We study the relation of AGN accretion, star formation rate (SFR), and stellar mass (M$_*$) using a sample of $approx$ 8600 star-forming galaxies up to z=2.5 selected with textit{Herschel} imaging in the GOODS and COSMOS fields. For each of them we d
erive SFR and M$_*$, both corrected, when necessary, for emission from an active galactic nucleus (AGN), through the decomposition of their spectral energy distributions (SEDs). About 10 per cent of the sample are detected individually in textit{Chandra} observations of the fields. For the rest of the sample we stack the X-ray maps to get average X-ray properties. After subtracting the X-ray luminosity expected from star formation and correcting for nuclear obscuration, we derive the average AGN accretion rate for both detected sources and stacks, as a function of M$_{*}$, SFR and redshift. The average accretion rate correlates with SFR and with M$_*$. The dependence on SFR becomes progressively more significant at z$>$0.8. This may suggest that SFR is the original driver of these correlations. We find that average AGN accretion and star formation increase in a similar fashion with offset from the star-forming main-sequence. Our interpretation is that accretion onto the central black hole and star formation broadly trace each other, irrespective of whether the galaxy is evolving steadily on the main-sequence or bursting.
We report the serendipitous detection of a significant overdensity of Herschel-SPIRE 250 micron sources in the vicinity of MRC1138-26. We use an adaptive kernel density estimate to quantify the significance, including a comparison with other fields.
The overdensity has a size of ~3.5-4 and stands out at ~5sigma with respect to the background estimate. No features with similar significance were found in four extragalactic control fields: GOODS-North, Lockman, COSMOS and UDS. The chance of having a similar overdensity in a field with the same number but randomly distributed sources is less than 2%. The clump is also visible as a low surface brightness feature in the Planck 857 GHz map. We detect 76 sources at 250 micron (with a signal-to-noise ratio greater than 3), in a region of 4 radius; 43 of those are above a flux density limit of 20 mJy. This is a factor of 3.6 in excess over the average in the four control fields, considering only the sources above 20 mJy. We also find an excess in the number counts of sources with 250 micron flux densities between 30 and 40 mJy, compared to deep extragalactic blank-field number counts. Assuming a fixed dust temperature (30 K) and emissivity (beta=1.5) a crude, blackbody-derived redshift distribution, zBB, of the detected sources is significantly different from the distributions in the control fields and exhibits a significant peak at zBB ~ 1.5, although the actual peak redshift is highly degenerate with the temperature. We tentatively suggest, based on zBB and the similar S250/S350 colours of the sources within the peak, that a significant fraction of the sources in the clump may be at a similar redshift. Since the overdensity lies ~7 south of the z=2.16 Spiderweb protocluster MRC1138-26, an intriguing possibility (that is presently unverifiable given the data in hand) is that it lies within the same large-scale structure.(abridged)
We use deep far-infrared data from the PEP/GOODS-Herschel surveys and rest frame ultraviolet photometry to study the evolution of the molecular gas mass function of normal star forming galaxies. Computing the molecular gas mass, M(mol), by scaling st
ar formation rates (SFR) through depletion timescales, or combining IR luminosity and obscuration properties as in Nordon et al., we obtain M(mol) for roughly 700, z=0.2-3.0 galaxies near the star forming main sequence. The number density of galaxies follows a Schechter function of M(mol). The characteristic mass M* is found to strongly evolve up to z~1, and then to flatten at earlier epochs, resembling the infrared luminosity evolution of similar objects. At z~1, our result is supported by an estimate based on the stellar mass function of star forming galaxies and gas fraction scalings from the PHIBSS survey. We compare our measurements to results from current models, finding better agreement with those that are treating star formation laws directly rather than in post-processing. Integrating the mass function, we study the evolution of the M(mol) density and its density parameter Omega(mol).
We present results from the deepest Herschel-PACS (Photodetector Array Camera and Spectrometer) far-infrared blank field extragalactic survey, obtained by combining observations of the GOODS (Great Observatories Origins Deep Survey) fields from the P
ACS Evolutionary Probe (PEP) and GOODS-Herschel key programmes. We describe data reduction and the construction of images and catalogues. In the deepest parts of the GOODS-S field, the catalogues reach 3-sigma depths of 0.9, 0.6 and 1.3 mJy at 70, 100 and 160 um, respectively, and resolve ~75% of the cosmic infrared background at 100um and 160um into individually detected sources. We use these data to estimate the PACS confusion noise, to derive the PACS number counts down to unprecedented depths and to determine the infrared luminosity function of galaxies down to LIR=10^11 Lsun at z~1 and LIR=10^12 Lsun at z~2, respectively. For the infrared luminosity function of galaxies, our deep Herschel far-infrared observations are fundamental because they provide more accurate infrared luminosity estimates than those previously obtained from mid-infrared observations. Maps and source catalogues (>3-sigma) are now publicly released. Combined with the large wealth of multi-wavelength data available for the GOODS fields, these data provide a powerful new tool for studying galaxy evolution over a broad range of redshifts.
