We discuss optical associations, spectral energy distributions (SEDs), and photometric redshifts for Spitzer WideArea Infrared Extragalactic (SWIRE) Survey sources in the European Large-Area ISO Survey (ELAIS) N1 area and the Lockman Validation Field (VF). The band-merged Infrared Array Camera (IRAC) (3.6, 4.5, 5.8, and 8.0 m) and Multiband Imaging Photometer for Spitzer (24, 70, and 160 m) data have been associated with optical UgriZ data from the Isaac Newton Telescope Wide Field Survey in ELAIS N1 and with our own optical Ugri data in Lockman-VF. Criteria for eliminating spurious infrared sources and for carrying out star-quasar-galaxy separation are discussed, and statistics of the identification rate are given. Thirty-two percent of sources in the ELAIS N1 field are found to be optically blank (to r ¼ 23:5) and 16% in Lockman-VF (to r ¼ 25).The SEDs of selected ELAIS sources in N1 detected by SWIRE, most with spectroscopic redshifts, are modeled in terms of a simple set of galaxy and quasar templates in the optical and near-infrared (NIR), and with a set of dust emission templates (cirrus, M82 starburst, Arp 220 starburst, and active galactic nucleus [AGN] dust torus) in the mid-infrared.The optical data, together with the IRAC 3.6 and 4.5 m data, have been used to determine photometric redshifts. For galaxies with known spectroscopic redshifts, there is a notable improvement in the photometric redshift when the IRAC data are used, with a reduction in the rms scatter from 10% in (1 þ z) to 7%. Although further spectroscopic data are needed to confirm this result, the prospect of determining good photometric redshifts for much of the SWIRE survey, expected to yield over 2 million extragalactic objects, is excellent. Some modifications to the optical templates were required in the previously uninvestigated wavelength region 2-5 m.The photometric redshifts are used to derive the 3.6 and 24 m redshift distribution and to compare this with the predictions of models. For those sources with a clear mid-infrared excess, relative to the galaxy starlight model used for the optical and NIR, the mid-and far-infrared data are modeled in terms of the same dust emission templates (cirrus, M82, Arp 220, and AGN dust torus). The proportions found of each template type are cirrus, 31%; M82, 29%; Arp 220, 10%; and AGN dust tori, 29%. The distribution of the different infrared SED types in the L IR =L opt versus L IR plane, where L IR and L opt are the infrared and optical bolometric luminosities, respectively, is discussed.There is an interesting population of luminous cool cirrus galaxies with L IR > L opt , implying a substantial dust optical depth. Galaxies with Arp 220-like SEDs, of which there are a surprising preponderance compared with preexisting source count models, tend to have high ratios of infrared to optical bolometric luminosity, consistent with having very high extinction. There is also a high proportion of galaxies whose mid-infrared SEDs are fitted by an AGN dust torus template (29%). Of these only 8...

We characterize the Spitzer Wide-area Infrared Extragalactic Legacy Survey (SWIRE) galaxy populations in the SWIRE validation field within the Lockman Hole, based on the 3.6-24 m Spitzer data and deep U, g 0 , r 0 , i 0 optical imaging within an area $1/3 deg 2 for $16,000 Spitzer SWIRE sources. The entire SWIRE survey will discover over 2.3 million galaxies at 3.6 m and almost 350,000 at 24 m; $70,000 of these will be five-band 3.6-24 m detections. The colors cover a broad range, generally well represented by redshifted spectral energy distributions of known galaxy populations; however, significant samples of unusually blue objects in the [3.6]À[4.5] color are found, as well as many objects very red in the 3.6À24 m mid-IR. Nine of these are investigated and are interpreted as star-forming systems, starbursts, and active galactic nuclei (AGNs) from z ¼ 0:37 to 2.8, with luminosities from L IR ¼ 10 10:3 to 10 13.7 L .

