Thursday, 9 August at 11.00am @ Gemini

ABSTRACT: Discerning the exact nature of the sub-mJy radio population has been historically difficult due to the low luminosity of these sources at most wavelengths. Using deep ground based follow-up and observations from the Spitzer Space Telescope we are able to disentangle the Active Galactic Nuclei (AGN) and star-forming populations for the first time in a deep multi-frequency VLA/GMRT/MERLIN Survey of the 13^H XMM-Newton/Chandra Deep Field.

The discrimination diagnostics include radio luminosity, radio morphology, radio to mid-IR flux density ratio, radio to optical flux density ratio and radio spectral index. Hence we can separate the faint radio sources into AGN and star-forming galaxies and re-derive the extra-galactic Euclidean normalised source counts by type. We find that while star-forming galaxies dominate at the faintest flux densities and account for the up-turn in the counts, AGN still make up 25-30% of the counts at ~50uJy. Using radio luminosity as an unbiased star-formation rate indicator we are then able to examine the star-formation history of the universe up to z=2.5 which is consistent with measures at other wavelengths.

This work confirms, and provides an alternative evidence for, the distribution of star-formation by mass: `downsizing'. The low `characteristic times' (inverse specific star-formation rates) are similar to local radio selected 6dF-NVSS star-forming galaxies. This result suggests that virtually all the galaxies selected in the 13^H field are in a `burst mode' and we speculate on what may trigger this burst of star-formation.

PLEASE NOTE: This Seminar is being hosted by Gemini at their Hilo offices next to the JAC.