Using Herschel data from the deepest SPIRE and PACS surveys (HerMES and PEP) in COSMOS and GOODS (N+S), we examine the dust properties of IR-luminous (L_IR>10^10 L_sun) galaxies at 0.1<z<2 and determine how these evolve with cosmic time. The unique a
ngle of this work is the rigorous analysis of survey selection effects, making this the first study of the star-formation-dominated, IR-luminous population within a framework almost entirely free of selection biases. We find that IR-luminous galaxies have SEDs with broad far-IR peaks characterised by cool/extended dust emission and average dust temperatures in the 25-45K range. Hot (T>45K) SEDs and cold (T<25K), cirrus-dominated SEDs are rare, with most sources being within the range occupied by warm starbursts such as M82 and cool spirals such as M51. We observe a luminosity-temperature (L-T) relation, where the average dust temperature of log[L_IR/L_sun]=12.5 galaxies is about 10K higher than that of their log[L_IR/L_sun]=10.5 counterparts. However, although the increased dust heating in more luminous systems is the driving factor behind the L-T relation, the increase in dust mass and/or starburst size with luminosity plays a dominant role in shaping it. Our results show that the dust conditions in IR-luminous sources evolve with cosmic time: at high redshift, dust temperatures are on average up to 10K lower than what is measured locally. This is manifested as a flattening of the L-T relation, suggesting that (U)LIRGs in the early Universe are typically characterised by a more extended dust distribution and/or higher dust masses than local equivalent sources. Interestingly, the evolution in dust temperature is luminosity dependent, with the fraction of LIRGs with T<35K showing a 2-fold increase from z~0 to z~2, whereas that of ULIRGs with T<35K shows a 6-fold increase.
(abridged) Far-infrared Herschel photometry from the PEP and HerMES programs is combined with ancillary datasets in the GOODS-N, GOODS-S, and COSMOS fields. Based on this rich dataset, we reproduce the restframe UV to FIR ten-colors distribution of g
alaxies using a superposition of multi-variate Gaussian modes. The median SED of each mode is then fitted with a modified version of the MAGPHYS code that combines stellar light, emission from dust heated by stars and a possible warm dust contribution heated by an AGN. The defined Gaussian grouping is also used to identify rare sources. The zoology of outliers includes Herschel-detected ellipticals, very blue z~1 Ly-break galaxies, quiescent spirals, and torus-dominated AGN with star formation. Out of these groups and outliers, a new template library is assembled, consisting of 32 SEDs describing the intrinsic scatter in the restframe UV-to-submm colors of infrared galaxies. This library is tested against L(IR) estimates with and without Herschel data included, and compared to eight other popular methods often adopted in the literature. When implementing Herschel photometry, these approaches produce L(IR) values consistent with each other within a median absolute deviation of 10-20%, the scatter being dominated more by fine tuning of the codes, rather than by the choice of SED templates. Finally, the library is used to classify 24 micron detected sources in PEP GOODS fields. AGN appear to be distributed in the stellar mass (M*) vs. star formation rate (SFR) space along with all other galaxies, regardless of the amount of infrared luminosity they are powering, with the tendency to lie on the high SFR side of the main sequence. The incidence of warmer star-forming sources grows for objects with higher specific star formation rates (sSFR), and they tend to populate the off-sequence region of the M*-SFR-z space.
We present spectroscopic observations for a sample of 36 Herschel-SPIRE 250-500um selected galaxies (HSGs) at 2<z<5 from the Herschel Multi-tiered Extragalactic Survey (HerMES). Redshifts are confirmed as part of a large redshift survey of Herschel-S
PIRE-selected sources covering ~0.93deg^2 in six extragalactic legacy fields. Observations were taken with the Keck I Low Resolution Imaging Spectrometer (LRIS) and the Keck II DEep Imaging Multi-Object Spectrograph (DEIMOS). Precise astrometry, needed for spectroscopic follow-up, is determined by identification of counterparts at 24um or 1.4GHz using a cross-identification likelihood matching method. Individual source luminosities range from log(L_IR/Lsun)=12.5-13.6 (corresponding to star formation rates 500-9000Msun/yr, assuming a Salpeter IMF), constituting some of the most intrinsically luminous, distant infrared galaxies yet discovered. We present both individual and composite rest-frame ultraviolet spectra and infrared spectral energy distributions (SEDs). The selection of these HSGs is reproducible and well characterized across large areas of sky in contrast to most z>2 HyLIRGs in the literature which are detected serendipitously or via tailored surveys searching only for high-z HyLIRGs; therefore, we can place lower limits on the contribution of HSGs to the cosmic star formation rate density at (7+-2)x10^(-3)Msun/yr h^3Mpc^(-3) at z~2.5, which is >10% of the estimated total star formation rate density (SFRD) of the Universe from optical surveys. The contribution at z~4 has a lower limit of 3x10^(-3)Msun/yr h^3 Mpc^(-3), ~>20% of the estimated total SFRD. This highlights the importance of extremely infrared-luminous galaxies with high star formation rates to the build-up of stellar mass, even at the earliest epochs.
