[Paper Review] The LOFAR Two-metre Sky Survey - II. First data release

T. W. Shimwell, C. Tasse+105 more|UvA-DARE (University of Amsterdam)|Nov 19, 2018

Radio Astronomy Observations and Technology96 references175 citations

TL;DR

This paper presents the first public data release of LoTSS, detailing 424 deg^2 observed at 120–168 MHz with ~6 arcsec resolution, yielding 325,694 detected sources and a median sensitivity of 71 μJy/beam.

ABSTRACT

The LOFAR Two-metre Sky Survey (LoTSS) is an ongoing sensitive, high-resolution 120-168MHz survey of the entire northern sky for which observations are now 20% complete. We present our first full-quality public data release. For this data release 424 square degrees, or 2% of the eventual coverage, in the region of the HETDEX Spring Field (right ascension 10h45m00s to 15h30m00s and declination 45$^\circ$00$'$00$''$ to 57$^\circ$00$'$00$''$) were mapped using a fully automated direction-dependent calibration and imaging pipeline that we developed. A total of 325,694 sources are detected with a signal of at least five times the noise, and the source density is a factor of $\sim 10$ higher than the most sensitive existing very wide-area radio-continuum surveys. The median sensitivity is S$_{ m 144 MHz} = 71\,μ$Jy beam$^{-1}$ and the point-source completeness is 90% at an integrated flux density of 0.45mJy. The resolution of the images is 6$''$ and the positional accuracy is within 0.2$''$. This data release consists of a catalogue containing location, flux, and shape estimates together with 58 mosaic images that cover the catalogued area. In this paper we provide an overview of the data release with a focus on the processing of the LOFAR data and the characteristics of the resulting images. In two accompanying papers we provide the radio source associations and deblending and, where possible, the optical identifications of the radio sources together with the photometric redshifts and properties of the host galaxies. These data release papers are published together with a further $\sim$20 articles that highlight the scientific potential of LoTSS.

Motivation & Objective

• Provide a high-quality, wide-area low-frequency radio survey of the northern sky with ~6 arcsec resolution and ~100 μJy/beam sensitivity.
• Deliver a fully automated direction-dependent calibration and imaging pipeline for LoTSS-DR1.
• Produce a catalogue of detected radio sources with positions, fluxes, and shapes, plus mosaic images for public use.
• Quantify the survey's imaging quality, sensitivity, completeness, and astrometric/photometric reliability in the HETDEX Spring Field region.

Proposed method

• Observe the northern sky in the 120–168 MHz band using LOFAR HBA with 8 hr dwell times and multi-beam capabilities.
• Apply a fully automated direction-dependent calibration and imaging pipeline (KillMS and ddfacet) to correct ionospheric and beam-model errors.
• Perform direction-independent calibration first, bootstrap flux density scale, then iteratively apply direction-dependent calibration and spectral deconvolution (SSD) across 45 facets.
• Use a two-stage deconvolution approach with subspace deconvolution (SSD) and EB Kalman filter-based Jones matrix estimation to handle wide-field, wide-band data.
• Mosaic images and a source catalog are produced, with astrometric corrections anchored to Pan-STARRS cross-matches.
• Cross-match a subset of sources with VLSSr and WENSS to derive flux scale corrections across six 10-SB blocks, and apply these corrections to the full dataset.

Experimental results

Research questions

• RQ1 What is the quality and scientific utility of a first public data release of LoTSS-DR1 in terms of sensitivity, resolution, and astrometric accuracy?
• RQ2 Can an automated direction-dependent calibration and imaging pipeline deliver high-fidelity, wide-area low-frequency radio images suitable for robust source cataloging?
• RQ3 How does LoTSS-DR1's source density and completeness compare to existing wide-area radio surveys at similar frequencies?
• RQ4 What is the achievable astrometric and flux-density accuracy when cross-matching LoTSS-DR1 with optical/other radio surveys?
• RQ5 What are the practical considerations and bottlenecks in processing, archiving, and distributing large LOFAR data sets for public release?

Key findings

• DR1 covers 424 deg^2 in the HETDEX Spring Field, with 325,694 sources detected at ≥5σ.
• Median sensitivity is 71 μJy/beam at 144 MHz, and image resolution is 6 arcsec.
• Source completeness is 90% at integrated flux density of 0.45 mJy.
• Image astrometric accuracy is within 0.2 arcsec and flux-density scales are bootstrapped against VLSSr and WENSS.
• The images reach a typical noise range of 60–160 μJy/beam in the high-resolution maps, and 100–400 μJy/beam in lower-resolution maps.
• The pipeline successfully produced 58 images (out of 63 data sets) for the HETDEX Spring Field, with five failures due to ionospheric conditions or very bright sources.