MeerKAT discovery of a high-redshift strongly-lensed hydroxyl gigamaser

Abstract

At low redshifts, hydroxyl megamasers (OHMs) have been shown to trace galaxy mergers, obscured starbursts, high molecular gas densities, and candidate dual supermassive black hole systems. Given this astrophysical utility, exploring these sources at larger cosmological look-back times is therefore of key interest. While previous OHM surveys have been limited to redshifts of $z \lesssim 0.25$, the ability to expand the OHM frontier is significantly enhanced with new high-sensitivity radio facilities such as MeerKAT. In this Letter, we report the discovery of an OHM in the gravitational lens system HATLAS J142935.3-002836 at $z = 1.027$, the most distant OHM source yet detected. The spectrum has blended 1667 and 1665 MHz emission and exhibits a highly complex profile, with spectral components ranging in widths of $<8$ km s$^{-1}$ to $\sim300$ km s$^{-1}$. The integrated (magnification uncorrected) luminosity of log($L_{\rm OH} / L_{\odot}$) = 5.51 $\pm$ 0.67 makes this the most apparently luminous OHM known to date. In the same wide-band dataset, we have also detected a previously unknown ${\rm H I}$ absorption line. The signal-to-noise ratio of over 150 with just a 4.7 h observation highlights the potential that MeerKAT and the future Square Kilometre Array mid-frequency array offer to explore the high-redshift OHM universe.

Figures (5)

• Figure 1: Left: Hubble Space Telescope near-infrared (F160W) image of H1429-0028, including the foreground disk lens. Right: Calanog2014 lens model of H1429-0028, oriented with north at the top and east to the left. The orange and black lines denote the critical and caustic curves, respectively.
• Figure 2: Observed OHM emission spectrum of the lensed system, HATLAS1429-0028. The 16.6 kHz resolution spectrum shows a complex emission profile with remarkably high integrated SNR. Bayesian model selection prefers the 5-Gaussian component model (blue curve). The residuals (data - model) are shown in the bottom panel.
• Figure 3: The relationship between magnification and radius for hypothetical OH maser components centred on the three NIR components identified in the Calanog2014 lens modelling with Keck K$_{\rm s}$ Adaptive-Optics imaging data. We centre circularly symmetric Gaussian components on these three locations and compute the magnifications for each. The dashed horizontal line shows the total NIR magnification derived from Calanog2014, which is consistent with the value derived in messias2014herschel. See the main text for the full discussion.
• Figure 4: Far-infrared and OH luminosity correlation Glowackijarvis2023discoveryzhang2024fashi. The three MeerKAT-discovered OHMs (blue, green, red) have doubled, tripled, and quadrupled the previous OHM redshift record held for almost two decades, respectively. The MIGHTEE (green) and highest redshift source (red) reported in this work are both strongly lensed, suggesting untargeted OHM searches as a way of identifying lensed systems Button2024.
• Figure 5: The H i absorption spectrum (light blue), which is centred on the systemic redshift ($V=0$ km s$^{-1}$), as derived from optical and low-J CO lines. The OH emission spectrum (dark blue), scaled by 0.04 for comparison, appears blue-shifted, potentially indicating a molecular outflow.