the spectrum within the estimated profile width is measured to be 2.3 sigma above the noise, while the integrated line profile Based on the systemic redshift measured by SoFiA, we calcu- over the same width has an SNR of 2.2. The calculations of the lated that the 1612 MHz satellite line of IRAS 10597+5926 integrated SNRs takes into account the correlated noise due to would also fall in the Apertif bandwidth,redshifted to 1347.9469 smoothing the spectra. Only the 1720 MHz satellite line remains MHz. We applied the same spline fitting technique to this data outside the frequency range of Apertif at 1438 MHz. cube to remove residual artefacts. The resulting cube has a noise Given our chosen cosmology, IRAS 10597+5926sits at a lu- of 1.30 mJy beam−1. We extracted the pixel spectrum at the posi- minosity distance of D = 963 Mpc. We assume isotropic emis- tion of the peak of the 1667 MHz total intensity map. The spec- L sion and measure the luminosity in the 1667 and 1665 MHz lines trum was then boxcar smoothed by 5 channels (bottom panel to be log(L/L⊙) = 3.83 ± 0.01 and 3.09 ± 0.04, respectively, of Figure 1). To calculate the line flux we integrate over ±8 and a 1612 MHz upper limit of log(L/L⊙) ≤ 2.2 ± 0.2. Our er- channels (138 kms−1) of the expected 1612 MHz line center. rors reflect the uncertainty in the flux measurements based on This corresponds, subjectively, to a symmetric extent roughly the noise in the spectra. The total luminosity in the 1665 and the width of the brightest part of the 1665 and 1667 MHz lines, 1667 lines, log(L/L⊙) = 3.90 ± 0.03, makes IRAS 10597+5926 and where we would expect the 1612 MHz line to be brightest if the 4th most luminous OH maser source known, and places it on it is present (see also the gray shaded region in Figure A.1). the margin of classification as an OH gigamaser. Our flux seems We have done a number of tests to understand the signifi- to be in good agreement with that of Willett (2012) who mea- cance of a measurement at the exact location of the predicted sure log(L/L⊙) = 3.85 for the 1667 MHz line using the GBT, al- 1612 MHz line, including correlating the Apertif spectrum with though the cosmological parameters used to calculate their value different matched filters, and conducting a Bayesian analysis are not published. with a strong redshift prior. These tests are described in Ap- Figure 2 shows an optical PanSTARRS5 (Chambers et al. pendix A. The matched filter tests modestly strengthen the case 2016) g-band image with the Apertif 1.36 GHz radio contin- for a detection of the 1612 MHz line. However, the Bayesian uum contours. The radio continuum image has an rms of 33 µJy analysis suggests that there is roughlya 1 in 3 chance of a detec- beam−1 at the location of the IRAS 10597+5926 and the contin- tion at that location given the noise of the spectrum. As a result, uum source associated with the maser is a 6±1 mJy point source we describe our measurement of the integrated flux of the 1612 at the resolution of Apertif, 13 × 17 arcsec. The ground-based MHz line as an upper limit. images show a disturbed optical morphology which we interpret as an advancedmergeras is seen in a majorityof ULIRG sources 3. Results for which there is sufficient imaging (e.g. Clements et al. 1996; Farrah et al. 2001). 2 We measure all OH maser emission lines of the Π3/2(J = 3/2) Among powerful OHMs, IRAS 10597+5926 is unique state that are within in the Apertif bandwidth: the 1612, 1665, in that the 1667 and 1665 MHz lines are relatively narrow and 1667 MHz lines, redshifted to z⊙ = 0.19612. The two main and distinct, whereas the brightest gigamasers show blended lines are clearly visible in the top panel Figure 1. They have emission features for the 1667 and 1665 MHz lines span- an integrated signal-to-noise ratios (SNR) of 12 and 57, respec- tively. The bottom panel of Figure 1 shows the spectrum and 5 https://panstarrs.stsci.edu/

