Millimeter observations of obscured Spitzer 24 μm sources

We present MAMBO 1.2 mm observations of 40 extragalactic sources from the Spitzer First Look Survey that are bright in the mid-IR (S-24 mu m > 1 mJy) but optically obscured (log [vF(v)(24 mu m)/vF(v)(0.7 mu m)] > 1). We use these observations to search for cold dust emission, probing the similarity of their spectral energy distributions to star-forming IR galaxies or obscured AGNs. The sample as a whole is well detected at mean mJy and S-1.2 mm = 0.74 +/- 0.09 mJy and S-1.2 mm/S-24 mu m = 0.15 +/- 0.03. Seven (three) of the sources are individually detected at > 3 sigma (> 5 sigma) levels. Mean millimeter fluxes are higher for sources with the reddest mid-IR/optical colors. Optically faint but with relatively low millimeter-to-mid-IR ratios, the typical SEDs are inconsistent with redshiftedSED shapes of local star-forming IR galaxies. They also differ from SEDs of typical submillimeter-selected galaxies, with the 24 mm sources that are individually detected by MAMBO possibly representing intermediate objects. Compared to star-forming galaxies, a stronger but optically obscured mid-IR component without associated strong far-IR emission has to be included. This component may be due to luminous optically obscured AGN, which would represent a significant part of the high-redshift AGN population.