The MeerKLASS L-band On-the-Fly Continuum Survey: Data Release 1

TL;DR

This paper presents the first public data release (DR1) of the MeerKLASS L-band continuum survey, obtained with MeerKAT in On-the-Fly (OTF) scanning and processed via visibility-domain mosaicking to yield deep wide-field 1284 MHz continuum images over ~268 deg$^2$ with a median RMS of ~33 μJy beam$^{-1}$ and a restoring beam of ~25.5''×7.8''. It delivers a Stokes I source catalogue of 34,874 objects and performs extensive cross-survey validation (flux scale within 5% and astrometry within ~1.5'') along with in-band spectral-index estimates from seven sub-bands. Rigorous completeness assessments through injection-recovery and differential source counts show excellent agreement with existing surveys, confirming the end-to-end reliability of the pipeline. The results demonstrate the effectiveness of scanning surveys combined with OTF imaging for large-area, high-fidelity radio astronomy and outline a path toward future SKA-Mid-scale surveys, including DR2 and a large UHF-wide component.

Abstract

The MeerKAT Large Area Synoptic Survey (MeerKLASS) collaboration has acquired multiple passes of L-band (856-1712 MHz) scanning observations over a 268 deg$^2$ sky region. This scanning enables efficient, large-area sky surveys by continuously scanning the MeerKAT array back and forth at fixed elevation while recording data at 2 sec intervals, progressively covering the survey region as the Earth rotates. We employ a novel on-the-fly (OTF) interferometric imaging technique to construct continuum images and catalogs from 16 hours of scan data. These data products, constituting the first MeerKLASS L-band data release (DR1), consist of high-fidelity radio continuum images and a catalogue of 34,874 radio sources detected with a SNR$>$9. The resulting Stokes I images achieve a median noise level of 33 $μ$Jy\,beam$^{-1}$ and a median angular resolution of approximately $25.5''\times 7.8''$. Cross-comparisons with previous surveys confirm the consistency of our flux density scale within 5\% and astrometric precision within $1.5''$. Additionally, flux densities measured across the seven sub-bands enable in-band spectral-index estimates for the detected sources, providing insights into their physical properties and the broader source population. We compute the differential source counts, finding good agreement with existing measurements and validating our end-to-end processing. This data release demonstrates the effectiveness of scanning surveys when combined with OTF interferometric imaging. Commensal intensity mapping and interferometric imaging offers a dramatic enhancement of survey science per invested hour of observations and could therefore be an appealing option for next generation facilities like SKA-Mid.

Figures (13)

• Figure 1: Sky footprint of the MeerKLASS L-band observing blocks included in this data release. Each blue point marks the pointing centre of the 2 sec snapshot visibility. This footprint overlaps with the $\text{KiDS-DR5}$KiDS_DR5 and upcoming $\text{DESI}$-DR11. This sky area is divided into $2.15\degr \times 2.15\degr$ tiles with $0.075\degr$ overlap between adjacent tiles. Each square represents a tile, and the tile ID is indicated at its centre. The layout ensures full coverage of the $\sim$268 deg$^2$ field.
• Figure 2: Illustration of snapshot selection for imaging a single MeerKLASS L-band tile. The blue points show all snapshot pointing centres in the field. The shaded square marks the $2\degr\times2\degr$ region around the chosen tile centre (Blue star). Red circles indicate the $10\,\text{dB}$ primary-beam attenuation contours at 1284 MHz for the snapshots whose centres fall within and around this region; these snapshots are jointly imaged with DDFacet to form the final tile image.
• Figure 3: Overview of the mosaic and image quality across the MeerKLASS L-band DR1 survey area used in this work. The top panel shows the full $\sim$268 deg$^2$ continuum mosaic constructed from 67 tiles (\ref{['fig:alltiles']}) at the L-band centred around 1284 MHz. Three example tiles are highlighted on the mosaic as A, B and C. The zoomed-in cutout of the respective tiles are shown in the lower panels. These illustrate the high dynamic range and low noise floor achieved across a range of sky positions. The selected fields span a variety of declinations and survey depths, and their structure reflects the uniformity and imaging fidelity delivered by the pipeline. The anisotropy of the synthesized beam is driven by a time smearing effect that has since been overcome with a new OTF-optimized observing mode at MeerKAT.
• Figure 4: Estimated background noise across the survey area, derived from the full residual mosaic at 1284 MHz. The RMS map is computed by applying sigma-clipped statistics within overlapping sliding windows ($100\times100$ pixels, stepped every 50 pixels) directly on the combined residual image. The map reveals significant spatial variation in noise level due to differences in integration time near the survey edge and residual imaging artefacts near bright sources.
• Figure 5: Histogram of restoring beam sizes across all 67 tiles. Top: The distributions of BMAJ (major axis) and BMIN (minor axis) are tightly clustered around median values of $\sim25.5\arcsec$ and $\sim7.8\arcsec$ respectively (dashed vertical lines), indicating uniform angular resolution over the full survey footprint. Bottom: The distribution of BPA (beam position angle) of the major axis clustered around median value of approximately $-88.9\degr$. Also, a median beam is shown in the right panel for reference.