We present the rest‐frame optical and infrared colours of a complete sample of 1114 z < 0.3 galaxies from the Spitzer Wide‐Area Infrared Extragalactic (SWIRE) Legacy Survey and the Sloan Digital Sky Survey (SDSS). We discuss the optical and infrared colours of our sample and analyse in detail the contribution of dusty star‐forming galaxies and active galactic nuclei (AGN) to optically selected red sequence galaxies. We propose that the optical (g−r) colour and infrared log(L24/L3.6) colour of galaxies in our sample are determined primarily by a bulge‐to‐disc ratio. The (g−r) colour is found to be sensitive to the bulge‐to‐disc ratio for disc‐dominated galaxies, whereas the log(L24/L3.6) colour is more sensitive for bulge‐dominated systems. We identify ∼18 per cent (195 sources) of our sample as having red optical colours and infrared excess. Typically, the infrared luminosities of these galaxies are found to be at the high end of star‐forming galaxies with blue optical colours. Using emission‐line diagnostic diagrams, 78 are found to have an AGN contribution and 117 are identified as star‐forming systems. The red (g−r) colour of the star‐forming galaxies could be explained by extinction. However, their high optical luminosities cannot. We conclude that they have a significant bulge component. The number densities of optically red star‐forming galaxies are found to correspond to ∼13 per cent of the total number density of our sample. In addition, these systems contribute ∼13 per cent of the total optical luminosity density, and 28 per cent of the total infrared luminosity density of our SWIRE/SDSS sample. These objects may reduce the need for ‘dry mergers’.

We present the optical‐to‐submillimetre spectral energy distributions (SEDs) for 33 radio and mid‐infrared (mid‐IR) identified submillimetre galaxies discovered via the SHADES 850‐μm SCUBA imaging in the Subaru‐XMM Deep Field (SXDF). Optical data for the sources come from the SXDF and mid‐ and far‐IR fluxes from SWIRE. We obtain photometric redshift estimates for our sources using optical and IRAC 3.6‐ and 4.5‐μm fluxes. We then fit SED templates to the longer wavelength data to determine the nature of the far‐IR emission that dominates the bolometric luminosity of these sources. The IR template fits are also used to resolve ambiguous identifications and cases of redshift aliasing. The redshift distribution obtained broadly matches previous results for submillimetre sources and on the SHADES SXDF field. Our template fitting finds that active galactic nuclei, while present in about 10 per cent of our sources, do not contribute significantly to their bolometric luminosity. Dust heating by starbursts, with either Arp220 or M82 type SEDs, appears to be responsible for the luminosity in most sources (23/33 are fitted by Arp220 templates, 2/33 by the warmer M82 templates). 8/33 sources, in contrast, are fitted by a cooler cirrus dust template, suggesting that cold dust has a role in some of these highly luminous objects. Three of our sources appear to have multiple identifications or components at the same redshift, but we find no statistical evidence that close associations are common among our SHADES sources. Examination of rest‐frame K‐band luminosity suggests that ‘downsizing’ is underway in the submillimetre galaxy population, with lower redshift systems lying in lower mass host galaxies. Of our 33 identifications six are found to be of lower reliability but their exclusion would not significantly alter our conclusions.

We fit a parametric model comprising a mixture of multi-dimensional Gaussian functions to the 3.6 to 8µm colour and optical photometric redshift distribution of galaxy populations in the ELAIS-N1 and Lockman Fields of the Spitzer Wide-area Infrared Extragalactic Legacy survey (SWIRE). For 16,698 sources in ELAIS-N1 we find our data are best modelled (in the sense of the Bayesian Information Criterion) by the sum of four Gaussian distributions or modes (C a , C b , C c and C d ).We compare the fit of our empirical model with predictions from existing semianalytic and phenomological models. We infer that our empirical model provides a better description of the mid-infrared colour distribution of the SWIRE survey than these existing models. This colour distribution test is thus a powerful model discriminator and is entirely complementary to comparisons of number counts. We use our model to provide a galaxy classification scheme and explore the nature of the galaxies in the different modes of the model. Population C a is found to consist of dusty star-forming systems such as ULIRG's, over a broad redshift range. Low redshift late-type spirals are found in population C b , where PAH emission dominates at 8µm, making these sources very red in longer wavelength IRAC colours. Population C c consists of dusty starburst systems with high levels of star-formation activity at intermediate redshifts. Low redshift early-type spirals and ellipticals are found to dominate Population C d . We thus find a greater variety of galaxy types than one can with optical photometry alone.-2 -Finally we develop a new technique to identify unusual objects, and find a selection of outliers with very red IRAC colours. These objects are not detected in the optical, but have very strong detections in the mid-infrared. These sources are modelled as dust-enshrouded, strongly obscured AGN, where the high midinfrared emission may either be attributed to dust heated by the AGN or substantial star-formation. These sources have z ph ∼ 2 − 4, making them incredibly infrared luminous, with a L ir ∼ 10 12.6−14.1 L ⊙ .