[Abridged] We study the evolution of the radio spectral index and far-infrared/radio correlation (FRC) across the star-formation rate-stellar masse (i.e. SFR-M*) plane up to z 2. We start from a M*-selected sample of galaxies with reliable SFR and redshift estimates. We then grid the SFR-M* plane in several redshift ranges and measure the infrared luminosity, radio luminosity, radio spectral index, and ultimately the FRC index (i.e. qFIR) of each SFR-M*-z bin. The infrared luminosities of our SFR-M*-z bins are estimated using their stacked far-infrared flux densities inferred from observations obtained with Herschel. Their radio luminosities and radio spectral indices (i.e. alpha, where Snu nu^-alpha) are estimated using their stacked 1.4GHz and 610MHz flux densities from the VLA and GMRT, respectively. Our far-infrared and radio observations include the most widely studied blank extragalactic fields -GOODS-N/S, ECDFS, and COSMOS- covering a sky area of 2deg^2. Using this methodology, we constrain the radio spectral index and FRC index of star-forming galaxies with M*>10^10Msun and 0<z<2.3. We find that alpha^1.4GHz_610MHz does not evolve significantly with redshift or with the distance of a galaxy with respect to the main sequence (MS) of the SFR-M* plane (i.e. Delta_log(SSFR)_MS=log[SSFR(galaxy)/SSFR_MS(M*,z)]). Instead, star-forming galaxies have a radio spectral index consistent with a canonical value of 0.8, which suggests that their radio spectra are dominated by non-thermal optically thin synchrotron emission. We find that qFIR displays a moderate but statistically significant redshift evolution as qFIR(z)=(2.35+/-0.08)*(1+z)^(-0.12+/-0.04), consistent with some previous literature. Finally, we find no significant correlation between qFIR and Delta_log(SSFR)_MS, though a weak positive trend, as observed in one of our redshift bins, cannot be firmly ruled out using our dataset.
Three billion years after the big bang (at redshift z=2), half of the most massive galaxies were already old, quiescent systems with little to no residual star formation and extremely compact with stellar mass densities at least an order of magnitude larger than in low redshift ellipticals, their descendants. Little is known about how they formed, but their evolved, dense stellar populations suggest formation within intense, compact starbursts 1-2 Gyr earlier (at 3<z<6). Simulations show that gas-rich major mergers can give rise to such starbursts which produce dense remnants. Sub-millimeter selected galaxies (SMGs) are prime examples of intense, gas-rich, starbursts. With a new, representative spectroscopic sample of compact quiescent galaxies at z=2 and a statistically well-understood sample of SMGs, we show that z=3-6 SMGs are consistent with being the progenitors of z=2 quiescent galaxies, matching their formation redshifts and their distributions of sizes, stellar masses and internal velocities. Assuming an evolutionary connection, their space densities also match if the mean duty cycle of SMG starbursts is 42 (+40/-29) Myr (consistent with independent estimates), which indicates that the bulk of stars in these massive galaxies were formed in a major, early surge of star-formation. These results suggests a coherent picture of the formation history of the most massive galaxies in the universe, from their initial burst of violent star-formation through their appearance as high stellar-density galaxy cores and to their ultimate fate as giant ellipticals.
We use deep Herschel PACS and SPIRE observations in GOODSS, GOODSN and COSMOS to estimate the average dust mass (Mdust) of galaxies on a redshift-stellar mass (Mstar)-SFR grid. We study the scaling relations between Mdust, Mstar and SFR at z<=2.5. No clear evolution of Mdust is observed at fixed SFR and Mstar. We find a tight correlation between SFR and Mdust, likely a consequence of the Schmidt-Kennicutt (S-K) law. The Mstar-Mdust correlation observed by previous works flattens or sometimes disappears when fixing the SFR. Most of it likely derives from the combination of the Mdust-SFR and Mstar-SFR correlations. We then investigate the gas content as inferred by converting Mdust by assuming that the dust/gas ratio scales linearly with the gas metallicity. All galaxies in the sample follow, within uncertainties, the same SFR-Mgas relation (integrated S-K law), which broadly agrees with CO-based results for the bulk of the population, despite the completely different approaches. The majority of galaxies at z~2 form stars with an efficiency (SFE=SFR/Mgas) ~5 times higher than at z~0. It is not clear what fraction of such variation is an intrinsic redshift evolution and what fraction arises from selection effects. The gas fraction (fgas) decreases with Mstar and increases with SFR, and does not evolve with z at fixed Mstar and SFR. We explain these trends by introducing a universal relation between fgas, Mstar and SFR, non-evolving out to z~2.5. Galaxies move across this relation as their gas content evolves in time. We use the 3D fundamental fgas-Mstar-SFR relation and the redshift evolution of the Main Sequence to estimate the evolution of fgas in the average population of galaxies as a function of z and Mstar, and we find evidence a downsizing scenario.
