Galaxy Evolution in the Reddest Possible Filter |
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Authors: | EA Richards |
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Institution: | (1) National Radio Astronomy Observatory and, University of Virginia, 520 Edgemont Road, Charlottesville, VA 22903, USA |
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Abstract: | We describe an observational programme aimed at understanding the radio emission from distant, rapidly evolving galaxy populations.
These observations were carried out at 1.4 and 8.5 GHz with the VLA, centred on the Hubble Deep Field, obtaining limiting
flux densities of 40 and 8 μJy respectively. The differential count of the radio sources is marginally sub-Euclidean to the
completeness limits(γ = − 2.4 ± 0.1) and fluctuation analysis suggests nearly 60 sources per arcmin2 at the 1 μJy level. Using high-resolution 1.4 GHz observations obtained with MERLIN, we resolve all radio sources detected
in the VLA complete sample and measure a median angular size for the microjansky radio population of 1-2". This clue, coupled
with the steep spectral index of the 1.4 GHz selected sample, suggests diffuse synchrotron radiation in z ∼ 1 galactic discs. The wide-field HST and ground-based optical exposures show that the radio sources are identified primarily
with disc systems composed of irregulars, peculiars, interacting/merging galaxies and a few isolated field spirals. Only 20%
of the radio sources can be attributed to AGN – the majority are probably associated with starburst activity. The available
redshifts range from 0.1 to 3, with a mean of about 0.8. We are plrobably witnessing a major episode of starburst activity
in these luminous (L > L
*) systems, occasionallyaccompanied by an embedded AGN.About 20% of the radio sources remain unidentified to I = 26-28 inthe HDF and flanking fields. Several of these objectshave extremely red counterparts. We suggestthat these are
high-redshift dusty protogalaxies.
This revised version was published online in July 2006 with corrections to the Cover Date. |
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