Article number, page 4 of 9 K. M. Hess et al.: Apertif view of an OH megamaser ning over 1000 kms−1 (Staveley-Smith et al. 1989; Baan et al. The lower limit to the total IR luminosity, which is calculated 1992b; Darling & Giovanelli 2001). The brightest known IR exclusively from the 60 and 100 µm far IR bands, firmly qualifies 14070+0525 spans 2400 kms−1. Baan et al. (1992b) suggests IRAS 10597+5926 as a ULIRG (Houck et al. 1985). Although this is either the result of a violent merger, an AGN outflow, or we calculate a rangeof star formationrates based onfar IR+FUV −1 −1 a 500 kms rotating disk around a supermassive blackhole in or mid-IR, both estimates put the rate at well over 100 M⊙ yr , which the 1667 and 1665 lines are blended (Baan & Klöckner and indeed the WISE 3.4-4.6 µm (W1-W2 = 0.987 ± 0.053) and 2001). We note that the 1667 MHz line appears to have two dis- 4.6-12 µm (W2-W3 = 4.315 ± 0.058) colours place it firmly in tinct peaks, however VLBI imaging is required to say anything the range of starbursting galaxies (Cluver et al. 2014). This is about the configuration of the masing material, or narrownessof consistent with the physical conditions responsible for OHM ac- the OH lines in IRAS 10597+5926. tivity.