[Abridged] We study the evolution of the dust temperatures of galaxies in the SFR-M* plane up to z~2 using observations from the Herschel Space Observatory. 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 Tdust of each SFR-M*-z bin. Dust temperatures are inferred using the stacked far-infrared flux densities of our SFR-M*-z bins. At all redshifts, Tdust increases with infrared luminosities (LIR), specific SFRs (SSFR; i.e., SFR/M*) and distances with respect to the main sequence (MS) of the SFR-M* plane (i.e., D_SSFR_MS=log[SSFR(galaxy)/SSFR_MS(M*,z)]). The Tdust-SSFR and Tdust-D_SSFR_MS correlations are statistically more significant than the Tdust-LIR one. While the slopes of these three correlations are redshift-independent, their normalizations evolve from z=0 and z~2. We convert these results into a recipe to derive Tdust from SFR, M* and z. The existence of a strong Tdust-D_SSFR_MS correlation provides us with information on the dust and gas content of galaxies. (i) The slope of the Tdust-D__SSFR_MS correlation can be explained by the increase of the star-formation efficiency (SFE; SFR/Mgas) with D_SSFR_MS as found locally by molecular gas studies. (ii) At fixed D_SSFR_MS, the constant Tdust observed in galaxies probing large ranges in SFR and M* can be explained by an increase or decrease of the number of star-forming regions with comparable SFE enclosed in them. (iii) At high redshift, the normalization towards hotter temperature of the Tdust-D_SSFR_MS correlation can be explained by the decrease of the metallicities of galaxies or by the increase of 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.
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 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 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.
We investigate the effect of the high-pass filter data reduction technique on the Herschel PACS PSF and noise of the PACS maps at the 70, 100 and 160 um bands and in medium and fast scan speeds. This branch of the PACS Photometer pipeline is the most used for cosmological observations and for point-source observations.The calibration of the flux loss due to the median removal applied by the PACS pipeline (high-pass filter) is done via dedicated simulations obtained by polluting real PACS timelines with fake sources at different flux levels. The effect of the data reduction parameter settings on the final map noise is done by using selected observations of blank fields with high data redundancy. We show that the running median removal can cause significant flux losses at any flux level. We analyse the advantages and disadvantages of several masking strategies and suggest that a mask based on putting circular patches on prior positions is the best solution to reduce the amount of flux loss. We provide a calibration of the point-source flux loss for several masking strategies in a large range of data reduction parameters, and as a function of the source flux. We also show that, for stacking analysis, the impact of the high-pass filtering effect is to reduce significantly the clustering effect. The analysis of the global noise and noise components of the PACS maps shows that the dominant parameter in determining the final noise is the high-pass filter width. We also provide simple fitting functions to build the error map from the coverage map and to estimate the cross-correlation correction factor in a representative portion of the data reduction parameter space.
Deep Herschel imaging and 12CO(2-1) line luminosities from the IRAM PdBI are combined for a sample of 17 galaxies at z>1 from the GOODS-N field. The sample includes galaxies both on and above the main sequence (MS) traced by star-forming galaxies in the SFR-M* plane. The far-infrared data are used to derive dust masses, Mdust. Combined with an empirical prescription for the dependence of the gas-to-dust ratio on metallicity (GDR), the CO luminosities and Mdust values are used to derive for each galaxy the CO-to-H2 conversion factor, alpha_co. Like in the local Universe, the value of alpha_co is a factor of ~5 smaller in starbursts compared to normal star-forming galaxies (SFGs). We also uncover a relation between alpha_co and dust temperature (Tdust; alpha_co decreasing with increasing Tdust) as obtained from modified blackbody fits to the far-infrared data. While the absolute normalization of the alpha_co(Tdust) relation is uncertain, the global trend is robust against possible systematic biases in the determination of Mdust, GDR or metallicity. Although we cannot formally distinguish between a step and a smooth evolution of alpha_co with the dust temperature, we can conclude that in galaxies of near-solar metallicity, a critical value of Tdust=30K can be used to determine whether the appropriate alpha_co is closer to the starburst value (1.0 Msun(K kms pc^2)^-1, if Tdust>30K) or closer to the Galactic value (4.35 Msun (K kms pc^2)^-1, if Tdust<30K). This indicator has the great advantage of being less subjective than visual morphological classifications of mergers/SFGs, which can be difficult at high z because of the clumpy nature of SFGs. In the absence of far-infrared data, the offset of a galaxy from the main sequence (i.e., log[SSFR(galaxy)/SSFR_MS(M*,z)]) can be used to identify galaxies requiring the use of an alpha_co conversion factor lower than the Galactic value.
