Spitzer spectroscopy has revealed that ~80% of submm galaxies (SMGs) are starburst (SB) dominated in the mid-infrared. Here we focus on the remaining ~20% that show signs of harboring powerful active galactic nuclei (AGN). We have obtained Spitzer-IR
S spectroscopy of a sample of eight SMGs which are candidates for harboring powerful AGN on the basis of IRAC color-selection (S8/S4.5>2; i.e. likely power-law mid-infrared SEDs). SMGs with an AGN dominating (>50%) their mid-infrared emission could represent `missing link sources in an evolutionary sequence involving a major merger. First of all, we detect PAH features in all of the SMGs, indicating redshifts from 2.5-3.4, demonstrating the power of the mid-infrared to determine redshifts for these optically faint dusty galaxies. Secondly, we see signs of both star-formation (from the PAH features) and AGN activity (from continuum emission) in our sample: 62% of the sample are AGN-dominated in the mid-infrared with a median AGN content of 56%, compared with <30% on average for typical SMGs, revealing that our IRAC color selection has successfully singled out sources with proportionately more AGN emission than typical SB-dominated SMGs. However, we find that only about 10% of these AGN dominate the bolometric emission of the SMG when the results are extrapolated to longer infrared wavelengths, implying that AGN are not a significant power source to the SMG population overall, even when there is evidence in the mid-infrared for substantial AGN activity. When existing samples of mid-infrared AGN-dominated SMGs are considered, we find that S8/S4.5>1.65 works well at selecting mid-infrared energetically dominant AGN in SMGs, implying a duty cycle of ~15% if all SMGs go through a subsequent mid-infrared AGN-dominated phase in the proposed evolutionary sequence.
We present results from a multi-wavelength study of 29 sources (false detection probabilities <5%) from a survey of the Great Observatories Origins Deep Survey-North field at 1.1mm using the AzTEC camera. Comparing with existing 850um SCUBA studies i
n the field, we examine differences in the source populations selected at the two wavelengths. The AzTEC observations uniformly cover the entire survey field to a 1-sigma depth of ~1mJy. Searching deep 1.4GHz VLA, and Spitzer 3--24um catalogues, we identify robust counterparts for 21 1.1mm sources, and tentative associations for the remaining objects. The redshift distribution of AzTEC sources is inferred from available spectroscopic and photometric redshifts. We find a median redshift of z=2.7, somewhat higher than z=2.0 for 850um-selected sources in the same field, and our lowest redshift identification lies at a spectroscopic redshift z=1.1460. We measure the 850um to 1.1mm colour of our sources and do not find evidence for `850um dropouts, which can be explained by the low-SNR of the observations. We also combine these observed colours with spectroscopic redshifts to derive the range of dust temperatures T, and dust emissivity indices $beta$ for the sample, concluding that existing estimates T~30K and $beta$~1.75 are consistent with these new data.
Many models that seek to explain the origin of the unresolved X-ray background predict that Compton-thick Active Galactic Nuclei (AGNs) are ubiquitious at high redshift. However, few distant Compton-thick AGNs have been reliably identified to date. H
ere we present Spitzer-IRS spectroscopy and 3.6-70um photometry of a z=2.2 optically identified AGN (HDF-oMD49) that is formally undetected in the 2Ms Chandra Deep Field-North (CDF-N) survey. The Spitzer-IRS spectrum and spectral energy distribution of this object is AGN dominated, and a comparison of the energetics at X-ray wavelengths to those derived from mid-infrared (mid-IR) and optical spectroscopy shows that the AGN is intrinsically luminous (L_X~3x10^44 erg/s) but heavily absorbed by Compton-thick material (N_H>>10^24 cm^{-2}); i.e., this object is a Compton-thick quasar. Adopting the same approach that we applied to HDF-oMD49, we found a further six objects at z~2-2.5 in the literature that are also X-ray weak/undetected but have evidence for AGN activity from optical and/or mid-IR spectroscopy, and show that all of these sources are also Compton-thick quasars with L_X>10^44 erg/s. On the basis of the definition of Daddi etal. (2007), these Compton-thick quasars would be classified as mid-IR excess galaxies, and our study provides the first spectroscopic confirmation of Compton-thick AGN activity in a subsample of these z~2 mid-IR bright galaxies. Using the four objects that lie in the CDF-N field, we estimate the space-density of Compton-thick quasars [Phi~(0.7-2.5)x10^-5 Mpc^-3 for L_X>10^44 erg/s objects at z~2-2.5] and show that Compton-thick accretion is as ubiquitious as unobscured accretion in the distant Universe.