4.2. OH line ratios 4. Discussion In local thermodynamic equilibrium (LTE), the OH lines are ex- This first serendipitous OHM detected from the AWES survey of i pectedto haveratios of 1612 : 1665 : 1667 : 1720 = 1:5:9:1 Apertif demonstrates that future wide-area H surveys will not (Radford 1964; Robinson & McGee 1967; Henkel et al. 1991). only be a useful way to discover new extragalactic OH masers However a broad range of values has been measured in mega- (Briggs 1998), but with their broad spectral bandwidth, provide masers with the highest resolution images revealing that com- simultaneous detections or constraints on OH satellite lines. Be- pact emission tends to have 1667/1665 emission ratios closer to low we present the host galaxy properties, we discuss the prop- R = 5 (Lonsdale 2002). erties of the OHM in the context of what is known about satellite H For IRAS 10597+5926, we find (limiting) line ratios of lines, and we present a prediction for the discovery space which 1:8.3 : 45.9. This corresponds to a hyperfine ratio between will be revealed with AWES. 1667/1665 of RH = 5.5±0.5. The 1667/1612 ratio, R1612 > 45.9, is the largest line ratio limit ever measured for this satellite 4.1. Host Galaxy Properties line. Figure 3, adapted from McBride et al. (2013), shows our upper limit (red arrow) versus all existing measurements and The infrared (IR) luminosity and star formation rate (SFR) of limits in the literature7. For direct comparison, we also calcu- the host galaxy are tied to the physical conditions thought to be late an upper limit (black arrow) based on the methodology + responsible for OHM activity. IRAS 10597 5926 is a very lu- of McBride et al. (2013), who use F1612 = σ1612 ∆ν1667 where minous OHM, so one would expect it to be very luminous in σ1612 is the rms of the 1612 MHz spectrum and ∆ν1667 is the the mid- and far-IR. We use publicly available archival data to width of the 1667 MHz line containing 75% of the line emis- confirm the extreme IR and SFR properties of this system. sion. For IRAS 10597+5926, ∆ν1667 is ±13 channels–more than IRAS 10597+5926 is detected in FUV by GALEX 60% wider than our estimate. The black arrow falls within the (Buat et al. 2007), in all four WISE infrared bandsimages at 3.4, uncertainty of our estimate, but we note that this method is (1) 4.6, 12, 22 µm (Zhang et al. 2014), and in IRAS 60 and 100 µm independent of the properties of spectrum at the predicted loca- band images (Moshir & et al. 1990). For the WISE bands we use tionof theline and(2) independentofthe shapeof the 1667MHz k-corrected fluxes based on a galaxy template and SED fitting line, which may suggest a more individualized approach. In fact, (Jarrett et al. 2019). For IRAS bands this is not available so for if we integrate the 1612 MHz spectrum over ±13 channels, we them and the GALEX data we take into account a zeroth order get the same lower limit as indicated by the red arrow. k-correction of 1/(1 + z) dependence with redshift and represent The solid black line in Figure 3 shows the predicted line 2 the additional uncertainty in the error. ratios in “color-color” space if all OH lines in the Π3/2(J = We estimate the star formation rate (SFR) using two meth- 3/2) state have equal excitation temperatures (Lockett & Elitzur ods from independent subsets of the available IR and UV data. 2008; McBride et al. 2013). If the R1612 ratio deviates from this First, we calculate the total IR (TIR) flux from IRAS as out- line, it may indicate secondary maser pumping mechanisms are lined in Sanders & Mirabel (1996). IRAS 12 and 25 µm fluxes important and should be explored in future models. On face are upper limits. Ignoring these values we find a lower limit to value, our measurement is among the strongest constraints that be log(LTIR/L⊙) > 12.24. By including them we find an up- OHMs may still be globally consistent with the equal excitation per limit to the total IR luminosity of log(LTIR/L⊙) < 12.37. temperature hypothesis, although a deviation from this is also In fact, the WISE W3 and W4 fluxes suggest the 12 and 25 within the uncertainties of the measurement. µm contribution may be an order of magntiude lower than that suggested by the above mentioned upper limit. To calculate the SFR, we combine Equations 11 and 12 of Kennicutt & Evans 4.3. Updated predictions for Apertif, and a word on 1612 6 co-detections and stacking. (2012) to derive a FUV + IR SFR from the LTIR lower limit of ± −1 179 40 M⊙ yr . Alternatively, using the k-corrected WISE The Apertif surveys have recently been guaranteed observing 22 µm band (W4) and calibration from Cluver et al. (2017), we time through December 2021, for a total of 30 months of sur- ± −1 find an SFR of 108 42 M⊙ yr . We note that W4 is a better vey operations. Previous papers have estimated the Apertif OHM estimate of SFR than W3 in ULIRGs because of, among other 1667 MHz detection rate based on specifications before the sur- things, strong Si dust absorption in the W3 band (T. Jarrett, pri- veyswere operational (e.g. McKean & Roy 2009). Here, we pro- vate communication). Finally, we estimate a galaxy stellar mass based on the WISE 3.4 µm (W1) and the mass-to- ratio from 7 We note that the original McBride et al. (2013) plot had two black Cluver et al. (2014) to be log(M∗/M⊙) = 10.65 ± 0.14. points despite their table only listing one detection. Ultimately a wig- gle in the spectrum of IRAS 11028+3130 was deemed unreliable and 6 See also Murphy et al. 2011; Hao et al. 2011 for the constants used updated in the table as an upper limit, but accidentally left in the plot in these equations. (J. McBride, private communication). We have corrected it here.

