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We present the largest spectroscopic follow-up performed in SWIRE ELAIS-N1. We were able to determine redshifts for 289 extragalactic sources. The values of spectroscopic redshifts of the latter have been compared with the estimated values from our photometric redshift code with very good agreement between the two for both galaxies and quasars. Six of the quasars are hyperluminous infrared (IR) galaxies all of which are broad-line active galactic nuclei. We have performed emission-line diagnostics for 30 sources using the [O iii]/Hβ, [N ii]/Hα and [S ii]/Hα line ratios in order to classify these 30 sources into star forming, Seyferts, composite and LINER and compare the results to the predictions from our spectral energy distribution template fitting methods and mid-IR selection methods. [ABSTRACT FROM AUTHOR]

We investigate the clustering of galaxies selected in the 3.6 μm band of the Spitzer Wide-area Infrared Extragalactic (SWIRE) legacy survey. The angular two-point correlation function is calculated for 11 samples with flux limits of S3.6≥ 4–400 μJy, over an 8 deg2 field. The angular clustering strength is measured at >5σ significance at all flux limits, with amplitudes of A= (0.49–29) × 10−3 at 1°, for a power-law model, Aθ−0.8. We estimate the redshift distributions of the samples using phenomological models, simulations and photometric redshifts, and so derive the spatial correlation lengths. We compare our results with the Galaxies In Cosmological Simulations (GalICS) models of galaxy evolution and with parametrized models of clustering evolution. The GalICS simulations are consistent with our angular correlation functions, but fail to match the spatial clustering inferred from the phenomological models or the photometric redshifts. We find that the uncertainties in the redshift distributions of our samples dominate the statistical errors in our estimates of the spatial clustering. At low redshifts (median z≤ 0.5), the comoving correlation length is approximately constant, Mpc, and then decreases with increasing redshift to a value of 2.9 ± 0.3 h−1 Mpc for the faintest sample, for which the median redshift is z∼ 1. We suggest that this trend can be attributed to a decrease in the average galaxy and halo mass in the fainter flux-limited samples, corresponding to changes in the relative numbers of early- and late-type galaxies. However, we cannot rule out strong evolution of the correlation length over . [ABSTRACT FROM AUTHOR]

We present the rest-frame optical and infrared colours of a complete sample of 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 colour and infrared colour of galaxies in our sample are determined primarily by a bulge-to-disc ratio. The colour is found to be sensitive to the bulge-to-disc ratio for disc-dominated galaxies, whereas the 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 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’. [ABSTRACT FROM AUTHOR]