The far infra-red SEDs of main sequence and starburst galaxies

We compare observed far infra-red/sub-millimetre (FIR/sub-mm) galaxy spectral energy distributions (SEDs) of massive galaxies ($M_{\star}\gtrsim10^{10}$ $h^{-1}$M$_{\odot}$) derived through a stacking analysis with predictions from a new model of galaxy formation. The FIR SEDs of the model galaxies are calculated using a self-consistent model for the absorption and re-emission of radiation by interstellar dust based on radiative transfer calculations and global energy balance arguments. Galaxies are selected based on their position on the specific star formation rate (sSFR) - stellar mass ($M_{\star}$) plane. We identify a main sequence of star-forming galaxies in the model, i.e. a well defined relationship between sSFR and $M_\star$, up to redshift $z\sim6$. The scatter of this relationship evolves such that it is generally larger at higher stellar masses and higher redshifts. There is remarkable agreement between the predicted and observed average SEDs across a broad range of redshifts ($0.5\lesssim z\lesssim4$) for galaxies on the main sequence. However, the agreement is less good for starburst galaxies at $z\gtrsim2$, selected here to have elevated sSFRs$>10\times$ the main sequence value. We find that the predicted average SEDs are robust to changing the parameters of our dust model within physically plausible values. We also show that the dust temperature evolution of main sequence galaxies in the model is driven by star formation on the main sequence being more burst-dominated at higher redshifts.Comment: 20 pages, 13 figures. Accepted to MNRA

Star formation properties of sub-mJy radio sources

We investigate the star formation properties of ~800 sources detected in one of the deepest radio surveys at 1.4 GHz. Our sample spans a wide redshift range (~0.1 - 4) and about four orders of magnitude in star formation rate (SFR). It includes both star forming galaxies (SFGs) and active galactic nuclei (AGNs), further divided into radio-quiet and radio-loud objects. We compare the SFR derived from the far infrared luminosity, as traced by Herschel, with the SFR computed from their radio emission. We find that the radio power is a good SFR tracer not only for pure SFGs but also in the host galaxies of RQ AGNs, with no significant deviation with redshift or specific SFR. Moreover, we quantify the contribution of the starburst activity in the SFGs population and the occurrence of AGNs in sources with different level of star formation. Finally we discuss the possibility of using deep radio survey as a tool to study the cosmic star formation history.Comment: 18 pages, 14 figures, 1 table (available in its entirety as ancillary data

ALMA observations of atomic carbon in z~4 dusty star-forming galaxies

We present ALMA [CI]($1-0$) (rest frequency 492 GHz) observations for a sample of 13 strongly-lensed dusty star-forming galaxies originally discovered at 1.4mm in a blank-field survey by the South Pole Telescope. We compare these new data with available [CI] observations from the literature, allowing a study of the ISM properties of $\sim 30$ extreme dusty star-forming galaxies spanning a redshift range $2 < z < 5$. Using the [CI] line as a tracer of the molecular ISM, we find a mean molecular gas mass for SPT-DSFGs of $6.6 \times 10^{10}$ M$_{\odot}$. This is in tension with gas masses derived via low-$J$ $^{12}$CO and dust masses; bringing the estimates into accordance requires either (a) an elevated CO-to-H$_2$ conversion factor for our sample of $\alpha_{\rm CO} \sim 2.5$ and a gas-to-dust ratio $\sim200$, or (b) an high carbon abundance $X_{\rm CI} \sim 7\times10^{-5}$. Using observations of a range of additional atomic and molecular lines (including [CI], [CII], and multiple transitions of CO), we use a modern Photodissociation Region code (3D-PDR) to assess the physical conditions (including the density, UV radiation field strength, and gas temperature) within the ISM of the DSFGs in our sample. We find that the ISM within our DSFGs is characterised by dense gas permeated by strong UV fields. We note that previous efforts to characterise PDR regions in DSFGs may have significantly underestimated the density of the ISM. Combined, our analysis suggests that the ISM of extreme dusty starbursts at high redshift consists of dense, carbon-rich gas not directly comparable to the ISM of starbursts in the local Universe.Comment: 21 pages, 12 figures. Accepted for publication in MNRA