We study a sample of 61 submillimetre galaxies (SMGs) selected from ground-based surveys, with known spectroscopic redshifts and observed with Herschel as part of the PACS Evolutionary Probe (PEP) and the Herschel Multi-tiered Extragalactic Survey (HerMES) key programmes. We use the broad far-infrared wavelength coverage (100-600um) provided by the combination of PACS and SPIRE observations. Using a power-law temperature distribution model to derive infrared luminosities and dust temperatures, we measure a dust emissivity spectral index for SMGs of beta=2.0+/-0.2. Our results unveil the diversity of the SMG population. Some SMGs exhibit extreme infrared luminosities of ~10^13 Lsun and relatively warm dust components, while others are fainter (~10^12 Lsun) and are biased towards cold dust temperatures. The extreme infrared luminosities of some SMGs (LIR>10^12.7 Lsun, 26/61 systems) imply SFRs of >500Msun yr^-1. 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 (~40K as opposed to ~25K), consistent with observations of local ULIRGs triggered by major mergers and with results from hydrodynamic simulations of major mergers combined with radiative transfer calculations. Luminous SMGs are also 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 thus might be evolving through a secular mode of star formation. [abridged]
We use deep observations taken with the Photodetector Array Camera and Spectrometer (PACS), on board the Herschel satellite as part of the PACS evolutionary probe (PEP) guaranteed project along with submm ground-based observations to measure the dust mass of a sample of high-z submillimeter galaxies (SMGs). We investigate their dust content relative to their stellar and gas masses, and compare them with local star-forming galaxies. High-z SMGs are dust rich, i.e. they have higher dust-to-stellar mass ratios compared to local spiral galaxies (by a factor of 30) and also compared to local ultraluminous infrared galaxies (ULIRGs, by a factor of 6). This indicates that the large masses of gas typically hosted in SMGs have already been highly enriched with metals and dust. Indeed, for those SMGs whose gas mass is measured, we infer dust-to-gas ratios similar or higher than local spirals and ULIRGs. However, similarly to other strongly star-forming galaxies in the local Universe and at high-z, SMGs are characterized by gas metalicities lower (by a factor of a few) than local spirals, as inferred from their optical nebular lines, which are generally ascribed to infall of metal-poor gas. This is in contrast with the large dust content inferred from the far-IR and submm data. In short, the metalicity inferred from the dust mass is much higher (by more than an order of magnitude) than that inferred from the optical nebular lines. We discuss the possible explanations of this discrepancy and the possible implications for the investigation of the metalicity evolution at high-z.
We use deep observations obtained with the Photodetector Array Camera and Spectrometer (PACS) onboard the Herschel space observatory to study the far-infrared (FIR) properties of submm and optically faint radio galaxies (SMGs and OFRGs). From literature we compiled a sample of 35 securely identified SMGs and nine OFRGs located in the GOODS-N and the A2218 fields. This sample is cross-matched with our PACS 100 um and 160 um multi-wavelength catalogs. About half of the galaxies in our sample are detected with PACS. The dust temperatures and the infrared luminosities of our galaxies are derived by fitting their PACS and SCUBA 850 um (only the upper limits for the OFRGs) flux densities with a single modified (beta=1.5) black body function. The median dust temperature of our SMG sample is T=36+/-8K while for our OFRG sample it is T=47+/-3K. For both samples, median dust temperatures derived from Herschel data agree well with previous estimates. In particular, Chapman et al. (2005) found a dust temperature of T=36+/-7K for a large sample of SMGs assuming the validity of the FIR/radio correlation. The agreement between our studies confirms that the local FIR/radio correlation effectively holds at high redshift even though we find <q>=2.17+/-0.19, a slightly lower value than that observed in local systems. The median IR luminosities of SMGs and OFRGs are 4.6*10^12 Lsun and 2.6*10^12 Lsun, respectively. We note that for both samples the IR luminosity estimates from the radio part of the spectral energy distribution are accurate, while estimates from the mid-IR are considerably (x3) more uncertain. Our observations confirm the remarkably high luminosities of SMGs and thus imply median star-formation rates of 960Msun yr^-1 for SMGs with S(850um)>5mJy and 460Msun yr^-1 for SMGs with S(850um)>2mJy, assuming a Chabrier IMF and no dominant AGN contribution to the far-infrared luminosity.
