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Register a new userThe Spectral Energy Distributions and Infrared Luminosities of z approx 2 Dust Obscured Galaxies from Herschel and Spitzer
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J. Melbourne
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Dust-obscured galaxies (DOGs) are a subset of high-redshift (z approx 2) optically-faint ultra-luminous infrared galaxies (ULIRGs, e.g. L_{IR} > 10^{12} Lsun). We present new far-infrared photometry, at 250, 350, and 500 um (observed-frame), from the Herschel Space Telescope for a large sample of 113 DOGs with spectroscopically measured redshifts. Approximately 60% of the sample are detected in the far-IR, confirming their high IR luminosities, which range from 10^{11.6} Lsun < L_{IR} (8-1000 um) <10^{13.6} Lsun. 90% of the Herschel detected DOGs in this sample are ULIRGs and 30% have L_{IR} > 10^{13} Lsun. The rest-frame near-IR (1 - 3 um) SEDs of the Herschel detected DOGs are predictors of their SEDs at longer wavelengths. DOGs with power-law SEDs in the rest-frame near-IR show observed-frame 250/24 um flux density ratios similar to the QSO-like local ULIRG, Mrk 231. DOGs with a stellar bump in their rest-frame near-IR show observed-frame 250/24 um flux density ratios similar to local star-bursting ULIRGs like NGC 6240. For the Herschel detected DOGs, accurate estimates (within approx 25%) of total IR luminosity can be predicted from their rest-frame mid-IR data alone (e.g. from Spitzer observed-frame 24 um luminosities). Herschel detected DOGs tend to have a high ratio of infrared luminosity to rest-frame 8 um luminosity (the IR8= L_{IR}(8-1000 um)/v L_{v}(8 um) parameter of Elbaz et al. 2011). Instead of lying on the z=1-2 infrared main-sequence of star forming galaxies (like typical LIRGs and ULIRGs at those epochs) the DOGs, especially large fractions of the bump sources, tend to lie in the starburst sequence. While, Herschel detected DOGs are similar to scaled
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We present SHARC-II 350um imaging of twelve 24um-bright (F_24um > 0.8 mJy) Dust-Obscured Galaxies (DOGs) and CARMA 1mm imaging of a subset of 2 DOGs, all selected from the Bootes field of the NOAO Deep Wide-Field Survey. Detections of 4 DOGs at 350um imply IR luminosities which are consistent within a factor of 2 of expectations based on a warm dust spectral energy distribution (SED) scaled to the observed 24um flux density. The 350um upper limits for the 8 non-detected DOGs are consistent with both Mrk231 and M82 (warm dust SEDs), but exclude cold dust (Arp220) SEDs. The two DOGs targeted at 1mm were not detected in our CARMA observations, placing strong constraints on the dust temperature: T_dust > 35-60 K. Assuming these dust properties apply to the entire sample, we find dust masses of ~3x10^8 M_sun. In comparison to other dusty z ~ 2 galaxy populations such as sub-millimeter galaxies (SMGs) and other Spitzer-selected high-redshift sources, this sample of DOGs has higher IR luminosities (2x10^13 L_sun vs. 6x10^12 L_sun for the other galaxy populations), warmer dust temperatures (>35-60 K vs. ~30 K), and lower inferred dust masses (3x10^8 M_sun vs. 3x10^9 M_sun). Herschel and SCUBA-2 surveys should be able to detect hundreds of these power-law dominated DOGs. We use HST and Spitzer/IRAC data to estimate stellar masses of these sources and find that the stellar to gas mass ratio may be higher in our 24um-bright sample of DOGs than in SMGs and other Spitzer-selected sources. Although larger sample sizes are needed to provide a definitive conclusion, the data are consistent with an evolutionary trend in which the formation of massive galaxies at z~2 involves a sub-millimeter bright, cold-dust and star-formation dominated phase followed by a 24um-bright, warm-dust and AGN-dominated phase.
We discuss optical associations, spectral energy distributions and photometric redshifts for SWIRE sources in the ELAIS-N1 area and the Lockman Validation Field. The band-merged IRAC (3.6, 4.5, 5.8 and 8.0 mu) and MIPS (24, 70, 160 mu) data have been associated with optical UgriZ data from the INT Wide Field Survey in ELAIS-N1, and with our own optical Ugri data in Lockman-VF. The spectral energy distributions of selected ELAIS sources in N1 detected by SWIRE, most with spectroscopic redshifts, are modelled in terms of a simple set of galaxy and quasar templates in the optical and near infrared, and with a set of dust emission templates (cirrus, M82 starburst, Arp 220 starburst, and AGN dust torus) in the mid infrared. The optical data, together with the IRAC 3.6 and 4.5 mu 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+z) to 7%. The photometric redshifts are used to derive the 3.6 and 24 mu 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 near infrared, the mid and far infrared data are modelled in terms of the same dust emission templates. The proportions found of each template type are: cirrus 31%, M82 29%, Arp 220 10%, 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, is discussed.