SPT0346-52: Negligible AGN Activity in a Compact, Hyper-starburst Galaxy at z = 5.7

We present Chandra ACIS-S and ATCA radio continuum observations of the strongly lensed dusty, star-forming galaxy SPT-S J034640-5204.9 (hereafter SPT0346-52) at $z$ = 5.656. This galaxy has also been observed with ALMA, HST, Spitzer, Herschel, APEX, and the VLT. Previous observations indicate that if the infrared (IR) emission is driven by star formation, then the inferred lensing-corrected star formation rate ($\sim$ 4500 M_{\sun} yr$^{-1}$) and star formation rate surface density $\Sigma_{\rm SFR}$ ($\sim$ 2000 M_{\sun} {yr^{-1}} {kpc^{-2}}) are both exceptionally high. It remained unclear from the previous data, however, whether a central active galactic nucleus (AGN) contributes appreciably to the IR luminosity. The {\it Chandra} upper limit shows that SPT0346-52 is consistent with being star-formation dominated in the X-ray, and any AGN contribution to the IR emission is negligible. The ATCA radio continuum upper limits are also consistent with the FIR-to-radio correlation for star-forming galaxies with no indication of an additional AGN contribution. The observed prodigious intrinsic IR luminosity of (3.6 $\pm$ 0.3) $\times$ 10$^{13}$ L_{\sun} originates almost solely from vigorous star formation activity. With an intrinsic source size of 0.61 $\pm$ 0.03 kpc, SPT0346-52 is confirmed to have one of the highest $\Sigma_{SFR}$ of any known galaxy. This high $\Sigma_{SFR}$, which approaches the Eddington limit for a radiation pressure supported starburst, may be explained by a combination of very high star formation efficiency and gas fraction.Comment: 8 pages, 6 figures, accepted for publication in Ap

The Cosmic Far-Infrared Background Buildup Since Redshift 2 at 70 and 160 microns in the COSMOS and GOODS fields

The Cosmic Far-Infrared Background (CIB) at wavelengths around 160 {\mu}m corresponds to the peak intensity of the whole Extragalactic Background Light, which is being measured with increasing accuracy. However, the build up of the CIB emission as a function of redshift, is still not well known. Our goal is to measure the CIB history at 70 {\mu}m and 160 {\mu}m at different redshifts, and provide constraints for infrared galaxy evolution models. We use complete deep Spitzer 24 {\mu}m catalogs down to about 80 {\mu}Jy, with spectroscopic and photometric redshifts identifications, from the GOODS and COSMOS deep infrared surveys covering 2 square degrees total. After cleaning the Spitzer/MIPS 70 {\mu}m and 160 {\mu}m maps from detected sources, we stacked the far-IR images at the positions of the 24 {\mu}m sources in different redshift bins. We measured the contribution of each stacked source to the total 70 and 160 {\mu}m light, and compare with model predictions and recent far-IR measurements made with Herschel/PACS on smaller fields. We have detected components of the 70 and 160 {\mu}m backgrounds in different redshift bins up to z ~ 2. The contribution to the CIB is maximum at 0.3 <= z <= 0.9 at 160{\mu}m (and z <= 0.5 at 70 {\mu}m). A total of 81% (74%) of the 70 (160) {\mu}m background was emitted at z < 1. We estimate that the AGN relative contribution to the far-IR CIB is less than about 10% at z < 1.5. We provide a comprehensive view of the CIB buildup at 24, 70, 100, 160 {\mu}m. IR galaxy models predicting a major contribution to the CIB at z < 1 are in agreement with our measurements, while our results discard other models that predict a peak of the background at higher redshifts. Our results are available online http://www.ias.u-psud.fr/irgalaxies/ .Comment: Accepted in Astronomy & Astrophysic

