No abstract

We analyze the dependence of galaxy structure (size and Sersic index) and mode of star formation (Σ SF R and SF R IR /SF R UV ) on the position of galaxies in the SFR versus Mass diagram. Our sample comprises roughly 640000 galaxies at z ∼ 0.1, 130000 galaxies at z ∼ 1, and 36000 galaxies at z ∼ 2. Structural measurements for all but the z ∼ 0.1 galaxies are based on HST imaging, and SFRs are derived using a Herschel-calibrated ladder of SFR indicators. We find that a correlation between the structure and stellar population of galaxies (i.e., a 'Hubble sequence') is already in place since at least z ∼ 2.5. At all epochs, typical star-forming galaxies on the main sequence are well approximated by exponential disks, while the profiles of quiescent galaxies are better described by de Vaucouleurs profiles. In the upper envelope of the main sequence, the relation between the SFR and Sersic index reverses, suggesting a rapid build-up of the central mass concentration in these starbursting outliers. We observe quiescent, moderately and highly star-forming systems to co-exist over an order of magnitude or more in stellar mass. At each mass and redshift, galaxies on the main sequence have the largest size. The rate of size growth correlates with specific SFR, and so does Σ SF R at each redshift. A simple model using an empirically determined SF law and metallicity scaling, in combination with an assumed geometry for dust and stars is able to relate the observed Σ SF R and SF R IR /SF R UV , provided a more patchy dust geometry is assumed for high-redshift galaxies.

We compare multi-wavelength star formation rate (SFR) indicators out to z ∼ 3 in the GOODS-South field. Our analysis uniquely combines U -to-8µm photometry from FIREWORKS, MIPS 24 µm and PACS 70, 100, and 160 µm photometry from the PEP survey, and Hα spectroscopy from the SINS survey. We describe a set of conversions that lead to a continuity across SFR indicators. A luminosityindependent conversion from 24 µm to total infrared luminosity yields estimates of L IR that are in the median consistent with the L IR derived from PACS photometry, albeit with significant scatter. Dust correction methods perform well at low to intermediate levels of star formation. They fail to recover the total amount of star formation in systems with large SF R IR /SF R UV ratios, typically occuring at the highest SFRs (SF R UV +IR 100 M ⊙ /yr) and redshifts (z 2.5) probed. Finally, we confirm that Hα-based SFRs at 1.5 < z < 2.6 are consistent with SF R SED and SF R UV +IR provided extra attenuation towards HII regions is taken into account (A V,neb = A V,continuum /0.44). With the cross-calibrated SFR indicators in hand, we perform a consistency check on the star formation histories inferred from SED modeling. We compare the observed SFR-M relations and mass functions at a range of redshifts to equivalents that are computed by evolving lower redshift galaxies backwards in time. We find evidence for underestimated stellar ages when no stringent constraints on formation epoch are applied in SED modeling. We demonstrate how resolved SED modeling, or alternatively deep UV data, may help to overcome this bias. The age bias is most severe for galaxies with young stellar populations, and reduces towards older systems. Finally, our analysis suggests that SFHs typically vary on timescales that are long (at least several 100 Myr) compared to the galaxies' dynamical time.

Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

Deep far-infrared photometric surveys studying galaxy evolution and the nature of the cosmic infrared background are a key strength of the Herschel mission. We describe the scientific motivation for the PACS Evolutionary Probe (PEP) guaranteed time key program and its role within the entire set of Herschel surveys, and the field selection that includes popular multiwavelength fields such as GOODS, COSMOS, Lockman Hole, ECDFS, and EGS. We provide an account of the observing strategies and data reduction methods used. An overview of first science results illustrates the potential of PEP in providing calorimetric star formation rates for high-redshift galaxy populations, thus testing and superseding previous extrapolations from other wavelengths, and enabling a wide range of galaxy evolution studies.

We present results from the deepest Herschel-Photodetector Array Camera and Spectrometer (PACS) far-infrared blank field extragalactic survey, obtained by combining observations of the Great Observatories Origins Deep Survey (GOODS) fields from the PACS 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 3σ depths of 0.9, 0.6 and 1.3 mJy at 70, 100 and 160 μm, respectively, and resolve ∼75% of the cosmic infrared background at 100 μm and 160 μm 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 L IR = 10 11 L at z ∼ 1 and L IR = 10 12 L 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 (>3σ) 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.

