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 μm} > 1 mJy) but optically obscured (log [vF_v(24 μm)/vF _v(0.7 μm)] > 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 S_{1.2 mm} = 0.74 ± 0.09 mJy and S _{1.2 mm}/S_{24 μm} = 0.15 ± 0.03. Seven (three) of the sources are individually detected at >3 σ (>5 σ) 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 redshifted SED shapes of local star-forming IR galaxies. They also differ from SEDs of typical submillimeter-selected galaxies, with the 24 μm 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.