The Rest-Frame Submillimeter Spectrum of High-Redshift, Dusty, Star-Forming Galaxies

We present the average rest-frame spectrum of high-redshift dusty, star-forming galaxies from 250-770GHz. This spectrum was constructed by stacking ALMA 3mm spectra of 22 such sources discovered by the South Pole Telescope and spanning z=2.0-5.7. In addition to multiple bright spectral features of 12CO, [CI], and H2O, we also detect several faint transitions of 13CO, HCN, HNC, HCO+, and CN, and use the observed line strengths to characterize the typical properties of the interstellar medium of these high-redshift starburst galaxies. We find that the 13CO brightness in these objects is comparable to that of the only other z>2 star-forming galaxy in which 13CO has been observed. We show that the emission from the high-critical density molecules HCN, HNC, HCO+, and CN is consistent with a warm, dense medium with T_kin ~ 55K and n_H2 >~ 10^5.5 cm^-3. High molecular hydrogen densities are required to reproduce the observed line ratios, and we demonstrate that alternatives to purely collisional excitation are unlikely to be significant for the bulk of these systems. We quantify the average emission from several species with no individually detected transitions, and find emission from the hydride CH and the linear molecule CCH for the first time at high redshift, indicating that these molecules may be powerful probes of interstellar chemistry in high-redshift systems. These observations represent the first constraints on many molecular species with rest-frame transitions from 0.4-1.2mm in star-forming systems at high redshift, and will be invaluable in making effective use of ALMA in full science operations.Comment: 19 pages, 10 figures (2 in appendices); accepted for publication in Ap

Planck's Dusty GEMS: Gravitationally lensed high-redshift galaxies discovered with the Planck survey

We present an analysis of 11 bright far-IR/submm sources discovered through a combination of the Planck survey and follow-up Herschel-SPIRE imaging. Each source has a redshift z=2.2-3.6 obtained through a blind redshift search with EMIR at the IRAM 30-m telescope. Interferometry obtained at IRAM and the SMA, and optical/near-infrared imaging obtained at the CFHT and the VLT reveal morphologies consistent with strongly gravitationally lensed sources. Additional photometry was obtained with JCMT/SCUBA-2 and IRAM/GISMO at 850 um and 2 mm, respectively. All objects are bright, isolated point sources in the 18 arcsec beam of SPIRE at 250 um, with spectral energy distributions peaking either near the 350 um or the 500 um bands of SPIRE, and with apparent far-infrared luminosities of up to 3x10^14 L_sun. Their morphologies and sizes, CO line widths and luminosities, dust temperatures, and far-infrared luminosities provide additional empirical evidence that these are strongly gravitationally lensed high-redshift galaxies. We discuss their dust masses and temperatures, and use additional WISE 22-um photometry and template fitting to rule out a significant contribution of AGN heating to the total infrared luminosity. Six sources are detected in FIRST at 1.4 GHz. Four have flux densities brighter than expected from the local far-infrared-radio correlation, but in the range previously found for high-z submm galaxies, one has a deficit of FIR emission, and 6 are consistent with the local correlation. The global dust-to-gas ratios and star-formation efficiencies of our sources are predominantly in the range expected from massive, metal-rich, intense, high-redshift starbursts. An extensive multi-wavelength follow-up programme is being carried out to further characterize these sources and the intense star-formation within them.Comment: A&A accepte

Sub-kiloparsec Imaging of Cool Molecular Gas in Two Strongly Lensed Dusty, Star-Forming Galaxies

