Optical R-band image (left) of the apparently faint sub-mJy radio galaxy PDF011255 detected in the course of the Phoenix Deep Radio Survey. The radio contours are overlaid on the optical image. On the right is the optical spectrum of the radio galaxy. This lies at a redshift of z=0.445 and shows strong narrow emission lines indicative of ongoing star-formation activity. The Phoenix survey has revealed a large number of apparently faint radio galaxies similar to PDF011255. Detailed studies suggest that these sources are likely to be intermediate and high redshift galaxies dominated by star-formation, although the presence of an obscured AGN is under debate.

Deep radio surveys reaching sub-mJy flux densities have revealed a new population of apparently faint radio sources below the flux density of 1mJy. This new radio population has properties significantly different form brighter radio sources (>10mJy) usually identified with quasars (QSOs) and giant ellipticals harboring a powerful Active Galactic Nucleus (AGN).

Indeed, photometric and spectroscopic studies of apparently faint (sub-mJy and µJy) radio sources reveal that these objects are usually identified with disc type galaxies at intermediate and high redshifts. Their optical spectra exhibit narrow emission lines indicative of star-formation activity, although a contribution from an obscured AGN cannot be excluded and is under debate. The similarity of these objects with the nearby population of starbursts revealed by IRAS suggest that the sub-mJy faint radio population is the more distant counterpart of these nearby galaxies.

The Phoenix Deep Survey is an ongoing project aiming to study the nature and the evolution of the apparently faint radio population. The observations comprise an ultra-deep and homogeneous radio survey (1.4GHz) covering an area of 3 deg² and reaching µJy sensitivities. This is combined with multiwavelength photometric data (UV, optical, near- and far- infrared) and optical spectroscopy. Moreover, deep X-ray observations (50ksec) using the XMM have recently been obtained. This unique multiwavelength data-set is used to address the following issues:

• elucidate the nature of the energising source (AGN or star-formation) that dominates the observed activity of the apparently faint sub-mJy radio sources.
• study the cosmological evolution of sub-mJy radio galaxies to constrain galaxy formation and evolution models.
• investigate the evolution of the global star-formation rate. If the sub-mJy faint radio population is indeed dominated by starburst activity (as present studies indicate) then deep radio surveys, like Phoenix, offer an efficient tool of identifying and selecting the most active starburst galaxies out to z=1-2. Moreover, the insensitivity of radio wavelengths to dust obscuration makes radio surveys an unbiased census of the star-formation rate evolution unlike UV/optical data.
• explore the significance of interactions and mergers in triggering the observed activity in sub-mJy radio sources.
• explore the association of the sub-mJy radio population with other classes of extra-galactic sources, also believed to be starburst and/or AGNs at intermediate and high redshifts. These include the X-ray detected Narrow Emission-Line Galaxies (NELGs), the Extremely Red Objects (EROs) and the excess blue galaxies detected in deep optical surveys. Such a study will provide direct links between potentially similar objects selected at different wavelengths, providing complementary information necessary to elucidate their nature.