Millimeter imaging of submillimeter galaxies in the COSMOS field: Redshift distribution

We present new IRAM PdBI 1.3mm continuum observations at ~1.5" resolution of 28 SMGs previously discovered with the 870um bolometer LABOCA at APEX within the central 0.7deg2 of the COSMOS field. 19 out of the 28 LABOCA sources were detected with the PdBI at a >~3sigma level of ~1.4mJy/b. A combined analysis of this new sample with existing interferometrically identified SMGs in the COSMOS field yields the following results: 1) >~15%, and possibly up to ~40% of single-dish detected SMGs consist of multiple sources, 2) statistical identifications of multi-wavelength counterparts to the single-dish SMGs yield that only ~50% of these single-dish SMGs have real radio or IR counterparts, 3) ~18% of interferometric SMGs have only radio or even no multi-wavelength counterpart at all, and 4) ~50-70% of z>~3 SMGs have no radio counterparts down to an rms of 7-12uJy at 1.4GHz. Using the exact interferometric positions to identify proper multi-wavelength counterparts allows us to determine accurate photometric redshifts for these sources. The redshift distributions of the combined and the individual 1.1mm and 870um selected samples have a higher mean and broader width than the redshift distributions derived in previous studies. Our sample supports the previous tentative trend that on average brighter and/or mm-selected SMGs are located at higher redshifts. There is a tentative offset between the mean redshift for the 1.1mm (=3.1+/-0.4) and 870um (=2.6+/-0.4) selected samples, with the 1.1mm sources lying on average at higher redshifts. Based on our nearly complete sample of AzTEC 1.1mm SMGs within a uniform 0.15deg2 area we infer a higher surface density of z>~4 SMGs than predicted by current cosmological models. In summary, our findings imply that (sub-)millimeter interferometric identifications are crucial to build statistically complete and unbiased samples of SMGs.Comment: 35 pages, 18 figures, 10 tables; accepted for publication in A&

Highly-magnified, Multiply-imaged radio counterparts of the Sub-mm Starburst Emission in the Cluster-Lens MS0451.6-0305

Previous authors have reported the detection of intrinsically faint sub-mm emission lensed by the cluster MS0451.6-0305. They suggest that this emission arises from a merging system composed of a Ly-break galaxy and a pair of extremely red objects which are multiply-imaged in the optical/NIR observations. Since the submm emission presents an unusually large angular extent (~1 arcmin), the possible radio emission asociatted with that system can help to identify optical/NIR counterparts due to the higher spatial resolution and astrometric accuracy of the radio observations. Archive VLA data (BnA configuration at 1.4 GHz) was reduced and analysed. A simple lens model was constructed to aid the interpretation of the radio and pre-existing sub-mm and optical/NIR data. We present a 1.4 GHz map of the central region of MS0451.6-0305 and report the detection of gravitationally lensed radio emission, coincident with the previously discovered sub-mm lensed emission. The overall morphology and scale of the radio and sub-mm emission are strikingly similar, extending ~1 arcmin across the sky. This observation strongly suggests that the radio and sub-mm emission arise from the same sources. Preliminary estimates of the total S_850 microns/S_1.4 GHz flux density ratio appear to be consistent with that expected from distant star forming galaxies. The radio emission is resolved into 7 distinct components, and the overall structure can be explained, using a simple lens model, with three multiply-imaged radio sources at z~2.9. One of these sources is predicted to lie in the middle of the previously mentioned system in the source plane, suggesting that it is related to the intense star formation generated during the merging process.Comment: 11 pages, 3 figures. Acepted for publication in A&A on 05/10/200

A backward evolution model for infrared surveys: the role of AGN- and Color-L_TIR distributions

