M. Béthermin

A dust-obscured massive maximum-starburst galaxy at a redshift of 6.34

Massive present-day early-type (elliptical and lenticular) galaxies probably gained the bulk of their stellar mass and heavy elements through intense, dust-enshrouded starbursts—that is, increased rates of star formation—in the most massive dark-matter haloes at early epochs. However, it remains unknown how soon after the Big Bang massive starburst progenitors exist. The measured redshift (z) distribution of dusty, massive starbursts has long been suspected to be biased low in z owing to selection effects, as confirmed by recent findings of systems with redshifts as high as ∼5 (refs 2–4). Here we report the identification of a massive starburst galaxy at z = 6.34 through a submillimetre colour-selection technique. We unambiguously determined the redshift from a suite of molecular and atomic fine-structure cooling lines. These measurements reveal a hundred billion solar masses of highly excited, chemically evolved interstellar medium in this galaxy, which constitutes at least 40 per cent of the baryonic mass. A ‘maximum starburst’ converts the gas into stars at a rate more than 2,000 times that of the Milky Way, a rate among the highest observed at any epoch. Despite the overall downturn in cosmic star formation towards the highest redshifts, it seems that environments mature enough to form the most massive, intense starbursts existed at least as early as 880 million years after the Big Bang.

THE EVOLVING INTERSTELLAR MEDIUM OF STAR-FORMING GALAXIES SINCE z = 2 AS PROBED BY THEIR INFRARED SPECTRAL ENERGY DISTRIBUTIONS

Using data from the mid-infrared to millimeter wavelengths for individual galaxies and for stacked ensembles at 0.5 1012 L ☉). For galaxies within the MS, we show that the variations of specific star formation rates (sSFRs = SFR/M *) are driven by varying gas fractions. For relatively massive galaxies like those in our samples, we show that the hardness of the radiation field, U, which is proportional to the dust-mass-weighted luminosity (L IR/M dust) and the primary parameter defining the shape of the IR spectral energy distribution (SED), is equivalent to SFE/Z. For MS galaxies with stellar mass log (M */M ☉) ≥ 9.7 we measure this quantity, U, showing that it does not depend significantly on either the stellar mass or the sSFR. This is explained as a simple consequence of the existing correlations between SFR-M *, M *-Z, and M gas-SFR. Instead, we show that U (or equally L IR/M dust) does evolve, with MS galaxies having harder radiation fields and thus warmer temperatures as redshift increases from z = 0 to 2, a trend that can also be understood based on the redshift evolution of the M *-Z and SFR-M * relations. These results motivate the construction of a universal set of SED templates for MS galaxies that are independent of their sSFR or M * but vary as a function of redshift with only one parameter, U.

The deepest Herschel-PACS far-infrared survey: number counts and infrared luminosity functions from combined PEP/GOODS-H observations

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 theconstruction 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.

The Herschel view of the dominant mode of galaxy growth from z = 4 to the present day