Article number, page 5 of 9 A&A proofs: manuscript no. ms_ohmaser_accepted

Table 2. OH line detection predictions for AWES to 2020 and beyond. 1 1 OH Line Line AWESb AWESb,c 1.4x AWESb,c a 2 2 2 1 MHz Ratio 1500 deg 2500 deg 1500 deg (1) (2) (3) (4) (5) +1 +1 +1 1612 20 2−1 4−1 5−1 +4 +6 +5 11 1665 5 17−3 28−5 25−4 +17 +27 +24 1667 1 76−13 127−21 103−19 +0.2 +0.4 +1 1720 20 0.5−0.2 0.9−0.3 2−1 111 1 Notes. aThe assumed line ratio is in relation to the primary 1667 MHz line and is the median line ratio from McBride et al. (2013). bThe num- ber of detections are calculated for the frequency range 1280-1430 c 11 MHz. For AWES projections beyond 2020 which may shift the fre- 1 11 quency coverage (see text), we predict Apertif would detect an addi- 11 tional ∼12-20 (4-6) OHMs in the 1667 (1665) MHz line and an ad- ditional ∼1-2 OHMs in each of the satellite lines between 1430-1530 Fig. 3. The OH color-color plot for the 1612 MHz line, adapted from MHz, on top of what is listed above. McBride et al. (2013). Blue squares come from the literature, collated by Baan et al. (1992a) in their Table 2(a); black circle and gray arrows are the detection and upper limits from McBride et al. (2013), respec- ing coverage of approximately 1500 deg2 to get 1.4 times more tively. The red arrow is the RH ratio and R1612 upper limit from this sensitivity in spectral line maps. In both future cases, we also work; cyan cross shows our uncertainties; the black arrow is the up- estimate the number of OHMs that would be detected between per limit calculated using the McBride et al. (2013) methodology (see 1430-1530 MHz in 1000-1500 deg2 assuming the Apertif fre- Section 4.2). The black line is the predicted ratios assuming equal ex- quency coverage will shift to avoid RFI beyond 1280 MHz (see citation temperatures for all OH lines, with varying 1667 MHz optical caption). depth (Lockett & Elitzur 2008; McBride et al. 2013). We estimate the total number of Apertif 1667 MHz OH megamaser detections to be of order 110-150 (depending on the vide an update on the expected number of detections for AWES 2021 strategy and including a broader freqeuency range in that based on Roberts, Darling, & Baker (2021; submitted). 12 months), increasing the number of known OHMs by over 100%. The large error bars are due to the limited number of The estimate combines the OH luminosity function and as- known OHMs so as more are discovered, these predictions can sumed merger evolution rate, with the redshift range, area, and be tightened. sensitivity of the radio survey. The average OH line has a width This prediction is much higher than the approximately 19 of about150km s−1 which covers roughly ∼23 Apertif channels. OHMs that have been reported by the Arecibo Legacy Fast We assume the data quality is consistent with the first data re- ALFA Survey (ALFALFA; Haynes et al. 2018) which covered lease which achieves an average noise of approximately 1.3 mJy ∼ 7000 deg2. However, it is very likely that ALFALFA has yet beam−1 in the H i cubes at 36.6 kHz spectral resolution, and we to discover all potential OHMs and Suess et al. (2016) present require the OH megamasers to be 5σ detections. Though there is 60 ambiguous sources detected by ALFALFA’s 40% data release much speculation of the exact value, we assume a alone. Further, Apertif’s sensitivity is about a factor of two im- evolution rate (γ) of 2.2 (Lotz et al. 2011). This value is associ- proved over ALFALFA (1.86 mJy beam−1 over 48.8 kHz after ated with major mergers which is the source of most OHMs. Hanning smoothing; Haynes et al. 2018): an important consid- In addition, an extrapolation of the calculation for the 1667 eration for detecting OHMs since they must be redshifted, and MHz detections can be done to determine the number of satel- therefore fainter, to be at the proper frequency for detection. lite lines that may be detected by Apertif. Assuming all OHMs For undetected 1612 MHz lines, one may attempt to stack produce every satellite line and the median line ratios from OHMs in bins of R , redshift, or other physically meaningful pa- / H McBride et al. (2013), we can say 1667 1665 line ratio is about rameters. Given the expected number of 1667 MHz detections, / / 5 and that the 1667 1612 and 1667 1720 ratios are both around and assuming the noise goes down as the square root of the num- 20. This means we need 5 times more sensitivity for the 1665 ber of objects, we expect stacking would modestly improve the MHz line and 20 times more sensitivity to detect 1612 and 1720 detection threshold by a factor of a few in each bin. MHz lines. Using this and the adjusted redshift ranges (see Table 1), we can predict the number of detections for each line. The exact survey plan for 2021 is under discussion, so we 5. Conclusions present the numbers for the survey observing to through the end of 2020, and two possible strategies for 2021 (Table 2). Column We present the Apertif Wide-area Extragalactic Survey OH (1) is the frequency of the OH line in MHz. Column (2) is the megamaser detection of IRAS 10597+5926. The main lines at assumed line ratio with respect to the 1667 MHz line and is the 1667 and 1665 MHz were found redshifted to z⊙ = 0.19612 by median line ratio from McBride et al. (2013). Column (3) is the an automated spectral line source finding pipeline. The optical number of OHMs expected from AWES observations in the first image suggests IRAS 10597+5926 is a late-stage merger. From 18 months of the survey through the end of 2020 covering about infrared imaging, we show that the host galaxy is a ULIRG, and 1500 deg2. Column (4) is the total number of OHMs assuming estimate a log stellar mass of 10.65 ± 0.14 and star formation −1 that AWES continues to build up sky coverage in the same way rate up to 179 ± 40 M⊙ yr . We measure the total OH lumi- as it has for the previous 18 months for an estimated coverage of nosity to log(L/L⊙) = 3.90 ± 0.03, putting it on the cusp of the 2500 deg2. Column (5) assumes that AWES re-observes exist- highest luminosity OH sources, OH gigamasers. The extreme

