LOFAR low-band antenna observations of the 3C295 and Bootes fields: source counts and ultra-steep spectrum sources

TL;DR

This study uses LOFAR low-frequency observations to analyze source counts and spectral properties, revealing spectral flattening at low frequencies and identifying ultra-steep spectrum sources potentially linked to high-redshift galaxies.

Contribution

It provides the deepest low-frequency images to date, derives source counts across multiple frequencies, and investigates spectral flattening and ultra-steep spectrum sources with potential high-redshift associations.

Findings

Source counts agree with previous surveys when scaled with a spectral index of -0.7.

Spectral index scaling of -0.5 matches low-frequency source counts, indicating spectral flattening.

Most ultra-steep spectrum sources are found at redshifts 0.7 to 2.5.

Abstract

We present LOFAR Low Band observations of the Bootes and 3C295 fields. Our images made at 34, 46, and 62 MHz reach noise levels of 12, 8, and 5 mJy beam$^{-1}$, making them the deepest images ever obtained in this frequency range. In total, we detect between 300 and 400 sources in each of these images, covering an area of 17 to 52 deg$^{2}$. From the observations we derive Euclidean-normalized differential source counts. The 62 MHz source counts agree with previous GMRT 153 MHz and VLA 74 MHz differential source counts, scaling with a spectral index of $-0.7$. We find that a spectral index scaling of $-0.5$ is required to match up the LOFAR 34 MHz source counts. This result is also in agreement with source counts from the 38 MHz 8C survey, indicating that the average spectral index of radio sources flattens towards lower frequencies. We also find evidence for spectral flattening using the individual flux measurements of sources between 34 and 1400 MHz and by calculating the spectral index averaged over the source population. To select ultra-steep spectrum ($\alpha < -1.1$) radio sources, that could be associated with massive high redshift radio galaxies, we compute spectral indices between 62 MHz, 153 MHz and 1.4 GHz for sources in the Bo\"otes field. We cross-correlate these radio sources with optical and infrared catalogues and fit the spectral energy distribution to obtain photometric redshifts. We find that most of these ultra-steep spectrum sources are located in the $ 0.7 \lesssim z \lesssim 2.5$ range.