A wide-area GMRT 610-MHz survey of ELAIS N1 field

In this paper we present a wide-area 610 MHz survey of the ELAIS\,N1 field with the GMRT, covering an area of 12.8 deg$^2$ at a resolution of 6 arcsec and with an rms noise of $\sim 40$ $\mu$Jy beam$^{-1}$. This is equivalent to $\sim 20$ $\mu$Jy beam$^{-1}$ rms noise at 1.4 GHz for a spectral index of $-0.75$. The primary goal of the survey was to study the polarised sky at sub-mJy flux densities at $<$ GHz frequencies. In addition, a range of other science goals, such as investigations in to the nature of the low-frequency $\mu$Jy source populations and alignments of radio jets. A total of 6,400 sources were found in this region, the vast majority of them compact. The sample jointly detected by GMRT at 610 MHz and by VLA FIRST at 1.4\,GHz has a median spectral index of $-0.85 \pm 0.05$ and a median 610 MHz flux density of 4.5 mJy. This region has a wealth of ancillary data which is useful to characterize the detected sources. The multi-wavelength cross matching resulted optical/IR counterparts to $\sim 90$ per~cent of the radio sources, with a significant fraction having at least photometric redshift. Due to the improved sensitivity of this survey over preceding ones, we have discovered six giant radio sources (GRS), with three of them at $z \sim 1$ or higher. This implies that the population of GRS may be more abundant and common than known to date and if true this has implications for the luminosity function and the evolution of radio sources. We have also identified several candidate extended relic sources

The nature of the faint low-frequency radio source population

We present a multiwavelength study into the nature of faint radio sources in a deep radio image with the Giant Meterwave Radio Telescope at 612 MHz covering 1.2 deg$^2$ of the ELAIS N1 region. We detect 2800 sources above 50 µJy beam$^{−1}$. By matching to multiwavelength data, we obtain a redshift estimate for 63 per cent, with 29 per cent based on spectroscopy. For 1526 of the sources with redshifts, we use radio and X-ray luminosity, optical spectroscopy, mid-infrared colours and 24 µm and IR to radio flux ratios to search for the presence of an active galactic nucleus (AGN). The analysis reveals a rapid change in the population as flux density decreases from ~500 µJy to ~100 µJy. We find that 80.3 per cent of the objects show no evidence of AGN and have multiwavelength properties consistent with radio emission from star-forming galaxies (SFG). We classify 11.4 per cent as radio-quiet (RQ) AGN and the remaining 8.3 per cent as radio-loud (RL) AGN. The redshift of all populations extends to $z$ > 3 with a median of ~1. The median radio and far-IR luminosity increases systematically from SFG, to RQ AGN and RL AGN. The median $q_\text{24 µm}$ for SFG, 0.89 ± 0.01, is slightly below that for RQ AGN, 1.05 ± 0.03, and both differ substantially from the value for RL AGN of −0.06 ± 0.07. However, SFG and RQ AGN show no significant difference in far-IR/radio ratios and have statistically indistinguishable star formation rates inferred from radio and farIR luminosities. We conclude that radio emission from host galaxies of RQ AGN in this flux density regime results primarily from star formation activity.The authors acknowledge support from the Square Kilometre Array South Africa project, the South African National Research Foundation and Department of Science and Technology. EFO acknowledges funding from the National Astrophysics and Space Science Programme. MV acknowledges support from the European Commission Research Executive Agency (FP7-SPACE-2013-1 GA 607254), the South African Department of Science and Technology (DST/CON 0134/2014) and the Italian Ministry for Foreign Affairs and International Cooperation (PGR GA ZA14GR02). We thank the staff of the GMRT that made these observations possible. GMRT is run by the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research. This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA

The nature of the faint low-frequency radio source population

We present a multiwavelength study into the nature of faint radio sources in a deep radio image with the Giant Meterwave Radio Telescope at 612 MHz covering 1.2 deg$^2$ of the ELAIS N1 region. We detect 2800 sources above 50 µJy beam$^{−1}$. By matching to multiwavelength data, we obtain a redshift estimate for 63 per cent, with 29 per cent based on spectroscopy. For 1526 of the sources with redshifts, we use radio and X-ray luminosity, optical spectroscopy, mid-infrared colours and 24 µm and IR to radio flux ratios to search for the presence of an active galactic nucleus (AGN). The analysis reveals a rapid change in the population as flux density decreases from ~500 µJy to ~100 µJy. We find that 80.3 per cent of the objects show no evidence of AGN and have multiwavelength properties consistent with radio emission from star-forming galaxies (SFG). We classify 11.4 per cent as radio-quiet (RQ) AGN and the remaining 8.3 per cent as radio-loud (RL) AGN. The redshift of all populations extends to $z$ > 3 with a median of ~1. The median radio and far-IR luminosity increases systematically from SFG, to RQ AGN and RL AGN. The median $q_\text{24 µm}$ for SFG, 0.89 ± 0.01, is slightly below that for RQ AGN, 1.05 ± 0.03, and both differ substantially from the value for RL AGN of −0.06 ± 0.07. However, SFG and RQ AGN show no significant difference in far-IR/radio ratios and have statistically indistinguishable star formation rates inferred from radio and farIR luminosities. We conclude that radio emission from host galaxies of RQ AGN in this flux density regime results primarily from star formation activity.The authors acknowledge support from the Square Kilometre Array South Africa project, the South African National Research Foundation and Department of Science and Technology. EFO acknowledges funding from the National Astrophysics and Space Science Programme. MV acknowledges support from the European Commission Research Executive Agency (FP7-SPACE-2013-1 GA 607254), the South African Department of Science and Technology (DST/CON 0134/2014) and the Italian Ministry for Foreign Affairs and International Cooperation (PGR GA ZA14GR02). We thank the staff of the GMRT that made these observations possible. GMRT is run by the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research. This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA