The nature of the Mid-IR faint radio sources from the Spitzer First Look Survey

Data from the Spitzer Space Telescope (the First Look Survey - FLS) have recently been made public. We have compared the 24 micron images with very deep WSRT 1.4 GHz observations, centred on the FLS verification strip (FLSv). Approximately 75% of the radio sources have corresponding 24 micron 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. However, a significant fraction of radio sources detected by WSRT (25%) have no mid-IR detection in the FLSv (implying a 24 micron flux density less than 0.1 mJy). 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.