arXiv:astro-ph/0411492v1 17 Nov 2004 Mem. S.A.It. Vol. , 1 c  SAIt 2002 Memorie della M. Orienti 1,2 , M.A. Garrett 3 , C. Reynolds 3 and R. Morganti 4 1 Istituto di Radioastronomia - CNR, via Gobetti 101, I-40129 Bologna, Italy e-mail: orienti@ira.cnr.it 2 Dipartimento di Astronomia, Universit`a di Bologna, via Ranzani 1, I-40127 Bologna, Italy 3 Joint Institute for VLBI in Europe, Postbus 2, 7990 AA, Dwingeloo, The Netherlands 4 Netherlands Foundation for Research in Astronomy, Postbus 2, 7990 AA, Dwingeloo, The Netherlands Abstract. 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, 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. However, a significant fracion of radio sources detected by WSRT (∼ 25%) have no mid-IR identification in the FLSv (implying a 24 μm flux density ≤ 100 μJy). 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. Key words. starburst galaxies – infrared galaxies – low-luminosity AGN 1. Introduction Deep radio surveys (S ≤ 1 mJy) have clearly indicated the emergence of a new population of radio sources at mJy and sub- Send offprint requests to : M. Orienti Correspondence to : IRA - CNR, via Gobetti 101, I-40129 Bologna, Italy mJy levels. Several class of objects have been invoked to explain the steep rise in the integral radio source counts at faint sub- mJy levels: star forming galaxies, similar to M 82 and Arp 220 (Rowan-Robinson et al. 1993), and low-luminosity AGN like M 84. The fact that the locally derived far-