A deep VLA survey at 6 cm in the Lockman Hole

Abstract

Abstract.We have obtained a deep radio image with the Very Large Array at 6 cm in the Lockman Hole. The noise level in the central part of the field is ∼11 µJy. From these data we have extracted a catalogue of 63 radio sources with a maximum distance of 10 arcmin from the field center and with peak flux density greater than 4.5 times the local rms noise. The di fferential source counts are in good agreement with those obtained by other surveys. The analysis of the radio spectral index suggests a flattening of the average radio spectra and an increase of the population of flat spectrum radio sources in the faintest flux bin. Cross correlation with the ROSAT/XMM X-ray sources list yields 13 reliable radio/X-ray associations, corresponding to ∼21% of the radio sample. Most of these associations (8 out of 13) are classified as type II AGN. Using optical CCD (V and I) and K band data with approximate limits of V ∼ 25.5 mag, I ∼ 24.5 mag and K ∼ 20.2 mag, we found an optical identification for 58 of the 63 radio sources. This corresponds to an identification rate of ∼92%, one of the highest percentages so far available. From the analysis of the colour-colour diagram and of the radio flux -optical magnitude diagram we have been able to select a subsample of radio sources whose optical counterparts are likely to be high redshift (z > 0.5) early-type galaxies, hosting an Active Galactic Nucleus responsible of the radio activity. This class of objects, rather than a population of star-forming galaxies, appears to be the dominant population ( 50%) in a 5 GHz selected sample with a flux limit as low as 50 µJy. We also find evidence that at these faint radio limits a large fraction (∼60%) of the faintest optical counterparts (i.e. sources in the magnitude range 22.5 < I < 24.5 mag) of the radio sources are Extremely Red Objects (EROs) with I − K > 4 and combining our radio data with existing ISO data we conclude that these EROs sources are probably associated with high redshift, passively evolving elliptical galaxies. The six radio selected EROs represent only ∼2% of the optically selected EROs present in the field. If their luminosity is indeed a sign of AGN activity, the small number of radio detections suggests that a small fraction of the EROS population contains an active nucleus.

The Bolocam Lockman Hole Millimeter‐Wave Galaxy Survey: Galaxy Candidates and Number Counts

“…The circles of Bolocam, SCUBA, and MAMBO observations correspond to 2 confidence regions of position, including both beam sizes and stated pointing errors. The 6 cm VLA radio sources of Ciliegi et al (2003) and unpublished 21 cm VLA sources of M. Yun (2004, private communication), with average noise levels of 11 and 10-15 Jy beam À1 , respectively, are identified. The M. Yun (2004, private communication) radio field covers the entire Bolocam good coverage region, while the center of Ciliegi et al (2003) observations is offset to the northwest, with an overlap of approximately 130 arcmin 2 .…”

“…The 6 cm VLA radio sources of Ciliegi et al (2003) and unpublished 21 cm VLA sources of M. Yun (2004, private communication), with average noise levels of 11 and 10-15 Jy beam À1 , respectively, are identified. The M. Yun (2004, private communication) radio field covers the entire Bolocam good coverage region, while the center of Ciliegi et al (2003) observations is offset to the northwest, with an overlap of approximately 130 arcmin 2 . Also shown are the 20 published radio sources from deep 21 cm VLA observations (average noise level of 4.8 Jy beam À1 ) from Ivison et al (2002) that are coincident with SCUBA sources, as well as an additional 21 cm VLA source discovered by Egami et al (2004) from the reexamination of the Ivison et al (2002) map.…”

Dusty star-forming galaxies at high redshift

“…The radial probability distribution of the separation between submm source and the shorter wavelength galaxies is denoted by f (r). This likelihood ratio (LR) method was developed in Sutherland & Saunders (1992) with additional improvements in Ciliegi et al (2003), Brusa et al (2007) and−in the context of submillimeter data−in Chapin et al (2011). This likelihood ratio estimator is similar to the calculation of "p-values," or the corrected-Poissonian probability.…”

Star formation properties of sub-mJy radio sources

“…The radio catalogue includes 883 sources. Using a likelihood ratio technique (Ciliegi et al 2003), the optical/infrared counterparts of the radio sources have been identified (Bonzini et al 2012). Excluding the outermost region as defined in Bonzini et al (2012), the wealth of multi-wavelength data available for the E-CDFS allows robust estimates of photometric redshifts (Santini et al 2009;Taylor et al 2009;Cardamone et al 2010;Rafferty et al 2011).…”