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Abstract OH megamasers (OHMs) are extragalactic masers found primarily in gas-rich galaxy major mergers. To date, only ∼120 OHMs have been cataloged since their discovery in 1982, and efforts to identify distinct characteristics of OHM host galaxies have remained inconclusive. As radio astronomy advances with next-generation telescopes and extensive 21 cm Hisurveys, precursors to the Square Kilometre Array are expected to detect the 18 cm OH masing line with significantly increased frequency, potentially expanding the known OHM population tenfold. These detections, however, risk confusion with lower-redshift Hiemitters unless accompanied by independent spectroscopic redshifts. Building on methods proposed by Roberts et al. for distinguishing these interloping OHMs via near- to mid-IR photometry and emission line frequencies, we apply these techniques to data from the Arecibo Legacy Fast ALFA [AreciboL-band Feed Array] (ALFALFA) survey and a preliminary Aperture Tile In Focus (Apertif) Hiemission line catalog from the Westerbork Synthesis Radio Telescope. Our study, utilizing the Apache Point Observatory 3.5 m telescope to obtain optical spectroscopic redshifts of 142 candidates (107 from ALFALFA and 35 from Apertif), confirms five new OHM host galaxies and reidentifies two previously catalogued OHMs misclassified as Hiemitters in ALFALFA. These findings support the predictions from Roberts et al. and underscore the evolving landscape of radio astronomy in the context of next-generation telescopes. 

Abstract Adding to the RESOLVE and ECO Gas in Galaxy Groups (G3) initiative, we examine possible drivers of group-integrated Hi-to-halo mass ratios (M_HI,grp/M_halo) and group X-ray emission, including group halo mass (M_halo), virialization as probed by crossing time (t_cross), presence of active galactic nuclei (AGN), and group-integrated fractional stellar mass growth rate (FSMGR_grp). G3 groups spanM_halo= 10¹¹−10^14.5M_⊙with comprehensive Higas and AGN information, which we combine with X-ray stacking of ROSAT All-Sky data. We detect hot gas emission exceeding AGN and X-ray binary backgrounds confidently forM_halo= 10^12.6−10¹⁴M_⊙and unambiguously forM_halo> 10¹⁴M_⊙, reflecting an inverse dependence ofM_HI,grp/M_haloand hot gas emission on halo mass. At fixed halo mass,M_HI,grp/M_halotransitions to greater spread belowt_cross∼ 2 Gyr. Dividing groups across this transition, lower-t_crossgroups show elevated X-ray emission compared to higher-t_crossgroups forM_halo> 10^13.3M_⊙, but this trend reverses forM_halo= 10^12.6−10^13.3M_⊙. Additionally, AGN-hosting halos belowM_halo∼ 10^12.1M_⊙exhibit a broad, ∼0.25 dex deep valley inM_HI,grp/M_halocompared to non-AGN-hosting halos with correspondingly reduced FSMGR_grp. When diluted by non-AGN-hosting halos, this valley becomes shallower and narrower, falling roughly between $M_{\mathrm{halo}}={10}^{11.5}{M}_{\odot }$and $M_{\mathrm{halo}}={10}^{12.1}{M}_{\odot }$in the overallM_HI,grp/M_halovs.M_halorelation. We may also detect a second, less easily interpreted valley atM_halo∼ 10¹³M_⊙. Neither valley matches theoretical predictions of a deeper valley located at or above $M_{\mathrm{halo}}={10}^{12.1}{M}_{\odot }$. 

