The MeerKLASS UHF On-the-Fly Continuum Survey -- Data Release I

TL;DR

Meerkat MeerKLASS DR1 demonstrates a viable path for wide-area, high-fidelity continuum imaging using on-the-fly interferometric scanning in the UHF band. A dedicated data-processing pipeline.flagging, calibration, and CHGCENTRE-based phase corrections, combined with visibility-domain imaging in DDFacet, delivers an $816~\mathrm{MHz}$ mosaic over ~${800}\ \mathrm{deg^2}$ with a typical resolution of $32''\times17''$ and an RMS of approximately $35~\mu\mathrm{Jy\ beam^{-1}}$. The resulting catalogue contains ~95,483 radio sources, with astrometry cross-validated against external surveys showing sub-arcsecond precision and a robust flux-scale agreement; differential counts at 816 MHz align with literature after completeness corrections. This DR1 validates the OTF approach, enabling efficient large-area surveys and providing a valuable resource for population studies, spectral-index mapping, and cross-matching with optical surveys like DESI. The release lays the groundwork for the full MeerKLASS program and future SKA-era wide-area surveys, including planned polarisation measurements and improvements in smearing via sidereal-tracking of the delay center.

Abstract

We present the first public data release (DR1) from the interferometric component of the MeerKAT Large Area Synoptic Survey (MeerKLASS) UHF survey, a legacy program demonstrating a novel on-the-fly (OTF) mapping technique. This release is based on 12 hours of early science observations covering approximately 800 deg$^2$ of the southern sky. We describe the data processing pipeline developed to calibrate and image these fast-scanning observations, producing high-fidelity continuum images at a central frequency of 816 MHz. The resulting mosaic reaches an RMS sensitivity of $\sim$35 $μ$Jy beam$^{-1}$ in its deepest regions, with a typical angular resolution of $\sim32'' \times 17''$. In these images, we identify $95483$ radio sources. We validate the catalogue through cross-matching with external surveys, confirming sub-arcsecond astrometric accuracy and a robust flux density scale. We compute the differential source counts, finding excellent agreement with existing measurements and validating our end-to-end processing. The success of this pilot study serves as a crucial proof of concept for the OTF observing strategy, and the public release of the images and source catalogue provides a valuable resource for a wide range of astrophysical studies. This work paves the way for the full MeerKLASS OTF survey and future large-area survey projects with the SKA.

Figures (15)

• Figure 1: Sky footprint of the eight MeerKLASS UHF observing blocks included in this data release. Each point marks the pointing centre of the reference antenna (m008) during the OTF scan. Rising and setting scans are shown with distinct orientations, producing a cross-hatched pattern that enhances coverage uniformity and sensitivity in overlapping regions. All fields lie within the DESI survey footprint.
• Figure 2: Sky coverage of the MeerKLASS UHF imaging region, divided into $3.2^\circ \times 3.2^\circ$ tiles with $0.1^\circ$ 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$800 deg$^2$ field. Background points denote the instantaneous pointing centres from OTF scan blocks.
• Figure 3: Schematic of an individual tile used in the imaging pipeline. For each tile, all measurement sets with pointings falling inside a $4^\circ \times 4^\circ$ selection region (dashed blue box) are used as input for imaging with DDFacet. A large $7^\circ$ image is produced (green dashed box) to allow clean deconvolution of sources near the edges. The central $3.2^\circ \times 3.2^\circ$ science cutout (solid black square) is extracted for catalogue generation. This buffer strategy minimises edge artefacts and ensures uniform noise in the final mosaics.
• Figure 4: Overview of the mosaic and image quality across the MeerKLASS UHF survey area used in this work. The top panel shows the full $\sim$800 deg$^2$ continuum mosaic constructed from 89 tiles (\ref{['fig:tile_layout']}) at the UHF band centre (816 MHz). Three example tiles are highlighted on the mosaic and displayed below as zoomed-in cutouts, shown from left to right: Tile 63, Tile 41, and Tile 20. 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.
• Figure 5: Estimated background noise across the survey area, derived from the full residual mosaic at 816 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, primary beam coverage, and residual imaging artefacts. The deepest regions, with RMS values as low as $\sim35\,\mu$Jy beam$^{-1}$, are located near the centre of the field where multiple blocks overlap.