Spitzer 24 μ m imaging of Faint Radio Sources in the FLSv : a new radio - loud , Mid - IR / optically obscured population ?

Data from the Spitzer Space Telescope (the First Look Survey-FLS) have recently been made public. We have compared the 24 µm images with very deep WSRT 1.4 GHz observations (Morganti et al. 2004), centred on the FLS verification strip (FLSv). Approximately 75% of the radio sources have corresponding 24 µm identifications. Such a close correspondence is expected, especially at the fainter radio flux density levels, where star forming galaxies are thought to dominate both the radio and mid-IR source counts. Spitzer detects many sources that have no counterpart in the radio. However, a significant fraction of radio sources detected by the WSRT (∼ 25%) have no mid-IR identification in the FLSv (implying a 24 µm flux density ≤ 100 µJy). The fraction of radio sources without a counterpart in the mid-IR appears to increase with increasing radio flux density, perhaps indicating that some fraction of the AGN population may be detected more readily at radio than Mid-IR wavelenghts. We present initial results on the nature of the radio sources without Spitzer identification, using data from various multi-waveband instruments, including the publicly available R-band data from the Kitt Peak 4-m telescope.