We present spectral energy distributions (SEDs) for 68 Herschel sources detected at 5-sigma at 250, 350 and 500 mu in the HerMES SWIRE-Lockman field. We explore whether existing models for starbursts, quiescent star-forming galaxies and for AGN dust tori are able to model the full range of SEDs measured with Herschel. We find that while many galaxies (~ 56 %) are well fitted with the templates used to fit IRAS, ISO and Spitzer sources, for about half the galaxies two new templates are required: quiescent (cirrus) models with colder (10-20 K) dust, and a young starburst model with higher optical depth than Arp 220. Predictions of submillimetre fluxes based on model fits to 4.5-24 mu data agree rather poorly with the observed fluxes, but the agreement is better for fits to 4.5-70 mu data. Herschel galaxies detected at 500 mu tend to be those with the very highest dust masses.
We gather infrared (IR) photometric data from 8 to 500 microns (Spitzer, WISE, IRAS and Herschel) for all of the HRS galaxies. Draine & Li (2007) models are fit to the data from which the stellar contribution has been carefully removed. We find that our photometric coverage is sufficient to constrain all of the models parameters and that a strong constraint on the 20-60 microns range is mandatory to estimate the relative contribution of the photo-dissociation regions to the IR SED. The SED models tend to systematically under-estimate the observed 500 microns flux densities, especially for low mass systems. We provide the output parameters for all of the galaxies: the minimum intensity of the interstellar radiation field (ISRF), the fraction of PAH, the relative contribution of PDR and evolved stellar population to the dust heating, the $M_{dust}$ and the $L_{IR}$. For a subsample of gas-rich galaxies, we analyze the relations between these parameters and the integrated properties of galaxies, such as $M_*$, SFR, metallicity, H$alpha$ and H-band surface brightness, and the FUV attenuation. A good correlation between the fraction of PAH and the metallicity is found implying a weakening of the PAH emission in galaxies with low metallicities. The intensity of the IRSF and the H-band and H$alpha$ surface brightnesses are correlated, suggesting that the diffuse dust component is heated by both the young stars in star forming regions and the diffuse evolved population. We use these results to provide a new set of IR templates calibrated with Herschel observations on nearby galaxies and a mean SED template to provide the z=0 reference for cosmological studies. For the same purpose, we put our sample on the SFR-$M_*$ diagram. The templates are compared to the most popular IR SED libraries, enlightening a large discrepancy between all of them in the 20-100 microns range.
Luminous and ultraluminous infrared galaxies ((U)LIRGs) are the most extreme star forming galaxies in the universe. The local (U)LIRGs provide a unique opportunity to study their multi-wavelength properties in detail for comparison to their more numerous counterparts at high redshifts. We present common large aperture photometry at radio through X-ray wavelengths, and spectral energy distributions (SEDs) for a sample of 53 nearby LIRGs and 11 ULIRGs spanning log (LIR/Lsun) = 11.14-12.57 from the flux-limited Great Observatories All-sky LIRG Survey (GOALS). The SEDs for all objects are similar in that they show a broad, thermal stellar peak and a dominant FIR thermal dust peak, where nuLnu(60um) / nuLnu(V) increases from ~2-30 with increasing LIR. When normalized at IRAS-60um, the largest range in the luminosity ratio, R(lambda)=log[nuLnu(lambda)/nuLnu(60um)] observed over the full sample is seen in the Hard X-rays (HX=2-10 keV). A small range is found in the Radio (1.4GHz), where the mean ratio is largest. Total infrared luminosities, LIR(8-1000um), dust temperatures, and dust masses were computed from fitting thermal dust emission modified blackbodies to the mid-infrared (MIR) through submillimeter SEDs. The new results reflect an overall ~0.02 dex lower luminosity than the original IRAS values. Total stellar masses were computed by fitting stellar population synthesis models to the observed near-infrared (NIR) through ultraviolet (UV) SEDs. Mean stellar masses are found to be log(M/Msun) = 10.79+/-0.40. Star formation rates have been determined from the infrared (SFR_IR~45Msun/yr) and from the monochromatic UV luminosities (SFR_UV~1.3Msun/yr), respectively. Multiwavelength AGN indicators have be used to select putative AGN: about 60% of the ULIRGs would have been classified as an AGN by at least one of the selection criteria.
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