Identification of faint radio sources with optically luminous interacting disk galaxies

THE question of when galaxies and stars started to form in the early Universe is one of the most fundamental in cosmology. Faint radio sources (with fluxes of a few microjanskys or less) may be these early galaxies; radio surveys have shown them to be increasingly common as fainter levels are reached1-7, suggesting that they are an important constituent of the early Universe. Many of these sources were identified with faint blue galaxies—some of which have peculiar morphology (perhaps arising from inter actions) 1,8-10 or with galaxies hosting enhanced star formation activity10-12, but lack of optical resolution has made the nature of these identifications rather uncertain. Here we present the results of a very deep radio survey, obtained with the Very Large Array, of a field imaged with the Wide Field Camera on the Bubble Space Telescope. We find that most of the microjansky sources are indeed faint blue galaxies, with light profiles resembling disk galaxies. More than half of these galaxies occur in pairs or small groups, compared to fewer than ten per cent of nearby galaxies. We conclude that the microjansky sources are luminous disk galaxies, in a starburst (or post-starburst) phase that was triggered by interactions or mergers. Only a few show evidence for an active nucleus2,13,14, which is the other possible power source for the emission.

[1]  S. Casertano,et al.  Calibration of HST wide field camera for quantitative analysis of faint galaxy images , 1994 .

[2]  T. Thuan,et al.  Near-infrared observations of submillijansky radio sources - Evidence for a population of starburst galaxies at intermediate redshifts , 1987 .

[3]  P. Wilkinson,et al.  Interferometer phase calibration sources – I. The region $35^{\circ} \leq \delta \leq 75^{\circ} $ , 1992 .

[4]  James J. Condon,et al.  Radio Emission from Normal Galaxies , 1992 .

[5]  R. B. Partridge,et al.  Limits to cosmic background radiation fluctuations at 8.44 GHz between angular scales 10″ and 200″ , 1993 .

[6]  James E. Gunn,et al.  Spectroscopy of galaxies in distant clusters. II: The population of the 3C 295 cluster , 1983 .

[7]  R. Kron,et al.  Sub-millijansky 1.4 GHz source counts and multicolor studies of weak radio galaxy populations , 1985 .

[8]  E. Fomalont,et al.  A Deep 6-Centimeter Radio Source Survey: Optical Identifications , 1987 .

[9]  M. Bershady,et al.  The Hubble Space Telescope Medium Deep Survey with the Wide Field and Planetary Camera. 1: Methodology and results on the field near 3C 273 , 1994 .

[10]  R. McMahon,et al.  Spectroscopy of faint radio sources: the nature of the sub-mJy radio-source population , 1993 .

[11]  R. Windhorst,et al.  The micro-jansky radio source population at 5 GHz , 1991 .

[12]  J. Condon.  The 1.4 gigahertz luminosity function and its evolution , 1989 .

[13]  J. Dunlop,et al.  Deep HST imaging of distant weak radio and field galaxies , 1994 .

[14]  B. Santiago,et al.  The morphology of faint galaxies in Medium Deep Survey images using WFPC2 , 1994 .

[15]  W. Keel,et al.  Evidence for Evolution in the Galaxy Merger Rate , 1994 .

[16]  K. Kellermann,et al.  Radio Source Counts and their Interpretation , 1987 .

[17]  J. Ostriker,et al.  Are cosmologically distant objects obscured by dust? - A test using quasars. , 1984 .

[18]  R. B. Partridge,et al.  Microjansky source counts and spectral indices at 8.44 GHz , 1993 .

[19]  R. B. Partridge,et al.  6 centimeter radio-source counts and spectral index studies down to 0.1 millijansky , 1987 .

[20]  J. J. Condon.  Cosmological evolution of radio sources , 1984 .

[21]  Hubble Space Telescope Medium Deep Survey. 2: Deconvolution of Wide Field Camera field galaxy images in the 13 hour + 43 deg field , 1994 .