Empirical "backward" galaxy evolution models for infrared bright galaxies are constrained using multi-band infrared surveys. We developed a new Monte-Carlo algorithm for this task, implementing luminosity dependent distribution functions for the galaxies' infrared spectral energy distributions (SEDs) and for the AGN contribution, allowing for evolution of these quantities. The adopted SEDs take into account the contributions of both starbursts and AGN to the infrared emission, for the first time in a coherent treatment rather than invoking separate AGN and star-forming populations. In the first part of the paper we consider the quantification of the AGN contribution for local universe galaxies, as a function of total infrared luminosity. It is made using a large sample of LIRGs and ULIRGs for which mid-infrared spectra are available in the Spitzer archive. In the second part we present the model. Our best-fit model adopts very strong luminosity evolution, $L=L_0(1+z)^{3.4}$, up to $z=2.3$, and density evolution, $\rho=\rho_0(1+z)^2$, up to $z=1$, for the population of infrared galaxies. At higher $z$, the evolution rates drop as $(1+z)^{-1}$ and $(1+z)^{-1.5}$ respectively. To reproduce mid-infrared to submillimeter number counts and redshift distributions, it is necessary to introduce both an evolution in the AGN contribution and an evolution in the luminosity-temperature relation. Our models are in plausible agreement with current photometry-based estimates of the typical AGN contribution as a function of mid-infrared flux, and well placed to be compared to upcoming Spitzer spectroscopic results. As an example of future applications, we use our best-fitting model to make predictions for surveys with Herschel.Comment: Model available at: (http://www.physics.ubc.ca/~valiante/model) ApJ accepte

Cosmic evolution of submillimeter galaxies and their contribution to stellar mass assembly

The nature of galaxies selected at submillimeter wavelengths (SMGs, S_850 > 3 mJy), some of the bolometrically most luminous objects at high redshifts, is still elusive. In particular their star formation histories and source of emission are not accurately constrained. In this paper we introduce a new approach to analyse the SMG data. Namely, we present the first self-consistent UV-to-radio spectral energy distribution fits of 76 SMGs with spectroscopic redshifts using all photometric datapoints from ultraviolet to radio simultaneously. We find that they are highly star-forming (median star formation rate 713 MSun yr^-1 for SMGs at z>0.5), moderately dust-obscured (median A_V~2 mag), hosting significant stellar populations (median stellar mass 3.7x10^11 MSun) of which only a minor part has been formed in the ongoing starburst episode. This implies that in the past, SMGs experienced either another starburst episode or merger with several galaxies. The properties of SMGs suggest that they are progenitors of present-day elliptical galaxies. We find that these bright SMGs contribute significantly to the cosmic star formation rate density (~20%) and stellar mass density (~30-50%) at redshifts 2-4. Using number counts at low fluxes we find that as much as 80% of the cosmic star formation at these redshifts took place in SMGs brighter than 0.1 mJy. We find evidence that a linear infrared-radio correlation holds for SMGs in an unchanged form up to redshift of 3.6, though its normalization is offset from the local relation by a factor of ~2.1 towards higher radio luminosities. We present a compilation of photometry data of SMGs and determinations of cosmic SFR and stellar mass densities.Comment: Accepted to A&A. 14 pages (+23 pages as appendix), 7 figures, 6 tables. Table A1-A5 can be found in the source file in the machine-readable form. For SED templates, see http://archive.dark-cosmology.dk/ or the source file. v3: major improvements: 1) the incompleteness correction applied; 2) the (higher) local q-value correctly assigned; 3) estimates of A_V adde

Physical properties of z>4 submillimeter galaxies in the COSMOS field

We study the physical properties of a sample of 6 SMGs in the COSMOS field, spectroscopically confirmed to lie at z>4. We use new GMRT 325 MHz and 3 GHz JVLA data to probe the rest-frame 1.4 GHz emission at z=4, and to estimate the sizes of the star-forming (SF) regions of these sources, resp. Combining our size estimates with those available in the literature for AzTEC1 and AzTEC3 we infer a median radio-emitting size for our z>4 SMGs of (0.63"+/-0.12")x(0.35"+/-0.05") or 4.1x2.3 kpc^2 (major times minor axis; assuming z=4.5) or lower if we take the two marginally resolved SMGs as unresolved. This is consistent with the sizes of SF regions in lower-redshift SMGs, and local normal galaxies, yet higher than the sizes of SF regions of local ULIRGs. Our SMG sample consists of a fair mix of compact and more clumpy systems with multiple, perhaps merging, components. With an average formation time of ~280 Myr, derived through modeling of the UV-IR SEDs, the studied SMGs are young systems. The average stellar mass, dust temperature, and IR luminosity we derive are M*~1.4x10^11 M_sun, T_dust~43 K, and L_IR~1.3x10^13L_sun, resp. The average L_IR is up to an order of magnitude higher than for SMGs at lower redshifts. Our SMGs follow the correlation between dust temperature and IR luminosity as derived for Herschel-selected 0.1=1.95+/-0.26 for our sample, compared to q~2.6 for IR luminous galaxies at z4 SMGs put them at the high end of the L_IR-T_dust distribution of SMGs, and that our SMGs form a morphologically heterogeneous sample. Thus, further in-depth analyses of large, statistical samples of high-redshift SMGs are needed to fully understand their role in galaxy formation and evolution