We present a 1200-micron image of the Great Observatories Origin Deep Survey North (GOODS-N) field, obtained with the Max Planck Millimeter Bolometer array (MAMBO) on the IRAM 30-m telescope. The survey covers a contiguous area of 287 square arcmin t
o a near-uniform noise level of ~0.7mJy/beam. After Bayesian flux deboosting, a total of 30 sources are recovered (>=3.5sigma). An optimal combination of our 1200-micron data and an existing 850-micron image from the Submillimetre Common-User Bolometer Array (SCUBA) yielded 33 sources (>=4sigma). We combine our GOODS-N sample with those obtained in the Lockman Hole and ELAIS-N2 fields (Scott et al. 2002; Greve et al. 2004) in order to explore the degree of overlap between 1200-micron- and 850-micron-selected galaxies (hereafter SMGs), finding no significant difference between their 850-micron to 1200-micron flux density distributions. However, a noise-weighted stacking analysis yields a significant detection of the 1200-micron-blank SCUBA sources, whereas no significant 850-micron signal is found for the 850-micron-blank MAMBO sources. The hypothesis that the 850/1200-micron flux density distribution of SCUBA sources is also representative of the MAMBO population is rejected at the ~4sigma level, via Monte Carlo simulations. Therefore, although the populations overlap, galaxies selected at 850 and 1200micron are different, and there is compelling evidence for a significant 1200-micron-detected population which is not recovered at 850micron. These are submm drop-outs (SDOs), with S_850/S_1200 = 0.7-1.7, requiring very cold dust or unusual spectral energy distributions (T_d ~ 10K; beta ~ 1), unless SDOs reside beyond the redshift range observed for radio-identified SMGs, i.e. at z > 4.
We present evidence for the existence of an IRAC excess in the spectral energy distribution (SED) of 5 galaxies at 0.6<z<0.9 and 1 galaxy at z=1.7. These 6 galaxies, located in the Great Observatories Origins Deep Survey field (GOODS-N), are star for
ming since they present strong 6.2, 7.7, and 11.3 um polycyclic aromatic hydrocarbon (PAH) lines in their Spitzer IRS mid-infrared spectra. We use a library of templates computed with PEGASE.2 to fit their multiwavelength photometry and derive their stellar continuum. Subtraction of the stellar continuum enables us to detect in 5 galaxies a significant excess in the IRAC band pass where the 3.3 um PAH is expected. We then assess if the physical origin of the IRAC excess is due to an obscured active galactic nucleus (AGN) or warm dust emission. For one galaxy evidence of an obscured AGN is found, while the remaining four do not exhibit any significant AGN activity. Possible contamination by warm dust continuum of unknown origin as found in the Galactic diffuse emission is discussed. The properties of such a continuum would have to be different from the local Universe to explain the measured IRAC excess, but we cannot definitively rule out this possibility until its origin is understood. Assuming that the IRAC excess is dominated by the 3.3 um PAH feature, we find good agreement with the observed 11.3 um PAH line flux arising from the same C-H bending and stretching modes, consistent with model expectations. Finally, the IRAC excess appears to be correlated with the star-formation rate in the galaxies. Hence it could provide a powerful diagnostic for measuring dusty star formation in z>3 galaxies once the mid-infrared spectroscopic capabilities of the James Webb Space Telescope become available.
