The LOFAR Two-metre Sky Survey

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. 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 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.

Unveiling the cause of hybrid morphology radio sources (HyMoRS)

“…The remaining 6 sources lack either the resolution or extended emission required to determine if such conditions exist. While follow up observations would be required to robustly confirm such structure, preliminary exploration of the LOFAR Two Meter Sky Survey value added catalogue (LoTSS; Shimwell et al 2019;Williams et al 2019) using the automated morphological classification of Mingo et al (2019) suggests that those sources where structure can be confidently identified contain similar morphological features to those presented here. It is therefore likely that this orientation based explanation can be extended to explain a significant fraction of hybrid population.…”

Morphological classification of radio galaxies: capsule networks versus convolutional neural networks

“…We use v1.0 of the value-added catalogue for the HETDEXarea data release of LoTSS. The first step in creating the value-added catalogue involved using PyBDSF 1 to produce a radio source catalogue for the field, after which a decision tree was used to further categorise the sources, with details provided in (Williams et al 2019). After filtering the 325,694 sources to only include those classified as resolved leaves 24,096 sources (Shimwell et al 2019).…”

“…The LoTSS association and cross-identification effort (Williams et al 2019) was a project in which expert astronomers were tasked with characterising the radio emission for sources larger than 15 . Indicated were the locations of the peaks and extents of the emission, and whether there was one or more sources present.…”

“…The 6 classes are Unresolved-1, FRI, FRII, Hybrid-1, Hybrid-2 and Unresolved-2, all of which are described in further detail as follows. After the host galaxy location had been identified through the LoTSS identification effort (Williams et al 2019), the distances, d1 and d2, were determined as the distances in pixels from the host galaxy to the brightest peaks of emission on both sides of the source (shown with points marked with Y/inverted Y in Figure 2). Similarly, Maxd1 and Maxd2 were determined as the maximum extents of the source in each direction (marked with triangles on the plots), out to the masked 4rms limit.…”

“…For the majority of sources a maximum-likelihood ratio test was adequate because the sources are small and unresolved. For sources that are too large or complex, a visual host identification has been applied (Williams et al 2019).…”