Lofar Low-Band Antenna Observations of the 3c 295 and Boötes Fields: Source Counts and Ultra-Steep Spectrum Sources

Abstract

We present Low Frequency Array (LOFAR) Low Band observations of the Boötes and 3C 295 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 17-52 deg 2 . From the observations, we derive Euclidean-normalized differential source counts. The 62 MHz source counts agree with previous GMRT 153 MHz and Very Large Array 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 toward 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 (α < −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 Boötes field. We cross-correlate these radio sources with optical and infrared catalogs 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 z 2.5 range.

Radio spectra of bright compact sources at z>4.5

“…A number of our targets were observed, but not detected, in the following large surveys: The VLA LowFrequency Sky Survey Redux (VLSSr, 74 MHz; Lane et al 2014), TGSS, WENSS, the Green Bank 4.85 GHz survey (GB6, 4850 MHz; Gregory et al 1996), the 62 MHz LowFrequency Array (LOFAR) image of the Boötes field made by Van Weeren et al (2014) and the 3 GHz Caltech-NRAO Stripe 82 Survey (CNSS; Mooley et al 2016). To determine consistent upper limits for these non-detections, we downloaded the survey images and measured σ local within the 10×10 arcmin area surrounding the VLBI position.…”

Parametrizing Epoch of Reionization foregrounds: a deep survey of low-frequency point-source spectra with the Murchison Widefield Array

“…At the frequencies of interest, the average SI of sources is generally found to be around −0.7 to −0.8 (Intema et al 2011;Ghosh et al 2012;van Weeren et al 2014). We estimate the in-band SIs of all the measured sources in our field by fitting a power law to the 40 kHz SEDs.…”

LOFAR discovery of a 700-kpc remnant radio galaxy at low redshift

“…The integrated flux densities of the source extracted from the maps are listed in Table 2. The errors on the measurements were computed taking into account different contributions: i) 2% due to the uncertainty in the flux scale (Scaife & Heald 2012); ii) 10% due to the uncertainty in the global beam model (van Weeren et al 2014); iii) 15% due to variations in the measured flux density when using apertures of different sizes; iv) ∼10% due to the statistical error (thermal noise) measured around the source following Klein et al (2003).…”

“…A few tentative searches have also been performed on individual deep fields (Sirothia et al 2009;van Weeren et al 2014). The strategy of the steep spectral index is best for identifying sources with very curved spectra at low frequencies that have experienced significant radiative losses and are typically not detectable at 1.4 GHz.…”