We present an analysis of the deepest Herschel images in four major extragalactic fields GOODS–North, GOODS–South, UDS, and COSMOS obtained within the GOODS–Herschel and CANDELS–Herschel key programs. The star formation picture provided by a total of 10 497 individual far-infrared detections is supplemented by the stacking analysis of a mass complete sample of 62 361 star-forming galaxies from the Hubble Space Telescope (HST) H band-selected catalogs of the CANDELS survey and from two deep ground-based Ks band-selected catalogs in the GOODS–North and the COSMOS-wide field to obtain one of the most accurate and unbiased understanding to date of the stellar mass growth over the cosmic history. We show, for the first time, that stacking also provides a powerful tool to determine the dispersion of a physical correlation and describe our method called “scatter stacking”, which may be easily generalized to other experiments. The combination of direct UV and far-infrared UV-reprocessed light provides a complete census on the star formation rates (SFRs), allowing us to demonstrate that galaxies at z = 4 to 0 of all stellar masses (M∗) follow a universal scaling law, the so-called main sequence of star-forming galaxies. We find a universal close-to-linear slope of the log 10(SFR)–log 10(M∗) relation, with evidence for a flattening of the main sequence at high masses (log 10(M∗/M⊙) > 10.5) that becomesless prominent with increasing redshift and almost vanishes by z ≃ 2. This flattening may be due to the parallel stellar growth of quiescent bulges in star-forming galaxies, which mostly happens over the same redshift range. Within the main sequence, we measure a nonvarying SFR dispersion of 0.3 dex: at a fixed redshift and stellar mass, about 68% of star-forming galaxies form stars at a universal rate within a factor 2. The specific SFR (sSFR = SFR/M∗) of star-forming galaxies is found to continuously increase from z = 0 to 4. Finally we discuss the implications of our findings on the cosmic SFR history and on the origin of present-day stars: more than two-thirds of present-day stars must have formed in a regime dominated by the “main sequence” mode. As a consequence we conclude that, although omnipresent in the distant Universe, galaxy mergers had little impact in shaping the global star formation history over the last 12.5 billion years.

A Herschel view of the far-infrared properties of submillimetre galaxies

We study a sample of 61submillimetre 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 ultra-luminous 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.

ALMA REDSHIFTS OF MILLIMETER-SELECTED GALAXIES FROM THE SPT SURVEY: THE REDSHIFT DISTRIBUTION OF DUSTY STAR-FORMING GALAXIES

Using the Atacama Large Millimeter/submillimeter Array, we have conducted a blind redshift survey in the 3 mm atmospheric transmission window for 26 strongly lensed dusty star-forming galaxies (DSFGs) selected with the South Pole Telescope. The sources were selected to have S_(1.4mm) > 20 mJy and a dust-like spectrum and, to remove low-z sources, not have bright radio (S_843MHz) 3. We discuss the effect of gravitational lensing on the redshift distribution and compare our measured redshift distribution to that of models in the literature.

THE HIDDEN “AGN MAIN SEQUENCE”: EVIDENCE FOR A UNIVERSAL BLACK HOLE ACCRETION TO STAR FORMATION RATE RATIO SINCE z ∼ 2 PRODUCING AN M BH-M * RELATION

Using X-ray stacking analyses we estimate the average amounts of supermassive black hole (SMBH) growth taking place in star-forming galaxies at z ~ 1 and z ~ 2 as a function of galaxy stellar mass (M *). We find that the average SMBH growth rate follows remarkably similar trends with M * and redshift as the average star formation rates (SFRs) of their host galaxies (i.e., ∝ M * 0.86 ± 0.39 for the z ~ 1 sample and ∝ M * 1.05 ± 0.36 for the z ~ 2 sample). It follows that the ratio of SMBH growth rate to SFR is (1) flat with respect to M *, (2) not evolving with redshift, and (3) close to the ratio required to maintain/establish an SMBH to stellar mass ratio of ≈10–3 as also inferred from todays M BH-M Bulge relationship. We interpret this as evidence that SMBHs have, on average, grown in step with their host galaxies since at least z ~ 2, irrespective of host galaxy mass and active galactic nucleus triggering mechanism. As such, we suggest that the same secular processes that drive the bulk of star formation are also responsible for the majority of SMBH growth. From this, we speculate that it is the availability of gas reservoirs that regulate both cosmological SMBH growth and star formation.