Article number, page 6 of 9 K. M. Hess et al.: Apertif view of an OH megamaser star formation rate and infrared luminosity are consistent with Baan, W. A., Haschick, A., & Henkel, C. 1992a, AJ, 103, 728 the properties of the brightest known OHMs to date. Baan, W. A. & Haschick, A. D. 1987, ApJ, 318, 139 Baan, W. A., Haschick, A. D., & Henkel, C. 1989, ApJ, 346, 680 The untargeted detection by AWES demonstrates the natu- Baan, W. A. & Klöckner, H.-R. 2001, in Astronomical Society of the Pacific ral ability of wide-area H i surveys with broad frequency band- Conference Series, Vol. 249, The Central Kiloparsec of Starbursts and AGN: width and complimentary redshift range to benefit spectral line The La Palma Connection, ed. J. H. Knapen, J. E. Beckman, I. Shlosman, & T. J. Mahoney, 639 searches for OH megamasers. We predict that of order 110-150 Baan, W. A., Rhoads, J., Fisher, K., Altschuler, D. R., & Haschick, A. 1992b, sources will be found in the AWES survey, and speculate that of ApJ, 396, L99 order 4-6 OH sources may be detected in satellite lines. This Baan, W. A., Wood, P. A. D., & Haschick, A. D. 1982, ApJ, 260, L49 Briggs, F. H. 1998, A&A, 336, 815 increases the number of both known OHMs and OH satellite Buat, V., Takeuchi, T. T., Iglesias-Páramo, J., et al. 2007, ApJS, 173, 404 lines by more than 100%. The future is bright for OHMs: wide- Chambers, K. C., Magnier, E. A., Metcalfe, N., et al. 2016, arXiv e-prints, area H i surveys in addition to AWES, such as WALLABY ex- arXiv:1612.05560 Chengalur, J. N. & Kanekar, N. 2003, Phys. Rev. Lett., 91, 241302 pects to detect more than 100 OHMs over a different area of Clements, D. L., Sutherland, W. J., McMahon, R. G., & Saunders, W. 1996, sky (Koribalski et al. 2020). In fact, Roberts, Darling, & Baker MNRAS, 279, 477 (2021; submitted), suggest WALLABY may find even more Cluver, M. E., Jarrett, T. H., Dale, D. A., et al. 2017, ApJ, 850, 68 Cluver, M. E., Jarrett, T. H., Hopkins, A. M., et al. 2014, ApJ, 782, 90 OHMs, and based on our results, WALLABY will detect even Darling, J. 2003, Phys. Rev. Lett., 91, 011301 more satellite line sources. Meanwhile, dedicated searchesin ab- Darling, J. 2004, ApJ, 612, 58 Darling, J. 2007, ApJ, 669, L9 sorption against background radio sources (Apertif SHARP; and Darling, J. & Giovanelli, R. 2001, AJ, 121, 1278 MALS, Gupta et al. 2020) will detect OH in different masing en- Darling, J. & Giovanelli, R. 2002, AJ, 124, 100 vironments. Farrah, D., Rowan-Robinson, M., Oliver, S., et al. 2001, MNRAS, 326, 1333 Fernandez, M. X., Momjian, E., Salter, C. J., & Ghosh, T. 2010, AJ, 139, 2066 Acknowledgements. We are grateful to Tom Jarrrett for providing k-corrected Flambaum, V. V. & Kozlov, M. G. 2007, Phys. Rev. Lett., 98, 240801 WISE magnitudes and additional insight into the IR nature of the source; to Frayer, D. T., Seaquist, E. R., & Frail, D. A. 1998, AJ, 115, 559 James Allison for help in running the FLASHfinder tool to better understand the Gupta, N., Jagannathan, P., Srianand, R., et al. 2020, arXiv e-prints, reliability of our measurements; and to James McBride for advice in recreating arXiv:2007.04347 Hao, C.-N., Kennicutt, R. C., Johnson, B. D., et al. 2011, ApJ, 741, 124 Figure 6b from his paper. We also thank the anonymous referee for their com- Haynes, M. P., Giovanelli, R., Kent, B. R., et al. 2018, ApJ, 861, 49 ments which improved the quality of this paper. HR and JD acknowledge support Henkel, C., Baan, W. A., & Mauersberger, R. 1991, A&A Rev., 3, 47 from the National Science Foundation grant AST-1814648. EAKA is supported Houck, J. R., Schneider, D. P., Danielson, G. E., et al. 1985, ApJ, 290, L5 by the WISE research programme, which is financed by the Netherlands Or- Jansen, P., Xu, L.-H., Kleiner, I., Ubachs, W., & Bethlem, H. L. 2011, ganization for Scientific Research (NWO). 