Abstract
Spectra have been obtained with the Infrared Spectrograph (IRS) on the Spitzer Space Telescope for 20 sources in the Lockman Hole field of the SWIRE survey. The sample is divided between sources with indicators of an obscured AGN, based primarily on X-ray detections of optically faint sources, and sources with indicators of a starburst, based on optical and near-infrared spectral energy distributions (SEDs), which show a luminosity peak from stellar photospheric emission. Ten of the 11 AGN sources have IRS spectra that show silicate absorption or are power laws; only one AGN source shows PAH emission features. All nine of the sources showing starburst SEDs in the near-infrared show PAH emission features in the IRS spectra. Redshifts are determined from the IRS spectra for all nine starbursts (1.0 < z < 1.9) and 8 of the 11 AGNs (0.6 < z < 2.5). Classification as AGN because of an X-ray detection, the classification as AGN or starburst derived from the photometric SED, and the IRS spectroscopic classification as AGN (silicate absorption) or starburst (PAH emission) are all consistent in 18 of 20 sources. The surface density for starbursts that are most luminous in the mid-infrared is less than that for the most luminous AGNs within the redshift interval 1.7 ≾ z ≾ 1.9. This result implies that mid-infrared source counts at high redshift are dominated by AGNs for f_ν(24 μm) ≳ 1.0 mJy.
Highlights
Imaging surveys with the Spitzer Space Telescope using the Multiband Imaging Photometer ( MIPS) ( Rieke et al 2004) detect sufficient sources at 24 m to resolve the cosmic background (Hauser et al 1998) at mid-infrared wavelengths (Papovich et al 2004; Dole et al 2006)
Using spectral energy distributions (SEDs) with a sufficiently dominant stellar component in near-infrared wavelengths to yield a photometric redshift, we have identified those sources within SWIRE survey fields that fall into various redshift ranges based on our estimates of the photometric redshift
Mid-infrared spectra obtained with the IRS on Spitzer are presented for a sample of 20 sources with f(24 m) ! 1:0 mJy derived from the SWIRE survey of the Lockman Hole field
Summary
Imaging surveys with the Spitzer Space Telescope using the Multiband Imaging Photometer ( MIPS) ( Rieke et al 2004) detect sufficient sources at 24 m to resolve the cosmic background (Hauser et al 1998) at mid-infrared wavelengths (Papovich et al 2004; Dole et al 2006). The source counts have been modeled to explain the infrared background as arising from the evolution of luminous, star-forming galaxies (starbursts) whose luminosity is primarily in the infrared and arises from emission by dust (Lagache et al 2004; Chary et al 2004). It is known from various infrared and X-ray studies that there are many obscured active galactic nuclei (AGNs) (Alexander et al 2001), so many of the Spitzer mid-infrared sources could be dusty AGNs instead of starbursts. This expectation has not been confirmed with Spitzer spectroscopic observations
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