Identifying Luminous AGN in Deep Surveys: Revised IRAC Selection Criteria

Spitzer IRAC selection is a powerful tool for identifying luminous AGN. For deep IRAC data, however, the AGN selection wedges currently in use are heavily contaminated by star-forming galaxies, especially at high redshift. Using the large samples of luminous AGN and high-redshift star-forming galaxies in COSMOS, we redefine the AGN selection criteria for use in deep IRAC surveys. The new IRAC criteria are designed to be both highly complete and reliable, and incorporate the best aspects of the current AGN selection wedges and of infrared power-law selection while excluding high redshift star-forming galaxies selected via the BzK, DRG, LBG, and SMG criteria. At QSO-luminosities of log L(2-10 keV) (ergs/s) > 44, the new IRAC criteria recover 75% of the hard X-ray and IRAC-detected XMM-COSMOS sample, yet only 38% of the IRAC AGN candidates have X-ray counterparts, a fraction that rises to 52% in regions with Chandra exposures of 50-160 ks. X-ray stacking of the individually X-ray non-detected AGN candidates leads to a hard X-ray signal indicative of heavily obscured to mildly Compton-thick obscuration (log N_H (cm^-2) = 23.5 +/- 0.4). While IRAC selection recovers a substantial fraction of luminous unobscured and obscured AGN, it is incomplete to low-luminosity and host-dominated AGN.Comment: 22 pages, 15 figures, accepted for publication in ApJ, full resolution version available at http://www.stsci.edu/~donley/iragn_paper

Submillimetre Source Counts in the Fields of High-Redshift Galaxy Clusters

We present a submillimetre survey of seven high-z galaxy clusters (0.64<z<1.0) using the Submillimetre Common-User Bolometer Array (SCUBA) at 850 and 450 um. The targets, of similar richness and redshift, are selected from the Red-sequence Cluster Survey (RCS). We use this sample to investigate the apparent excess of submillimetre source counts in the direction of cluster fields compared to blank fields. The sample consists of three galaxy clusters that exhibit multiple optical arcs due to strong gravitational lensing, and a control group of four clusters with no apparent strong lensing. A tentative excess of 2.7-sigma is seen in the number density of submillimetre luminous galaxies (SMGs) within the lensing cluster fields compared to that in the control group. Ancillary observations at radio, mid-infrared, optical, and X-ray wavelengths allow for the identification of counterparts to many of the SMGs. Utilizing photometric redshifts, we conclude that at least three of the galaxies within the lensing fields have redshifts consistent with the clusters and implied infrared luminosities of ~10^12 Lsol. The existence of SMG cluster members may therefore be boosting source counts in the lensing cluster fields, which might be an effect of the dynamical state of those clusters. However, we find that the removal of potential cluster members from the counts analysis does not entirely eliminate the difference between the cluster samples. We also investigate possible occurrences of lensing between background SMGs and lower-z optical galaxies, though further observations are required to make any conclusive claims. Although the excess counts between the two cluster samples have not been unambiguously accounted for, these results warrant caution for interpreting submillimetre source counts in cluster fields and point source contamination for Sunyaev-Zel'dovich surveys. [Abridged]Comment: 33 pages, 23 figures, 5 tables; accepted for publication in MNRA