VLBA+GBT observations of the COSMOS field and radio source counts at 1.4 GHz

Abstract

We present very long baseline interferometry (VLBI) observations of 179 radio sources in the COSMOS field with extremely high sensitivity using the Green Bank Telescope (GBT) together with the Very Long Baseline Array (VLBA) (VLBA+GBT) at 1.4 GHz, to explore the faint radio population in the flux density regime of tens of $\mu$Jy. Here, the identification of active galactic nuclei (AGN) is based on the VLBI detection of the source, i.e., it is independent of X-ray or infrared properties. The milli-arcsecond resolution provided by the VLBI technique implies that the detected sources must be compact and have large brightness temperatures, and therefore they are most likely AGN (when the host galaxy is located at z$\ge$0.1). On the other hand, this technique allows us to only positively identify when a radio-active AGN is present, i.e., we cannot affirm that there is no AGN when the source is not detected. For this reason, the number of identified AGN using VLBI should be always treated as a lower limit. We present a catalogue containing the 35 radio sources detected with the VLBA+GBT, 10 of which were not previously detected using only the VLBA. We have constructed the radio source counts at 1.4 GHz using the samples of the VLBA and VLBA+GBT detected sources of the COSMOS field to determine a lower limit for the AGN contribution to the faint radio source population. We found an AGN contribution of >40-75% at flux density levels between 150 $\mu$Jy and 1 mJy. This flux density range is characterised by the upturn of the Euclidean-normalised radio source counts, which implies a contribution of a new population. This result supports the idea that the sub-mJy radio population is composed of a significant fraction of radio-emitting AGN, rather than solely by star-forming galaxies, in agreement with previous studies.

arXiv comment14 pages, 8 figures, 4 tables. Accepted for publication in A&A

References in corpus (40)

• Astropy: A Community Python Package for Astronomy
• The Cosmic Evolution Survey (COSMOS) -- Overview
• The COSMOS2015 Catalog: Exploring the 1<z<6 Universe with half a million galaxies
• zCOSMOS: A Large VLT/VIMOS redshift survey covering 0 < z < 3 in the COSMOS field
• The COSMOS Survey: Hubble Space Telescope / Advanced Camera for Surveys (HST/ACS) Observations and Data Processing
• DiFX2: A more flexible, efficient, robust and powerful software correlator
• The Chandra COSMOS Legacy survey: overview and point source catalog
• A Public Ks-selected Catalog in the COSMOS/UltraVISTA Field: Photometry, Photometric Redshifts and Stellar Population Parameters
• The VLA-COSMOS 3 GHz Large Project: Continuum data and source catalog release
• DiFX: A software correlator for very long baseline interferometry using multi-processor computing environments
• The VLA-COSMOS Survey: II. Source Catalog of the Large Project
• The VLA-COSMOS Survey. IV. Deep Data and Joint Catalog
• AGN Host Galaxy Morphologies in COSMOS
• Radio imaging of the Subaru/XMM-Newton Deep Field - I. The 100-microJy catalogue, optical identifications, and the nature of the faint radio source population
• The VLA-COSMOS 3 GHz Large Project: Multiwavelength counterparts and the composition of the faint radio population
• The faint radio sky: radio astronomy becomes mainstream
• The VLA-COSMOS 3~GHz Large Project: AGN and host-galaxy properties out to z$\lesssim$6
• Pixel-based correction for Charge Transfer Inefficiency in the Hubble Space Telescope Advanced Camera for Surveys
• The zCOSMOS Redshift Survey: the role of environment and stellar mass in shaping the rise of the morphology-density relation from z~1
• The Phoenix Deep Survey: The 1.4 GHz microJansky catalogue
• A new method to separate star forming from AGN galaxies at intermediate redshift: The submillijansky radio population in the VLA-COSMOS survey
• The VLA Survey of the Chandra Deep Field South. V. Evolution and Luminosity Functions of sub-mJy radio sources and the issue of radio emission in radio-quiet AGN
• The VLA-COSMOS Survey: III. Further Catalog Analysis and the Radio Source Counts
• The Caltech-NRAO Stripe 82 Survey (CNSS) Paper I: The Pilot Radio Transient Survey In 50 deg${}^2$
• Radio faint AGN: a tale of two populations
• Exploring the infrared/radio correlation at high redshift
• The VLA Survey of the Chandra Deep Field South. IV. Source Population
• Compton Thick AGN in the XMM-COSMOS survey
• Radio Observations of the 13hr XMM/ROSAT Deep X-ray Survey Area
• The accretion history of the Universe with the SKA
• The Radio/Optical Catalog of the SSA13 Field
• Variability of sub-mJy radio sources
• The faint radio sky: VLBA observations of the COSMOS field
• The Radio/Mid-Infrared Correlation and the Contribution of 15-micron Galaxies to the 1.4-GHz Source Counts
• Mosaiced wide-field VLBI observations of the Lockman Hole/XMM
• The Chandra Deep Fields: Lifting the Veil on Distant Active Galactic Nuclei and X-Ray Emitting Galaxies
• Wide-field VLBI Imaging
• The Contribution of Active Galactic Nuclei to the Microjansky Radio Population
• Are the hosts of VLBI selected radio-AGN different to those of radio-loud AGN?
• A wider audience: Turning VLBI into a survey instrument

Cited by in corpus (7)

• The infrared-radio correlation of star-forming galaxies is strongly M${}_{\star }$-dependent but nearly redshift-invariant since z$\sim$4
• Bubbles and outflows: the novel JWST/NIRSpec view of the z=1.59 obscured quasar XID2028
• Fundamental Reference AGN Monitoring Experiment (FRAMEx) I: Jumping Out of the Plane with the VLBA
• Nature of Faint Radio Sources in GOODS-North and GOODS-South Fields - I. Spectral Index and Radio-FIR Correlation
• A super-linear "radio-AGN main sequence'' links mean radio-AGN power and galaxy stellar mass since z$\sim$3
• SPARCS-North Wide-field VLBI Survey: Exploring the resolved MicroJy extra-galactic radio source population with EVN+e-MERLIN
• Faint polarised sources in the Lockman Hole field at 1.4 GHz