We present Keck spectroscopic observations and redshifts for a sample of 767 Herschel-SPIRE selected galaxies (HSGs) at 250, 350, and 500um, taken with the Keck I Low Resolution Imaging Spectrometer (LRIS) and the Keck II DEep Imaging Multi-Object Sp
ectrograph (DEIMOS). The redshift distribution of these SPIRE sources from the Herschel Multitiered Extragalactic Survey (HerMES) peaks at z=0.85, with 731 sources at z<2 and a tail of sources out to z~5. We measure more significant disagreement between photometric and spectroscopic redshifts (<delta_z>/(1+z)>=0.29) than is seen in non-infrared selected samples, likely due to enhanced star formation rates and dust obscuration in infrared-selected galaxies. We estimate that the vast majority (72-83%) of z<2 Herschel-selected galaxies would drop out of traditional submillimeter surveys at 0.85-1mm. We estimate the luminosity function and implied star-formation rate density contribution of HSGs at z<1.6 and find overall agreement with work based on 24um extrapolations of the LIRG, ULIRG and total infrared contributions. This work significantly increased the number of spectroscopically confirmed infrared-luminous galaxies at z>>0 and demonstrates the growing importance of dusty starbursts for galaxy evolution studies and the build-up of stellar mass throughout cosmic time. [abridged]
We discuss the clustering properties of galaxies with signs of ongoing star formation detected by the Spitzer Space Telescope at 24mum band in the SWIRE Lockman Hole field. The sample of mid-IR-selected galaxies includes ~20,000 objects detected abov
e a flux threshold of S24mum=310muJy. We adopt optical/near-IR color selection criteria to split the sample into the lower-redshift and higher-redshift galaxy populations. We measure the angular correlation function on scales of theta=0.01-3.5 deg, from which, using the Limber inversion along with the redshift distribution established for similarly selected source populations in the GOODS fields (Rodighiero et al. 2010), we obtain comoving correlation lengths of r0=4.98+-0.28 h^-1 Mpc and r0 =8.04+-0.69 h^-1 Mpc for the low-z (<z>=0.7) and high-z (<z>=1.7) subsamples, respectively. Comparing these measurements with the correlation functions of dark matter halos identified in the Bolshoi cosmological simulation (Klypin et al. 2011}, we find that the high-redshift objects reside in progressively more massive halos reaching Mtot>3e12 h^-1 Msun, compared to Mtot>7e11 h^-1 Msun for the low-redshift population. Approximate estimates of the IR luminosities based on the catalogs of 24mum sources in the GOODS fields show that our high-z subsample represents a population of distant ULIRGs with LIR>10^12Lsun, while the low-z subsample mainly consists of LIRGs, LIR~10^11Lsun. The comparison of number density of the 24mum selected galaxies and of dark matter halos with derived minimum mass Mtot shows that only 20% of such halos may host star-forming galaxies.
Gravitational lensing by massive galaxy clusters allows study of the population of intrinsically faint infrared galaxies that lie below the sensitivity and confusion limits of current infrared and submillimeter telescopes. We present ultra-deep PACS
100 and 160 microns observations toward the cluster lens Abell 2218, to penetrate the Herschel confusion limit. We derive source counts down to a flux density of 1 mJy at 100 microns and 2 mJy at 160 microns, aided by strong gravitational lensing. At these levels, source densities are 20 and 10 beams/source in the two bands, approaching source density confusion at 160 microns. The slope of the counts below the turnover of the Euclidean-normalized differential curve is constrained in both bands and is consistent with most of the recent backwards evolutionary models. By integrating number counts over the flux range accessed by Abell 2218 lensing (0.94-35 mJy at 100 microns and 1.47-35 mJy at 160 microns, we retrieve a cosmic infrared background (CIB) surface brightness of ~8.0 and ~9.9 nW m^-2 sr^-1, in the respective bands. These values correspond to 55% (+/- 24%) and 77% (+/- 31%) of DIRBE direct measurements. Combining Abell 2218 results with wider/shallower fields, these figures increase to 62% (+/- 25%) and 88% (+/- 32%) CIB total fractions, resolved at 100 and 160 microns, disregarding the high uncertainties of DIRBE absolute values.
