[Paper Review] The LOFAR Two-metre Sky Survey (LoTSS) III. First Data Release: optical/IR identifications and value-added catalogue

W. L. Williams, M. J. Hardcastle+39 more|Open Research Online (The Open University)|Nov 19, 2018

Gamma-ray bursts and supernovae3 references52 citations

TL;DR

The paper presents the first data release value-added catalogue for LoTSS, detailing optical/IR identifications and the cross-matching methodology using likelihood ratios and visual classifications to associate radio sources with Pan-STARRS/WISE counterparts.

ABSTRACT

The LOFAR Two-metre Sky Survey (LoTSS) is an ongoing sensitive, high-resolution 120-168 MHz survey of the Northern sky with diverse and ambitious science goals. Many of the scientific objectives of LoTSS rely upon, or are enhanced by, the association or separation of the sometimes incorrectly catalogued radio components into distinct radio sources, and the identification and characterisation of the optical counterparts to these sources. Here we present the source associations and optical and/or IR identifications for sources in the first data release, which are made using a combination of statistical techniques and visual association and identification. We document in detail the colour- and magnitude-dependent likelihood ratio method used for statistical identification as well as the Zooniverse project, called LOFAR Galaxy Zoo, used for the visual classification. We describe the process used to select which of these two different methods is most appropriate for each LoTSS source. The final LoTSS-DR1-IDs value-added catalogue presented contains 318,520 radio sources, of which 231,716 (73%) have optical and/or IR identifications in Pan-STARRS and WISE. The value-added catalogue is available online at https://lofar-surveys.org/, as part of this data release.

Motivation & Objective

• Identify optical/IR counterparts to LoTSS radio sources to enable photometric/spectroscopic redshift estimates.
• Differentiate correctly associated radio components from misclassified or blended components.
• Develop and apply a robust cross-matching workflow combining statistical LR techniques with human visual classification.
• Produce a value-added catalogue (LoTSS-DR1-IDs) with identified counterparts for a large radio source sample.

Proposed method

• Use a likelihood ratio (LR) approach incorporating magnitude and colour (m, c) to cross-match LoTSS sources with a combined Pan-STARRS–AllWISE catalogue.
• Model f(r) as the offset distribution considering major/minor axis radio positional uncertainties and astrometric errors; combine with n(m) and q(m) or n(m,c) and q(m,c) to compute LR.
• Iteratively estimate q(m,c) and Q0 to account for clustering and colour biases in host galaxy identification.
• Apply a decision tree to select sources suitable for LR matching (≤30 arcsec) and use LOFAR Galaxy Zoo (LGZ) visual classifications for complex, larger sources.
• Adopt LR thresholds to balance completeness and reliability of identifications (L_thr with C and R evaluated).
• Produce a colour- and magnitude-dependent LR analysis by combining Pan-STARRS i-band and WISE W1 data for robust identifications.

Experimental results

Research questions

• RQ1 How many LoTSS-DR1 sources can be reliably identified with optical/IR counterparts using LR methods versus visual classification?
• RQ2 What fraction of LoTSS-DR1 radio sources have Pan-STARRS/WISE counterparts, and what are their colour/magnitude properties?
• RQ3 How can complex, extended radio sources be optimally associated or separated into true components using statistical and visual approaches?
• RQ4 What are the completeness and reliability of LR-based identifications at a chosen threshold for LoTSS-DR1?
• RQ5 How can the LoTSS-DR1 identifications enable subsequent photometric redshift and rest-frame colour analyses (DR1-III)?

Key findings

• The final LoTSS-DR1-IDs value-added catalogue contains 318,520 radio sources.
• 231,716 (73%) of these have optical and/or IR identifications in Pan-STARRS and WISE.
• The value-added catalogue is available online as part of the LoTSS-DR1 data release.
• LR matching was applied to radio sources smaller than 30 arcsec, with a colour/magnitude LR method using Pan-STARRS i-band and WISE W1 data.
• A LOFAR Galaxy Zoo visual classification (LGZ) process was used for larger/complex sources where LR matching is unreliable.
• The combined Pan-STARRS–WISE catalogue together with LoTSS-DR1 counterparts enables photometric redshift estimates (DR1-III) and supports future WEAVE-LOFAR spectroscopy.