The LOFAR LBA Sky Survey: Deep Fields

Abstract

We present the first sub-mJy (≈ 0.7 mJy beam −1 ) survey to be completed below 100 MHz, which is over an order of magnitude deeper than previously achieved for widefield imaging of any field at these low frequencies. The high-resolution (15 × 15 arcsec) image of the Boötes field at 34-75 MHz is made from 56 hours of observation with the LOw Frequency ARray (LOFAR) Low Band Antenna (LBA) system. The observations and data reduction, including direction-dependent calibration, are described here. We present a radio source catalogue containing 1, 948 sources detected over an area of 23.6 deg 2 , with a peak flux density threshold of 5σ . Using existing datasets, we characterise the astrometric and flux density uncertainties, finding a positional uncertainty of ∼ 1.2 arcsec and a flux density scale uncertainty of about 5 per cent. Using the available deep 144-MHz data, we identified 144-MHz counterparts to all the 54-MHz sources, and produced a matched catalogue within the deep optical coverage area containing 829 sources. We calculate the Euclidean-normalised differential source counts and investigate the low-frequency radio source spectral indices between 54 and 144 MHz. Both show a general flattening in the radio spectral indices for lower flux density sources, from ∼ −0.75 at 144-MHz flux densities between 100 and 1000 mJy to ∼ −0.5 at 144-MHz flux densities between 5 and 10 mJy. Such flattening is attributable to a growing population of star forming galaxies and compact core-dominated AGN.

Magnetic fields and relativistic electrons fill entire galaxy cluster

“…The inspection of the direction-independent images produced by LiLF revealed that none of the 18 4-hour runs had to be discarded because of bad data quality (e.g., owing to severe ionospheric corruption). Thus, direction-dependent calibration and subsequent imaging were done with a slightly modified version of LiLF using the entire 72-hour dataset and the best model obtained from a single 4-hour observation, similarly to the strategy adopted for other LOFAR deep observations ( 40 , 46 , 47 ).…”

The LOFAR Two-metre Sky Survey

“…However, these effects are also likely entangled with real spectral index variations as a function of flux density and source population (see e.g. Williams et al 2021 andde Gasperin, Intema, &Frail 2018). Furthermore, these issues clearly highlight the degree of uncertainty in the most appropriate spectral index to assume for the alignment process (as previously stated we assume α = −0.783) and thus a further significant limitation of our present in-band spectral index measurements -altering the assumed spectral index has the effect of shifting our derived inband spectral index distribution in α.…”

MIGHTEE: deep 1.4 GHz source counts and the sky temperature contribution of star-forming galaxies and active galactic nuclei

“…For each of these different input source models, we follow the approach of many previous works (see e.g. Williams et al 2018, Hale et al 2019, Williams et al 2021, Shimwell et al 2022) and inject simulated sources into images of the corresponding field and determine how successful source detection is.…”