We present spectral energy distributions (SEDs), Spitzer colours, and infrared (IR) luminosities for 850‐μm selected galaxies in the Great Observatories Origins Deep Survey Northern (GOODS‐N) field. Using the deep Spitzer Legacy images and new data and reductions of the Very Large Array‐Hubble Deep Field (VLA‐HDF) radio data, we find statistically secure counterparts for 60 per cent (21/35) of our submillimetre (submm) sample, and identify tentative counterparts for another 12 objects. This is the largest sample of submm galaxies with statistically secure counterparts detected in the radio and with Spitzer. Half of the secure counterparts have spectroscopic redshifts, while the other half have photometric redshifts. We find that in most cases the 850‐μm emission is dominated by a single 24‐μm source, with a median flux density of 241 μJy, leading to a median 24‐to‐850‐μm flux density ratio of 0.040. A composite rest‐frame SED shows that the submm sources peak at longer wavelengths than those of local ultraluminous infrared galaxies (ULIRGs). Using a basic grey‐body model, 850‐μm selected galaxies appear to be cooler than local ULIRGs of the same luminosity. This demonstrates the strong selection effects, both locally and at high redshift, which may lead to an incomplete census of the ULIRG population. The SEDs of submm galaxies are also different from those of their high‐redshift neighbours, the near‐IR selected BzK galaxies, whose mid‐IR‐to‐radio SEDs are more like those of local ULIRGs. Using 24‐μm, 850‐μm and 1.4‐GHz observations, we fit templates that span the mid‐IR through radio to derive the integrated IR luminosity (LIR) of the submm galaxies and find a median value of LIR(8–1000 μm) = 6.0 × 1012 L⊙. By themselves, 24‐μm and radio fluxes are able to predict LIR reasonably well because they are relatively insensitive to temperature. However, the submm flux by itself consistently overpredicts LIR when using spectral templates which obey the local ULIRG temperature–luminosity relation. The shorter Spitzer wavelengths sample the stellar bump at the redshifts of the submm sources, and we find that the Spitzer photometry alone provides a model‐independent estimate of the redshift, σ[Δz/(1 +z)]= 0.07. The median redshift for our secure submm counterparts is 2.0. Using X‐ray and mid‐IR data, only 5 per cent of our secure counterparts (1/21) show strong evidence for an active galactic nucleus dominating the LIR.

Selection of active galactic nuclei (AGN) in the infrared allows the discovery of AGN whose optical emission is extinguished by dust. In this paper, we use the Spitzer Space Telescope First Look Survey (FLS) to assess what fraction of AGN with mid-infrared luminosities comparable to quasars are missed in optical quasar surveys due to dust obscuration. We begin by using the Sloan Digital Sky Survey (SDSS) database to identify 54 quasars within the 4 deg 2 extragalactic FLS. These quasars occupy a distinct region in mid-infrared color space by virtue of their strong, red, continua. This has allowed us to define a mid-infrared color criterion for selecting AGN candidates. About 2000 FLS objects have colors consistent with them being AGN, but most are much fainter in the mid-infrared than the SDSS quasars, which typically have 8µm flux densities, S 8.0 , ∼ 1mJy. We have investigated the properties of the 43 objects with S 8.0 ≥ 1mJy satisfying our AGN color selection. This sample should contain both unobscured quasars, and AGN which are absent from the SDSS survey due to extinction in the optical. After removing 16 known quasars, three probable normal quasars, and eight spurious or confused objects from the initial sample of 43, we are left with 16 objects which are likely to be obscured quasars or luminous Seyfert-2 galaxies. This suggests the numbers of obscured and unobscured AGN are similar in samples selected in the mid-infrared at S 8.0 ∼ 1mJy.

