MeerKAT Detects Most Distant, Most Luminous OH “Radio Laser” — About 8 Billion Light-Years Away

A razor-thin spike of radio emission, tuned to a wavelength near 18 centimetres, has been detected from a merging galaxy roughly eight billion light-years away by the South African radio array MeerKAT. The signal is hydroxyl (OH) megamaser emission from HATLAS J142935.3–002836 (H1429-0028) at redshift z = 1.027 — the highest-redshift OH megamaser reported to date.

Discovery and Significance

OH megamasers are sometimes called “space lasers” because they amplify radiation through stimulated emission, but they operate at radio frequencies rather than visible light. They are most often found in the dense, dusty centers of luminous and ultra-luminous infrared galaxies, frequently in major mergers where a strong far-infrared radiation field pumps the hydroxyl molecules and sustains the population inversion needed for maser action.

Left: Hubble Space Telescope near-infrared (F160W) image of H1429-0028, including the foreground disk lens. Right: Calanog et al. (2014) 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. (CREDIT: arXiv)

Why This Detection Stands Out

Two aspects make this detection remarkable: distance and brightness. The integrated apparent OH luminosity is reported as log(μ L_OH / L⊙) = 5.51 ± 0.67. Even after correcting for a plausible near-infrared lensing magnification of roughly μ ≈ 10, the intrinsic OH luminosity would remain among the most powerful known (log(L_OH / L⊙) ≈ 4.5).

The spectral profile is complex. Bayesian model selection favored a five-Gaussian decomposition of the OH line: one very narrow component with full-width at half-maximum ΔV = 7.05 ± 1.27 km s⁻1, plus broader components including one with ΔV = 315 ± 10 km s⁻1. Such structure can reflect multiple maser sites, outflowing gas, or differential magnification by a gravitational lens.

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. (CREDIT: arXiv)

Gravitational Lensing And Observational Details

The radio beam’s path to Earth passed through an unrelated foreground disk galaxy at z = 0.218 that acts as a gravitational lens, further amplifying the background source. The lens model adopted from Calanog et al. (2014) uses a Single Isothermal Ellipsoid profile with Einstein radius θE = 0.738 arcsec, axis ratio q = 0.792 and position angle θ = −51.0° (east of north). Previous imaging showed an almost-complete Einstein ring with an angular scale near 0.7 arcseconds.

MeerKAT observed H1429-0028 from 13–16 April 2021, using 62 of 64 antennas across the 544–1088 MHz band. The team recorded a single six-hour track (≈4.7 hours on source) and split the data into 32,768 spectral channels to achieve very fine frequency resolution. After calibration and imaging, the reported circularized point-spread function was 32.08 arcseconds FWHM, with a median per-channel rms noise σ = 362 μJy beam⁻1 at 16.6 kHz resolution.

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. (CREDIT: arXiv)

Extracting the faint line required careful processing: the Oxkat pipeline for calibration and radio-frequency interference excision, “peeling” two bright continuum sources using CubiCal, and spectral-line imaging with wsclean. The OH emission is unresolved at MeerKAT’s angular resolution, which limits precise localization within the merging system and complicates estimates of differential magnification for individual components.

“We are seeing the radio equivalent of a laser halfway across the universe. Not only that, during its journey to Earth, the radio waves are further amplified by a perfectly aligned, yet unrelated foreground galaxy,” said Dr. Thato Manamela, lead author and postdoctoral researcher at the University of Pretoria.

“This result is a powerful demonstration of what MeerKAT can do when paired with advanced computational infrastructure, fit-for-purpose data processing pipelines, and highly trained software support personnel,” added Prof. Roger Deane (IDIA).

Complementary Gas Tracers and Interpretation

The MeerKAT data also reveal H i absorption best described by two components with centroid velocities Vc = −36.3 ± 2.0 and 18.1 ± 3.7 km s⁻1 and FWHM ≈ 38–44 km s⁻1. Assuming a spin temperature Ts = 100 K and covering factor fc = 1, the column densities are estimated at ≈1.2×10²¹ and 1.1×10²¹ cm⁻2. The two brightest OH peaks are blueshifted relative to colder gas tracers (CO and [C i]), which could indicate a warm molecular outflow, multiple maser sites tied to merging nuclei, or effects from differential lensing magnification.

The paper notes uncertainty about the galaxy’s primary power source. The far-infrared to radio ratio qFIR = 2.2 ± 0.2 lies within 2σ of the median for star-forming galaxies, but contamination from the foreground lens or differential magnification of an active nucleus cannot be ruled out. The measured dust temperature Tdust ≈ 40.7 K may not reflect nuclear conditions where OH emission arises.

Implications And Next Steps

Bright, lensed OH megamasers like H1429-0028 are valuable signposts of extreme galaxy mergers at epochs when star formation and black hole growth were more common. The discovery highlights the scientific payoff of combining sensitive radio arrays, careful data pipelines, and lens modeling. The team plans systematic surveys to find many more such systems, paving the way for larger searches with the Square Kilometre Array (SKA).

The study has been accepted to Monthly Notices of the Royal Astronomical Society Letters and a preprint is available on arXiv.