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

Herrera Ruiz, N. and Middelberg, E. and Deller, A. and Smolčić, V. and Norris, R. P. and Novak, M. and Delvecchio, I. and Best, P. N. and Schinnerer, E. and Momjian, E. and Dettmar, R.-J. and Brisken, W. and Koekemoer, A. M. and Scoville, N. Z. (2018) VLBA+GBT observations of the COSMOS field and radio source counts at 1.4 GHz. Astronomy and Astrophysics, 616 . Art. No. A128. ISSN 0004-6361.

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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 μJy. Here, the identification of active galactic nuclei (AGN) is based on the VLBI detection of the source, meaning that 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 ≥ 0.1). On the other hand, this technique only allows us to positively identify when a radio-active AGN is present, in other words, 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, ten 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 μJy 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.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Smolčić, V.0000-0002-3893-8614
Delvecchio, I.0000-0001-8706-2252
Schinnerer, E.0000-0002-3933-7677
Momjian, E.0000-0003-3168-5922
Koekemoer, A. M.0000-0002-6610-2048
Scoville, N. Z.0000-0002-0438-3323
Additional Information:© ESO 2018. Received 6 March 2018 / Accepted 18 April 2018. N.H.R. acknowledges support from the Deutsche Forschungsgemeinschaft through project MI 1230/4-1. V.S., M.N. and I.D. acknowledge the European Union’s Seventh Framework programme under grant agreement 337595 (ERC Starting Grant, “CoSMass”). P.N.B. is grateful for support from STFC via grant ST/M001229/1. We thank the anonymous referee for the helpful and constructive comments, which have improved this paper. This work made use of the Swinburne University of Technology software correlator, developed as part of the Australian Major National Research Facilities Programme and operated under licence. This work made use of Topcat (Taylor 2005), available at This work also made use of APLpy, an open-source plotting package for Python hosted at, and Astropy, a community-developed core Python package for Astronomy (Astropy Collaboration et al. 2013). We wish to thank the staff of the VLBA and the GBT who greatly supported the experimental observations in this project. The VLBA and the GBT are operated by the Long Baseline Observatory and the Green Bank Observatory, respectively, facilities of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.
Funding AgencyGrant Number
Deutsche Forschungsgemeinschaft (DFG)MI 1230/4-1
European Research Council (ERC)337595
Science and Technology Facilities Council (STFC)ST/M001229/1
Associated Universities, Inc.UNSPECIFIED
Subject Keywords:catalogs / galaxies: active / radio continuum: galaxies
Record Number:CaltechAUTHORS:20180912-122527893
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Official Citation:VLBA+GBT observations of the COSMOS field and radio source counts at 1.4 GHz N. Herrera Ruiz, E. Middelberg, A. Deller, V. Smolčić, R. P. Norris, M. Novak, I. Delvecchio, P. N. Best, E. Schinnerer, E. Momjian, R.-J. Dettmar, W. Brisken, A. M. Koekemoer and N. Z. Scoville A&A, 616 (2018) A128 DOI:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:89576
Deposited By: George Porter
Deposited On:12 Sep 2018 19:37
Last Modified:12 Sep 2018 19:57

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