Millimeter Observations of Obscured Spitzer 24 μm Sources

Abstract

We present MAMBO 1.2 mm observations of 40 extragalactic sources from the Spitzer First Look Survey that are bright in the mid-IR (S24 μm > 1 mJy) but optically obscured (log > 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 S1.2 mm = 0.74 ± 0.09 mJy and S1.2 mm/S24 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.