We study the evolution of the dust temperature of galaxies in the SFR−M * plane up to z ∼ 2 using far-infrared and submillimetre observations from the Herschel Space Observatory taken as part of the PACS Evolutionary Probe (PEP) and Herschel Multi-tiered Extragalactic Survey (HerMES) guaranteed time key programmes. Starting from a sample of galaxies with reliable star-formation rates (SFRs), stellar masses (M * ) and redshift estimates, we grid the SFR−M * parameter space in several redshift ranges and estimate the mean dust temperature (T dust ) of each SFR-M * −z bin. Dust temperatures are inferred using the stacked far-infrared flux densities (100-500 μm) of our SFR-M * −z bins. At all redshifts, the dust temperature of galaxies smoothly increases with rest-frame infrared luminosities (L IR ), specific SFRs (SSFR; i.e., SFR/M * ), and distances with respect to the main sequence (MS) of the SFR−M * plane (i.e., Δ log (SSFR) MS = log [SSFR(galaxy)/SSFR MS (M * , z)]). The T dust −SSFR and T dust − Δ log (SSFR) MS correlations are statistically much more significant than the T dust −L IR one. While the slopes of these three correlations are redshiftindependent, their normalisations evolve smoothly from z = 0 and z ∼ 2. We convert these results into a recipe to derive T dust from SFR, M * and z, valid out to z ∼ 2 and for the stellar mass and SFR range covered by our stacking analysis. The existence of a strong T dust −Δ log (SSFR) MS correlation provides us with several pieces of information on the dust and gas content of galaxies. Firstly, the slope of the T dust −Δ log (SSFR) MS correlation can be explained by the increase in the star-formation efficiency (SFE; SFR/M gas ) with Δ log (SSFR) MS as found locally by molecular gas studies. Secondly, at fixed Δ log (SSFR) MS , the constant dust temperature observed in galaxies probing wide ranges in SFR and M * can be explained by an increase or decrease in the number of star-forming regions with comparable SFE enclosed in them. And thirdly, at high redshift, the normalisation towards hotter dust temperature of the T dust −Δ log (SSFR) MS correlation can be explained by the decrease in the metallicities of galaxies or by the increase in the SFE of MS galaxies. All these results support the hypothesis that the conditions prevailing in the star-forming regions of MS and far-above-MS galaxies are different. MS galaxies have star-forming regions with low SFEs and thus cold dust, while galaxies situated far above the MS seem to be in a starbursting phase characterised by star-forming regions with high SFEs and thus hot dust.

We study a sample of 61 submillimetre galaxies (SMGs) selected from ground-based surveys, with known spectroscopic redshifts and observed with the Herschel Space Observatory as part of the PACS Evolutionary Probe (PEP) and the Herschel Multi-tiered Extragalactic Survey (HerMES) guaranteed time key programmes. Our study makes use of the broad far-infrared and submillimetre wavelength coverage (100−600 μm) only made possible by the combination of observations from the PACS and SPIRE instruments aboard the Herschel Space Observatory. Using a power-law temperature distribution model to derive infrared luminosities and dust temperatures, we measure a dust emissivity spectral index for SMGs of β = 2.0 ± 0.2. Our results unambiguously unveil the diversity of the SMG population. Some SMGs exhibit extreme infrared luminosities of ∼10 13 L and relatively warm dust components, while others are fainter (a few times 10 12 L ) and are biased towards cold dust temperatures. Although at z ∼ 2 classical SMGs (>5 mJy at 850 μm) have large infrared luminosities (∼10 13 L ), objects only selected on their submm flux densities (without any redshift informations) probe a large range in dust temperatures and infrared luminosities. The extreme infrared luminosities of some SMGs (L IR 10 12.7 L , 26/61 systems) imply star formation rates (SFRs) of >500 M yr −1 (assuming a Chabrier IMF and no dominant AGN contribution to the FIR luminosity). Such high SFRs are difficult to reconcile with a secular mode of star formation, and may instead correspond to a merger-driven stage in the evolution of these galaxies. Another observational argument in favour of this scenario is the presence of dust temperatures warmer than that of SMGs of lower luminosities (∼40 K as opposed to ∼25 K), consistent with observations of local ultraluminous infrared galaxies triggered by major mergers and with results from hydrodynamic simulations of major mergers combined with radiative transfer calculations. Moreover, we find that luminous SMGs are systematically offset from normal star-forming galaxies in the stellar mass-SFR plane, suggesting that they are undergoing starburst events with short duty cycles, compatible with the major merger scenario. On the other hand, a significant fraction of the low infrared luminosity SMGs have cold dust temperatures, are located close to the main sequence of star formation, and therefore might be evolving through a secular mode of star formation. However, the properties of this latter population, especially their dust temperature, should be treated with caution because at these luminosities SMGs are not a representative sample of the entire star-forming galaxy population.