We present mid-IR spectral decomposition of a sample of 48 Spitzer-selected ULIRGs spanning z ∼ 1 -3 and likely L IR ∼ 10 12 -10 13 L ⊙ . Our study aims at quantifying the star-formation and AGN processes in these sources which recent results suggest have evolved strongly between the observed epoch and today. To do this, we study the mid-IR contribution of PAH emission, continuum, and extinction. About 3/4 of our sample are continuum-(i.e. AGN) dominated sources, but ∼ 60% of these show PAH emission, suggesting the presence of star-formation activity. These sources have redder mid-IR colors than typical optically-selected quasars. About 25% of our sample have strong PAH emission, but none are likely to be pure starbursts as reflected in their relatively high 5 µm hot dust continua. However, their steep 30 µm -to -14 µm slopes suggest that star-formation might dominate the total infrared luminosity. Six of our z ∼ 2 sources have EW 6.2 > ∼ 0.3 µm and L 14µm > ∼ 10 12 L ⊙ (implying L IR > ∼ 10 13 L ⊙ ). At these luminosities, such high EW 6.2 ULIRGs do not exist in the local Universe. We find a median optical depth at 9.7 µm of τ 9.7µm = 1.4. This is consistent with local IRAS-selected ULIRGs, but differs from early results on SCUBAselected z ∼ 2 ULIRGs. Similar to local ULIRGs about 25% of our sample show extreme obscuration (τ 9.7µm > ∼ 3) suggesting buried nuclei. In general, we find that our sources are similar to local ULIRGs, but are an order of magnitude more luminous. It is not clear whether our z ∼ 2 ULIRGs are simply scaled-up versions of local ULIRGs, or subject to fundamentally different physical processes.

We present optical spectroscopy of a sample of 77 luminous AGN and quasars selected on the basis of their mid-infrared colors. Our objects are selected from the Spitzer Extragalactic First Look Survey and SWIRE XMM-LSS fields, with a typical 24µm flux density of 5mJy. The median redshift is 0.6, with a range of ∼ 0.05 − 4. Only 33% (25/77) of these objects are normal type-1 quasars, with no obscuration. 44% (34/77) are type-2 objects, with highionization, narrow emission lines, 14% (11/77) are dust-reddened type-1 quasars, showing broad lines but a dust-reddened or unusually weak quasar continuum. 9% (7/77) show no sign of an AGN in the optical spectrum, having either starburst spectra or spectra which could be of either starburst or LINER types. These latter objects are analogous to the X-ray detected population of AGN with weak or non-existent optical AGN emission (the "XBONGs"). 21 of our objects from the SWIRE field fall within moderately-deep XMM exposures. All the unobscured quasars, and about half the obscured quasars are detected in these exposures. This sample, when taken together with other samples of Spitzer selected AGN and quasars, and results from X-ray studies, confirms that obscured AGN dominate the AGN and quasar number counts of all rapidly-accreting supermassive black hole systems, at least for z < ∼ 4. This implies a high radiative efficiency for the black hole accretion process.

We characterize infrared spectral energy distributions of 343 (Ultra) Luminous Infrared Galaxies from z = 0.3 − 2.8. We diagnose the presence of an AGN by decomposing individual Spitzer mid-IR spectroscopy into emission from star-formation and an AGN-powered continuum; we classify sources as star-forming galaxies (SFGs), AGN, or composites. Composites comprise 30% of our sample and are prevalent at faint and bright S 24 , making them an important source of IR AGN emission. We combine spectroscopy with multiwavelength photometry, including Herschel imaging, to create three libraries of publicly available templates (2-1000 µm). We fit the far-IR emission using a two temperature modified blackbody to measure cold and warm dust temperatures (T c and T w ). We find that T c does not depend on mid-IR classification, while T w shows a notable increase as the AGN grows more luminous. We measure a quadratic relationship between mid-IR AGN emission and total AGN contribution to L IR . AGN, composites, and SFGs separate in S 8 /S 3.6 and S 250 /S 24 , providing a useful diagnostic for estimating relative amounts of these sources. We estimate that >40% of IR selected samples host an AGN, even at faint selection thresholds (S 24 > 100 µJy). Our decomposition technique and color diagnostics are relevant given upcoming observations with the James Webb Space Telescope.

