The LOFAR Two-meter Sky Survey: Deep Fields Data Release 1

Abstract

The LOFAR Two-metre Sky Survey (LoTSS) will cover the full northern sky and, additionally, aims to observe the LoTSS deep fields to a noise level of 10µJy beam −1 over several tens of square degrees in areas that have the most extensive ancillary data. This paper presents the ELAIS-N1 deep field, the deepest of the LoTSS deep fields to date. With an effective observing time of 163.7 hours, it reaches a root mean square (RMS) noise level of 20 µJy beam −1 in the central region (and below 30 µJy beam −1 over 10 square degrees). The resolution is ∼6 arcsecs and 84862 radio sources were detected in the full area (68 square degrees) with 74127 sources in the highest quality area at less than 3 degrees from the pointing centre. The observation reaches a sky density of more than 5000 sources per square degree in the central region (∼5 square degrees). We present the calibration procedure, which addresses the special configuration of some observations and the extended bandwidth covered (115 to 177 MHz; central frequency 146.2 MHz) compared to standard LoTSS. We also describe the methods used to calibrate the flux density scale using cross-matching with sources detected by other radio surveys in the literature. We find the flux density uncertainty related to the flux density scale to be ∼6.5 per cent. By studying the variations of the flux density measurements between different epochs, we show that relative flux density calibration is reliable out to about a 3 degree radius, but that additional flux density uncertainty is present for all sources at about the 3 per cent level; this is likely to be associated with residual calibration errors, and is shown to be more significant in datasets with poorer ionosphere conditions. We also provide intra-band spectral indices, which can be useful to detect sources with unusual spectral properties. The final uncertainty in the flux densities is estimated to be ∼10 per cent for ELAIS-N1.

The multiwavelength properties of red QSOs: Evidence for dusty winds as the origin of QSO reddening

“…In total the SEDs for this study consist of 25−30 photometric data points, from the FIR to the UV. We note that we can reliably discard a possible contribution from synchrotron emission to the total SEDs based on available deep low-frequency radio imaging of these fields (LoTSS, Tasse et al 2021;Sabater et al 2021;Kondapally et al 2021). We expand on the radio properties of our sample in a companion paper (Calistro-Rivera, in prep.).…”

“…First, they have rich multiwavelength data from multiple surveys from the FIR to the UV, including deep Herschel coverage, some of which were recently compiled by the Herschel Extragalactic Legacy Program (HELP; see more details in Sect. 2.3), and second, all these fields have deep LOFAR surveys, including the LoTSS Deep Fields surveys (Tasse et al 2021;Sabater et al 2021) and the LOFAR GAMA surveys (Williams, Hardcastle et al, in prep.). The total number of selected QSOs defined as red or control QSOs in each field are summarised in Fig.…”

The LOFAR Two-metre Sky Survey

“…As highlighted though, the separation cannot be done definitively and a balance must be chosen between the completeness and reliability of the resolved/unresolved classification. Due to the large number of sources that are close to the separation boundary, small differences in the approach can lead to large differences in the outcome (particularly for faint sources) and our conservative 99.9% percentile separation results in a lower fraction of resolved sources than that found in LoTSS-DR1 (14%; Shimwell et al 2019) and LoTSS-deep (be-tween 11.3% and ∼30% depending on the adopted method -see Sabater et al 2021 andMandal et al 2021 for details) but a correspondingly higher level of confidence in their genuine extension.…”

“…To assess the scope/limitations for further improving the alignment of the flux density scale of our individual pointings we make use of the deep field dataset presented in Sabater et al (2021) where 22 epochs totaling over 160 hrs of data were synthesized together using the same data processing pipeline as used for LoTSS-DR2 to form a single deep image of the European Large-Area ISO Survey-North 1 (ELAIS-N1) region. As part of that work, maps were also made of the individual epochs which were all independently calibrated from the same sky model and with the same calibration parameters (i.e.…”

Radio spectral properties of star-forming galaxies in the MIGHTEE-COSMOS field and their impact on the far-infrared-radio correlation

“…To improve quality and sample size in studying the radio spectral properties at both low and high frequency, observations with similar spatial resolutions and high sensitivity at multiple frequencies are essential. The recently released high-sensitivity lowfrequency (< 150 MHz) data from the Low Frequency Array (LO-FAR) large surveys (e.g., de Gasperin et al 2021;Tasse et al 2021;Sabater et al 2021) and the undergoing joint project, SuperMIGH-TEE, which combines the MeerKAT and uGMRT telescopes to observe the MIGHTEE survey regions at 0.5-2.7 GHz with the same angular resolution (∼ 5 ′′ ) and similar sensitivity, will dramatically advance the studies of the radio spectrum.…”