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Appendix A: Reliability of the 1612 MHz line Appendix A.2: FLASHfinder Appendix A.1: Matched filter We performed a second test using the Bayesian line fitting tool FLASHfinder9 (Allison et al. 2012, 2014) with a strong redshift In order to test the reliability of the 1612 MHz detection we constraint. As with the matched filter, the tool was tested on the performed a test using matched filtering. Here we present the 1665 and 1667 MHz spectrum, in this case either assuming two matched filtering results on the full spectral resolution (un- Gaussian components, or assuming three Gaussian components smoothed) data, however we have also performed the exact same with redshift priors of z⊙ = 0.19612±0.01.The three component tests on the smoothed data presented in this work and found the Gaussian was overwhelmingly favoured with a difference in log- resulting plots are essentially identical. probability of 17.61 ± 0.07, in order to fit the double peak in the The shape of the 1612 MHz line is not known a priori, how- 1667MHz line. ever we start with the assumption that the line shapes are approx- FLASHfinder was then run on the 1612 MHz spectrum with imately the same for all the OH maser lines. This implies that all an even stronger redshift prior of z⊙ = 0.19612 ± 0.001. This lines trace the same gas components, although in practice that corresponds to an error of ∼ 300 kms−1, although the error in may not be the case. We take the 1667 MHz profile over the the redshift uncertainty measured by SoFiA for the 1667 MHz channel range indicated in Figure 1, and reproduced in Figure line is only 1 km s−1. Unfortunately, the Bayesian tool disfavors A.1, as an initial guess for the matched filter and apply it using a Gaussian componentat the predicted location of the 1612 MHz scipy.signal.filtfilt (Virtanen et al. 2020). To check the line with a log probability of -0.27. More optimistically phrased, implementation, we first apply the matched filter to the 1667 and the log probability estimates that there is a roughly 1 in 3 chance 1665 MHz line spectrum. The signal-to-noise is maximized ex- that we have detected the spectral line. actly where the 1667 MHz line is detected, as expected,although As a result of both the matched filter and the Bayesian anal- we note that the 1665 MHz signal is strongly blended with the ysis, we take the conservative approach throughout the paper of 1667 MHz signal (blue line in Panel B of Figure A.1). calling the 1612 MHz line measurement an upper limit rather We attempted to improve on this by truncating the matched than a tentative detection. filter at the estimated width of the 1612 MHz line (the line center ±8 channels corresponding to the gray shaded region of Figure A.1). When we applythe truncatedmatched filter to the 1665and 1667 MHz spectrum (black line in Figure A.1), the 1665 MHz signal is now distinct and its peak is also very well localized with the predicted frequency of the 1665 MHz line. The Panel D of Figure A.1 shows the result of cross corre- lating the full matched filter with the 1612 MHz spectrum. The most significant feature, positive or negative, is associated with where we expect the redshift 1612 MHz line to fall. (The rise at lowest frequencies is an edge effect.) In fact, using the entire 1667 MHz profile for the matched filter may be too broad: while this filter maximizes the SNR, its localization is poor. Panel E of Figure A.1 shows the cross correlation of 1612 MHz spectrum with this truncated matched filter. The strongest overall feature in the matched filter correlation is negative, and its profile shape appears noise-like. The strongest positive fea- ture is still associated with where we expect the 1612 MHz line to fall, but now the localization is significantly better, and re- markably well aligned with the predicted position of the 1612 MHz line. The shape of this positive feature also appears quali- tatively different than the noise features around it. Finally, we repeated the work described above with a Gaus- sian kernel fit to the 1667 MHz profile and a truncated Gaussian profile, which is simply the peak of the same Gaussian, now re- stricted to the inner ±8 channels. We find that the results for the full Gaussian are exactly the same as for the full matched fil- ter. For the truncated Gaussian, the result is to smooth out the very small (channel by channel) variations seen in Panel E but it does not change the large scale variations seen in the plot, or our results. What we conclude from the six combinations of applying a full or truncated matched filter, or a Gausssian filter to both the full resolution and the smoothed pixel spectra are that the exact shape of the filter doesnot matter in termsof identifyingthe most significant features in the spectra. However, the width of the filter is important in localizing these features (e.g. Panel B). Panel (E) is suggestive that there is emission detected from the 1612 MHz line, but the conclusions are tentative and the measurement is still dominated by noise. 9 https://github.com/drjamesallison/flash_finder