GOODS-HERSCHEL AND CANDELS: THE MORPHOLOGIES OF ULTRALUMINOUS INFRARED GALAXIES AT z ∼ 2*

Using deep 100 and 160 μm observations in GOODS-South from GOODS-Herschel, combined with high-resolution HST/WFC3 near-infrared imaging from CANDELS, we present the first detailed morphological analysis of a complete, far-infrared (FIR) selected sample of 52 ultraluminous infrared galaxies (ULIRGs; L IR > 1012 L ☉) at z ~ 2. We also make use of a comparison sample of galaxies with lower IR luminosities but with the same redshift and H-band magnitude distribution. Our visual classifications of these two samples indicate that the fractions of objects with disk and spheroid morphologies are roughly the same but that there are significantly more mergers, interactions, and irregular galaxies among the ULIRGs (72+5 – 7% versus 32 ± 3%). The combination of disk and irregular/interacting morphologies suggests that early-stage interactions, minor mergers, and disk instabilities could play an important role in ULIRGs at z ~ 2. We compare these fractions with those of a z ~ 1 sample selected from GOODS-H and COSMOS across a wide luminosity range and find that the fraction of disks decreases systematically with L IR while the fraction of mergers and interactions increases, as has been observed locally. At comparable luminosities, the fraction of ULIRGs with various morphological classifications is similar at z ~ 2 and z ~ 1, though there are slightly fewer mergers and slightly more disks at higher redshift. We investigate the position of the z ~ 2 ULIRGs, along with 70 z ~ 2 LIRGs, on the specific star formation rate versus redshift plane, and find 52 systems to be starbursts (i.e., they lie more than a factor of three above the main-sequence relation). We find that many of these systems are clear interactions and mergers (~50%) compared to only 24% of systems on the main sequence relation. If irregular disks are included as potential minor mergers, then we find that up to ~73% of starbursts are involved in a merger or interaction at some level. Although the final coalescence of a major merger may not be required for the high luminosities of ULIRGs at z ~ 2 as is the case locally, the large fraction (50%-73%) of interactions at all stages and potential minor mergers suggests that these processes contribute significantly to the high star formation rates of ULIRGs at z ~ 2.

A UNIFIED EMPIRICAL MODEL FOR INFRARED GALAXY COUNTS BASED ON THE OBSERVED PHYSICAL EVOLUTION OF DISTANT GALAXIES

We reproduce the mid-infrared to radio galaxy counts with a new empirical model based on our current understanding of the evolution of main-sequence (MS) and starburst (SB) galaxies. We rely on a simple Spectral Energy Distribution (SED) library based on Herschel observations: a single SED for the MS and another one for SB, getting warmer with redshift. Our model is able to reproduce recent measurements of galaxy counts performed with Herschel, including counts per redshift slice. This agreement demonstrates the power of our 2 Star-Formation Modes (2SFM) decomposition for describing the statistical properties of infrared sources and their evolution with cosmic time. We discuss the relative contribution of MS and SB galaxies to the number counts at various wavelengths and flux densities. We also show that MS galaxies are responsible for a bump in the 1.4 GHz radio counts around 50 {\mu}Jy. Material of the model (predictions, SED library, mock catalogs...) is available online at this http URL

THE CONTRIBUTION OF STARBURSTS AND NORMAL GALAXIES TO INFRARED LUMINOSITY FUNCTIONS AT z < 2

We present a parameter-less approach to predict the shape of the infrared (IR) luminosity function (LF) at redshifts z < 2. It requires no tuning and relies on only three observables: (1) the redshift evolution of the stellar mass function for star-forming galaxies, (2) the evolution of the specific star formation rate (sSFR) of main-sequence galaxies, and (3) the double-Gaussian decomposition of the sSFR-distribution at fixed stellar mass into a contribution (assumed redshift- and mass-invariant) from main-sequence and starburst activity. This self-consistent and simple framework provides a powerful tool for predicting cosmological observables: observed IR LFs are successfully matched at all z < 2, suggesting a constant or only weakly redshift-dependent contribution (8-14%) of starbursts to the star formation rate density. We separate the contributions of main-sequence and starburst activity to the global IR LF at all redshifts. The luminosity threshold above which the starburst component dominates the IR LF rises from log(LIR/Lsun) = 11.4 to 12.8 over 0 < z < 2, reflecting our assumed (1+z)^2.8-evolution of sSFR in main-sequence galaxies.