We present spatially-resolved imaging obtained with the Australia Telescope Compact Array (ATCA) of three CO lines in two high-redshift gravitationally lensed dusty star-forming galaxies, discovered by the South Pole Telescope. Strong lensing allows us to probe the structure and dynamics of the molecular gas in these two objects, at z=2.78 and z=5.66, with effective source-plane resolution of less than 1kpc. We model the lensed emission from multiple CO transitions and the dust continuum in a consistent manner, finding that the cold molecular gas as traced by low-J CO always has a larger half-light radius than the 870um dust continuum emission. This size difference leads to up to 50% differences in the magnification factor for the cold gas compared to dust. In the z=2.78 galaxy, these CO observations confirm that the background source is undergoing a major merger, while the velocity field of the other source is more complex. We use the ATCA CO observations and comparable resolution Atacama Large Millimeter/submillimeter Array dust continuum imaging of the same objects to constrain the CO-H_2 conversion factor with three different procedures, finding good agreement between the methods and values consistent with those found for rapidly star-forming systems. We discuss these galaxies in the context of the star formation - gas mass surface density relation, noting that the change in emitting area with observed CO transition must be accounted for when comparing high-redshift galaxies to their lower redshift counterparts.Comment: 14 pages, 7 figures; accepted for publication in Ap

Merger driven star-formation activity in Cl J1449+0856 at z=1.99 as seen by ALMA and JVLA

We use ALMA and JVLA observations of the galaxy cluster Cl J1449+0856 at z=1.99, in order to study how dust-obscured star-formation, ISM content and AGN activity are linked to environment and galaxy interactions during the crucial phase of high-z cluster assembly. We present detections of multiple transitions of 12CO, as well as dust continuum emission detections from 11 galaxies in the core of Cl J1449+0856. We measure the gas excitation properties, star-formation rates, gas consumption timescales and gas-to-stellar mass ratios for the galaxies. We find evidence for a large fraction of galaxies with highly-excited molecular gas, contributing >50% to the total SFR in the cluster core. We compare these results with expectations for field galaxies, and conclude that environmental influences have strongly enhanced the fraction of excited galaxies in this cluster. We find a dearth of molecular gas in the galaxies' gas reservoirs, implying a high star-formation efficiency (SFE) in the cluster core, and find short gas depletion timescales τ<0.1-0.4 Gyrs for all galaxies. Interestingly, we do not see evidence for increased specific star-formation rates (sSFRs) in the cluster galaxies, despite their high SFEs and gas excitations. We find evidence for a large number of mergers in the cluster core, contributing a large fraction of the core's total star-formation compared with expectations in the field. We conclude that the environmental impact on the galaxy excitations is linked to the high rate of galaxy mergers, interactions and active galactic nuclei in the cluster core

GOODS-Herschel: star formation, dust attenuation, and the FIR-radio correlation on the main sequence of star-forming galaxies up to z=4

We use deep panchromatic data sets in the GOODS-N field, from GALEX to the deepest Herschel far-infrared (FIR) and VLA radio continuum imaging, to explore the evolution of star-formation activity and dust attenuation properties of star-forming galaxies to z sime 4, using mass-complete samples. Our main results can be summarized as follows: (i) the slope of the star-formation rate–M* correlation is consistent with being constant sime0.8 up to z sime 1.5, while its normalization keeps increasing with redshift; (ii) for the first time we are able to explore the FIR–radio correlation for a mass-selected sample of star-forming galaxies: the correlation does not evolve up to z sime 4; (iii) we confirm that galaxy stellar mass is a robust proxy for UV dust attenuation in star-forming galaxies, with more massive galaxies being more dust attenuated. Strikingly, we find that this attenuation relation evolves very weakly with redshift, with the amount of dust attenuation increasing by less than 0.3 mag over the redshift range [0.5–4] for a fixed stellar mass; (iv) the correlation between dust attenuation and the UV spectral slope evolves with redshift, with the median UV slope becoming bluer with redshift. By z sime 3, typical UV slopes are inconsistent, given the measured dust attenuations, with the predictions of commonly used empirical laws. (v) Finally, building on existing results, we show that gas reddening is marginally larger (by a factor of around 1.3) than the stellar reddening at all redshifts probed. Our results support a scenario where the ISM conditions of typical star-forming galaxies evolve with redshift, such that at z ≥ 1.5 Main Sequence galaxies have ISM conditions moving closer to those of local starbursts