Astronomy & Astrophysics

Issue		A&A Volume 463, Number 2, February IV 2007
Page(s)		519 - 527
Section		Extragalactic astronomy
DOI		10.1051/0004-6361:20066428

A&A 463, 519-527 (2007)
DOI: 10.1051/0004-6361:20066428

The VVDS-VLA deep field

III. GMRT observations at 610 MHz and the radio spectral index properties of the sub-mJy population

M. Bondi¹, P. Ciliegi², T. Venturi¹, D. Dallacasa^{3, 1}, S. Bardelli², E. Zucca², R. M. Athreya⁴, L. Gregorini^{5, 1}, A. Zanichelli¹, O. Le Fèvre⁶, T. Contini⁷, B. Garilli⁸, A. Iovino⁹, S. Temporin⁹, and D. Vergani⁸

¹ INAF - Istituto di Radioastronomia, via Gobetti 101, 40129 Bologna, Italy
² INAF - Osservatorio Astronomico di Bologna, via Ranzani 1, 40127, Bologna, Italy
³ Università degli Studi di Bologna, Dipartimento di Astronomia, via Ranzani 1, 40127 Bologna, Italy
⁴ NCRA, Tata Institute of Fundamental Research Pune University Campus, Post Bag 3, Ganeshkhind Pune 411007, India
⁵ Università degli Studi di Bologna, Dipartimento di Fisica, via Irnerio 46, 40126 Bologna, Italy
⁶ Laboratoire d'Astrophysique de Marseille, UMR 6110 CNRS-Université de Provence, BP 8, 13376 Marseille Cedex 12, France
⁷ Laboratoire d'Astrophysique de l'Observatoire Midi-Pyrénées (UMR 5572), 14 avenue E. Belin, 31400 Toulouse, France
⁸ IASF-INAF - via Bassini 15, 20133 Milano, Italy
⁹ INAF - Osservatorio Astronomico di Brera, via Brera 28, Milan, Italy

(Received 22 September 2006 / Accepted 17 October 2006 )

Abstract
Aims.We present the low frequency (610 MHz) radio source counts of the VVDS-VLA field and investigate the radio spectral index properties of the sub-mJy population.
Methods.We use new deep ( ${\rm rms}\simeq 50~\mu$ Jy/beam) observations of the VVDS-VLA field obtained at 610 MHz with the GMRT and matched in resolution (6 arcsec) with already available VLA data at 1.4 GHz on the same field.
Results.We find evidence of a change of the dominant population of radio sources below 0.5 mJy (at 1.4 GHz): between 0.15 and 0.5 mJy the median spectral index is significantly flatter ( $\alpha=-0.46\pm 0.03$ ) than that of brighter sources ( $\alpha=-0.67\pm 0.05$ ). A relevant contribution below 0.5 mJy from a population of flat spectrum low luminosity compact AGNs and radio quiet QSOs could explain this effect. At even fainter flux density, between 0.10 and 0.15 mJy at 1.4 GHz, the median spectral index steepens again ( $\alpha=-0.61\pm 0.04$ ) suggesting that the contribution of starburst galaxies becomes important below $\sim$ 0.2 mJy. Finally we present a sample of 58 candidate ultra-steep sources with radio flux density from one to two orders of magnitude lower than any other sample of such objects.

Key words: surveys -- radio continuum: galaxies -- methods: data analysis

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