We present the mid-infrared spectra obtained with the Spitzer IRS for 52 sources, selected as infrared luminous, z k1 candidates in the Extragalactic First Look Survey. The sample selection criteria are f 24 m k 0:9 mJy, f (24 m)/ f (8 m) k 3:16, and f (24 m)/f (0:7 m) k 10. Of the 52 spectra, 47 (90%) produced measurable redshifts based solely on the mid-IR spectral features, with 35/47 (74%) at 1:5 P z P 3:2. Keck spectroscopy of a subsample (17/47) agrees with the mid-IR redshift measurements. The observed spectra fall into three categories: (1) 33% (17/52) have strong PAH emission and are probably powered by star formation with total IR luminosity roughly a factor of 5 higher than the local starburst ULIRGs; (2) 33% (17/52) have only deep silicate absorption at 9.8 m, indicatiing deeply embedded dusty systems -these data alone cannot determine the energetic nature of the heating sources in these systems; and (3) the remaining 34% are mid-IR continuum-dominated systems with weak PAH emission and/or silicate absorption. This subsample is probably AGNs. We derived monochromatic, rest-frame 5.8 m, continuum luminosities (L ), ranging from 10 10.3 to 10 12.6 L . Our spectra have mid-IR slope 5Y15 m k 2:1, much redder than the median value of 1.3 for the optically selected PG quasars. From the silicate absorption feature, we estimate that roughly two-thirds of the sample have optical depth 9:8 m > 1. Their L 1600 8 and L IR suggest that our sample is among the most luminous and most dust-enshrouded systems of its epoch. Our study has revealed a significant population of dust-enshrouded galaxies at z $ 2, whose enormous energy output, comparable to that of quasars, is generated by AGNs, as well as starbursts. This IR-luminous population has very little overlap with submillimeter and UV-selected populations.

We present the far-IR, millimeter, and radio photometry as well as optical and near-IR spectroscopy of a sample of 48 z ∼ 1 -3 Spitzer-selected ULIRGs with IRS mid-IR spectra. Our goals are to compute their bolometric emission, and to determine both the presence and relative strength of their AGN and starburst components. We find that strong-PAH sources tend to have higher MIPS160 µm and MAMBO 1.2 mm fluxes than weak-PAH sources. The depth of the 9.7 µm silicate feature does not affect MAMBO detectability. We fit the far-IR SEDs of our sample and find an average L IR ∼ 7 × 10 12 L ⊙ for our z > 1.5 sources. Our spectral decomposition suggests that strong-PAH sources typically have ∼ 20 -30% AGN fractions of L IR . The weak-PAH sources by contrast tend to have > ∼ 70% AGN fractions, with a few sources having comparable contributions of AGN and starbursts. The optical line diagnostics support the presence of AGN in the bulk of the weak-PAH sources. With one exception, our sources are narrow-line sources, show no obvious correspondence between the available optical extinction and the silicate feature depth, and, in two cases, show some evidence for outflows. Radio AGN are present in both strong-PAH and weak-PAH sources. This is supported by our sample's far-IR-to-radio ratios (q) being consistently below the average value of 2.34 for local star-forming galaxies. We use survival analysis to include the lower-limits given by the radio-undetected sources, arriving at q = 2.07 ±0.01 for our z > 1.5 sample. In total, radio and, where available, optical line diagnostics support the presence of AGN in 57% of the z > 1.5 sources, independent of IR-based diagnostics. For higher-z sources, the AGN luminosities alone are estimated to be > 10 12 L ⊙ , which, supported by the available [Oiii] luminosities, implies that the bulk of our sources host obscured quasars. 7 (1) R(24, 8) ≡ log 10 (νfν (24µm)/νfν (8µm) ∼ > 0.5; (2) R(24, 0.7) ≡ log 10 (νfν (24µm)/νfν (0.7µm) ∼ > 1.0.

We use a simple parameterization of the mid-IR spectra of a wide range of galaxy types in order to predict their distribution in the Infrared Array Camera (IRAC) 3.6 µm, 4.5 µm, 5.8 µm, and 8.0 µm and Multiband Photometer for Spitzer (MIPS) 24 µm color-color diagrams. We distinguish three basic spectral types by the energetically dominant component in the 3-12 µm regime: stellardominated; PAH-dominated; and continuum-dominated. We have used a Markov chain Monte Carlo (MCMC) approach to arrive at a more systematic and robust representation of the mid-IR spectra of galaxies than more traditional approaches. We find that IRAC color-color plots are well suited to distinguishing the above spectral types, while the addition of 24 µm data allows us to suggest practical 3color cuts which preferentially select higher redshift sources of specific type. We compare our simulations with the color-color plot obtained by the Spitzer First Look Survey (FLS) and find reasonable agreement. Lastly, we discuss other applications as well as future directions for this work.