We present spectroscopic follow-up observations of 70{mu}m selected galaxies from the SWIRE XMMLSS and Lockman Hole fields. We have measured spectroscopic redshifts for 293 new sources down to a 70{mu}m flux limit of 9mJy and R < 22. The redshift dis
tribution peaks at z ~ 0.3 and has a high redshift tail out to z = 3.5. We perform emission line diagnostics for 91 sources where [OIII], H{beta}, [NII], H{alpha} and [SII] emission lines are available to determine their power source. We find in our sample 13 QSOs, 1 Seyfert II galaxy, 33 star forming galaxies, 30 composite galaxies, 5 LINERs and 21 ambiguous galaxies. We fit single temperature dust spectral energy distributions (SEDs) to 81 70{mu}m sources with 160{mu}m photometry to estimate dust temperatures and masses. Assuming the dust emissivity factor ({beta}) as 1.5, we determine temperatures in the range ~ 20-60K and dust masses with a range of 10{^6}-10{^9} M{_odot}. Plotting these objects in the luminosity-temperature diagram suggests that these objects have lower dust temperatures than local IR luminous galaxies. The Herschel Space Observatory will be crucial in understanding the nature of these sources and to accurately determining the shape of the Rayleigh-Jeans tail of the dust SED. We then model SEDs from optical to far-IR for each source using a set of galaxy and quasar templates in the optical and near-IR (NIR) and with a set of dust emission templates (cirrus, M82 starburst, Arp 220 starburst and AGN dust torus) in the mid-IR (MIR) to far-IR (FIR). The number of objects fit with each dust template are: 57 Arp 220, 127 M82, 9 cirrus, 1 AGN dust torus, 70 M82 and cirrus, 26 M82 and AGN dust torus and 3 Arp 220 and AGN dust torus. We determine the total IR luminosity (LIR) in range 10{^8}-10{^{15}} L{_odot} by integrating the SED models from 8 to 1000{mu}m.
We explore the nature of Infrared Excess sources (IRX), which are proposed as candidates for luminous L_X(2-10keV)>1e43erg/s Compton Thick (N_H>2e24cm^{-2}$) QSOs at z~2. Lower redshift, z~1, analogues of the distant IRX population are identified by
firstly redshifting to z=2 the SEDs of all sources with secure spectroscopic redshifts in the AEGIS (6488) and the GOODS-North (1784) surveys and then selecting those that qualify as IRX sources at that redshift. A total of 19 galaxies are selected. The mean redshift of the sample is $zapprox1$. We do not find strong evidence for Compton Thick QSOs in the sample. For 9 sources with X-ray counterparts, the X-ray spectra are consistent with Compton Thin AGN. Only 3 of them show tentative evidence for Compton Thick obscuration. The SEDs of the X-ray undetected population are consistent with starburst activity. There is no evidence for a hot dust component at the mid-infrared associated with AGN heated dust. If the X-ray undetected sources host AGN, an upper limit of L_X(2-10keV) =1e43erg/s is estimated for their intrinsic luminosity. We propose that a large fraction of the $zapprox2$ IRX population are not Compton Thick QSOs but low luminosity [L_X(2-10keV)<1e43erg/s], possibly Compton Thin, AGN or dusty starbursts. It is shown that the decomposition of the AGN and starburst contribution to the mid-IR is essential for interpreting the nature of this population, as star-formation may dominate this wavelength regime.
We explore the nature of X-ray sources with 70 micron counterparts selected in the SWIRE fields ELAIS-N1, Lockman Hole and Chandra Deep Field South, for which Chandra X-ray data are available. A total of 28 X-ray/70 micron sources in the redshift int
erval 0.5<z<1.3 are selected. The X-ray luminosities and the shape of the X-ray spectra show that these sources are AGN. Modelling of the optical to far-infrared Spectral Energy Distribution indicates that most of them (27/28) have a strong starburst component (>50 solar masses per year) that dominates in the infrared. It is found that the X-ray and infrared luminosities of the sample sources are broadly correlated, consistent with a link between AGN activity and star-formation. Contrary to the predictions of some models for the co-evolution of AGN and galaxies, the X-ray/70 micron sources in the sample are not more obscured at X-ray wavelengths compared to the overall X-ray population. It is also found that the X-ray/70 micron sources have lower specific star-formation rates compared to the general 70 micron population, consistent with AGN feedback moderating the star-formation in the host galaxies.