Abstract We present measurements of the neutral atomic hydrogen (Hi) mass function (HiMF) and cosmic Hidensity (Ω_{H I}) at 0 ≤z≤ 0.088 from the Looking at the Distant Universe with MeerKAT Array (LADUMA) survey. Using LADUMA Data Release 1 (DR1), we analyze the HiMF via a new “recovery matrix” method that we benchmark against a more traditional modified maximum likelihood (MML) method. Our analysis, which implements a forward modeling approach, corrects for survey incompleteness and uses extensive synthetic source injections to ensure robust estimates of the HiMF parameters and their associated uncertainties. This new method tracks the recovery of sources in mass bins different from those in which they were injected and incorporates a Poisson likelihood in the forward modeling process, allowing it to correctly handle uncertainties in bins with few or no detections. The application of our analysis to a high-purity subsample of the LADUMA DR1 spectral line catalog in turn mitigates any possible biases that could result from the inconsistent treatment of synthetic and real sources. For the surveyed redshift range, the recovered Schechter function normalization, low-mass slope, and “knee” mass are ${\varphi }_{*}=3.56_{-1.92}^{+0.97}\times 10^{-3}$Mpc⁻³dex⁻¹, $\alpha =-1.18_{-0.19}^{+0.08}$, and $\log (M_{*}/M_{\odot })=10.01_{-0.12}^{+0.31}$, respectively, which together imply a comoving cosmic Hidensity of ${\Omega }_{\mathrm{H}\mathrm{I}}=3.09_{-0.47}^{+0.65}\times 10^{-4}$. Our results show consistency between recovery matrix and MML methods and with previous low-redshift studies, giving confidence that the cosmic volume probed by LADUMA, even at low redshifts, is not an outlier in terms of its Hicontent. 

Abstract We present a four-step group-finding algorithm for the Gas in Galaxy Groups (G3) initiative, a spin-off of thez∼ 0 REsolved Spectroscopy Of a Local VolumE (RESOLVE) and Environmental COntext (ECO) surveys. In preparation for future comparisons to intermediate redshift (e.g., the LADUMA survey), we design the group finder to adapt to incomplete, shallow, or nonuniform data. We use mock catalogs to optimize the group finder’s performance. Compared to friends-of-friends (with false-pair splitting), the G3 algorithm offers improved completeness and halo-mass recovery with minimal loss of purity. Combining it with the volume-limited Hicensus data for RESOLVE and ECO, we examine the Hicontent of galaxy groups as a function of group halo mass. Group-integrated HimassM_{H I,grp}rises monotonically over halo massesM_halo∼ 10¹¹–10^14.5M_⊙, pivoting in slope atM_halo∼ 10^11.4M_⊙, the gas-richness threshold scale. We present the first measurement of the scatter in this relation, which has a median of ∼0.3 dex and is asymmetric toward lowerM_{H I,grp}. We discuss interesting tensions with theoretical predictions and prior measurements of theM_{H I,grp}–M_halorelation. In an appendix, we release RESOLVE DR4 and ECO DR3, including updates to survey redshifts, photometry, and group catalogs, as well as a major expansion of the ECO Hiinventory with value-added data products. 

ABSTRACT We present the discovery 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) survey. 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 × 104 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 × 1010 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 offset relative to the centroid of its host galaxy’s optical light. This discovery demonstrates that spectral line mapping with the new generation of radio interferometers may provide important information on the cosmic merger history of galaxies. 

Abstract In the local universe, OH megamasers (OHMs) are detected almost exclusively in infrared-luminous galaxies, with a prevalence that increases with IR luminosity, suggesting that they trace gas-rich galaxy mergers. Given the proximity of the rest frequencies of OH and the hyperfine transition of neutral atomic hydrogen (Hi), radio surveys to probe the cosmic evolution of Hiin galaxies also offer exciting prospects for exploiting OHMs to probe the cosmic history of gas-rich mergers. Using observations for the Looking At the Distant Universe with the MeerKAT Array (LADUMA) deep Hisurvey, we report the first untargeted detection of an OHM atz> 0.5, LADUMA J033046.20−275518.1 (nicknamed “Nkalakatha”). The host system, WISEA J033046.26−275518.3, is an infrared-luminous radio galaxy whose optical redshiftz≈ 0.52 confirms the MeerKAT emission-line detection as OH at a redshiftz_OH= 0.5225 ± 0.0001 rather than Hiat lower redshift. The detected spectral line has 18.4σpeak significance, a width of 459 ± 59 km s⁻¹, and an integrated luminosity of (6.31 ± 0.18 [statistical] ± 0.31 [systematic]) × 10³L_⊙, placing it among the most luminous OHMs known. The galaxy’s far-infrared luminosityL_FIR= (1.576 ±0.013) × 10¹²L_⊙marks it as an ultraluminous infrared galaxy; its ratio of OH and infrared luminosities is similar to those for lower-redshift OHMs. A comparison between optical and OH redshifts offers a slight indication of an OH outflow. This detection represents the first step toward a systematic exploitation of OHMs as a tracer of galaxy growth at high redshifts.