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Unsmoothed data (A) Fitted 1667 MHz Predicted 1665 MHz

/beam] 1

1

2 4

1 1 1 1 1 1 1 11 11 1

0.010 Full 1667 MHz profile matched filter applied (B) Truncated 1667 MHz profile matched filter applied 0.008 Fitted 1667 MHz Predicted 1665 MHz 0.006

0.004

0.002 Arbitrary Units Arbitrary

0.000

1.375 1.380 1.385 1.390 1.395 1.400 1.405 1.410 1.415 1e9 Unsmoothed data (C) Predicted 1612 MHz

/beam]

4

4

112 4

4 1

Full 1667 MHz profile matched filter applied (D) Predicted 1612 MHz

4 .2.−

4

1

Truncated 1667 MHz profile matched filter applied (E) Predicted 1612 MHz

.2.−

1.335 1.340 1.345 1.350 1.355 1.360 1.365 1.370 1.375 Observed Frequency [Hz] 1e9

Fig. A.1. Panel A: Full resolution (unsmoothed) pixel spectrum showing the 1665 and 1667 MHz lines. Orange shaded region shows the extent of the OH 1667 MHz profile for the full matched filter. Gray shaded region shows extent of the OH 1667 MHz profile that has been used for the truncated filter. It matches the width over which the 1612 MHz line was measured in Section 3. Panel B: The matched filters applied to the unsmoothed data. Panel C: Full resolution pixel spectrum covering the predicted frequency of the 1612 MHz line. Panel D: The full 1667 profile matched filter applied to the unsmoothed 1612 MHz data. Panel E: The truncated 1667 MHz profile matched filter applied to the unsmoothed 1612 MHz data. Article number, page 9 of 9