Deep radio imaging of the UKIDSS Ultra Deep Survey field : the nature of the faint radio population, and the star-formation history of the Universe
The centrepiece of this thesis is a deep, new, high-resolution 1.4-GHz image covering the United Kingdom Infrared (IR) Telescope IR Deep Sky Survey (UKIDSS) Ultra Deep Survey (UDS) legacy field. Deep pseudo-continuum observations were made using the Very Large Array, prior to its recent upgrade, in its A, B and DnC configurations. The resulting mosaic has a full-width-at-half-maximum synthesised beam width of ≈ 1.7 arcsec and a point-source sensitivity of ≈ 60μJy (6σ ) across the central 0.6 deg2, while conserving flux from sources of extended emission. The full image covers 1.3 deg2. I also present a catalogue containing over 1,000 radio emitters, having chosen the 6-σ threshold by maximising the number of radio sources with secure optical/near-IR counterparts. Most of the sources in the catalogue (≈ 90 per cent) lie in the sub-mJy flux density regime. Deep, complementary data covering a wide range of wavelengths was used to explore this faint radio population, whose nature remains controversial. It was found that 53 per cent of the sample comprise active galactic nuclei (AGN). AGN dominate at & 0.2mJy and remain a significant population down to 0.1mJy; at lower fluxes – the so-called μJy radio population – star-forming galaxies become dominant. The radio sample presented here was also matched to Hubble Space Telescope imaging of the UDS field (which is part of the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey – CANDELS) to classify the faint radio population morphologically. These classifications were done using the Gini–M20 method. It was found that a low fraction of AGN and SFGs are undergoing interactions and mergers, 33 ± 9 and 13 ± 7 per cent, respectively. The merger fraction does not appear to have evolved significantly since z ∼ 3. This suggests that mergers have played a relatively minor role in the assembly of galaxies and super-massive black holes – certainly less significant than previously thought. Finally, I present a study of cosmic star-formation activity as a function of stellar mass and redshift, exploiting panchromatic stacking. Mid-IR–through–radio images, including new data from Herschel, are stacked at the positions of a K-selected (i.e. an approximately mass-selected) sample in the UDS field. Specific star-formation rates (SSFR, i.e. star-formation rate per stellar mass, or the rate at which a galaxy is converting its gas into stars) were derived from UDS radio luminosities measured here and stellar masses from the literature. The SSFR was found to be poorly correlated with stellar mass; it decreases with decreasing redshift; at a given mass, SSFR rises with redshift. These results indicate that at early epochs, galaxies were forming stars more efficiently and at a higher rate.