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Register a new userFar-infrared spectroscopy of a lensed starburst: a blind redshift from Herschel
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We report the redshift of HATLAS J132427.0+284452 (hereafter HATLAS J132427), a gravitationally lensed starburst galaxy, the first determined blind by the Herschel Space Observatory. This is achieved via the detection of [C II] consistent with z = 1.68 in a far-infrared spectrum taken with the SPIRE Fourier Transform Spectrometer. We demonstrate that the [C II] redshift is secure via detections of CO J = 2 - 1 and 3 - 2 using the Combined Array for Research in Millimeter-wave Astronomy and the Institut de Radioastronomie Millimetriques Plateau de Bure Interferometer. The intrinsic properties appear typical of high-redshift starbursts despite the high lensing-amplified fluxes, proving the ability of the FTS to probe this population with the aid of lensing. The blind detection of [C II] demonstrates the potential of the SAFARI imaging spectrometer, proposed for the much more sensitive SPICA mission, to determine redshifts of multiple dusty galaxies simultaneously without the benefit of lensing.
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The most intensively star-forming galaxies are extremely luminous at far-infrared (FIR) wavelengths, highly obscured at optical and ultraviolet wavelengths, and lie at $zge 1-3$. We present a programme of ${it Herschel}$ FIR spectroscopic observations with the SPIRE FTS and photometric observations with PACS, both on board ${it Herschel}$, towards a sample of 45 gravitationally lensed, dusty starbursts across $zsim 1-3.6$. In total, we detected 27 individual lines down to 3-$sigma$, including nine $[rm C{small II}]$ 158-$mu$m lines with confirmed spectroscopic redshifts, five possible $[rm C{small II}]$ lines consistent with their far-infrared photometric redshifts, and in some individual sources a few $[rm O{small III}]$ 88-$mu$m, $[rm O{small III}]$ 52-$mu$m, $[rm O{small I}]$ 145-$mu$m, $[rm O{small I}]$ 63-$mu$m, $[rm N{small II}]$ 122-$mu$m, and OH 119-$mu$m (in absorption) lines. To derive the typical physical properties of the gas in the sample, we stack all spectra weighted by their intrinsic luminosity and by their 500-$mu$m flux densities, with the spectra scaled to a common redshift. In the stacked spectra, we detect emission lines of $[rm C{small II}]$ 158-$mu$m, $[rm N{small II}]$ 122-$mu$m, $[rm O{small III}]$ 88-$mu$m, $[rm O{small III}]$ 52-$mu$m, $[rm O{small I}]$ 63-$mu$m, and the absorption doublet of OH at 119-$mu$m, at high fidelity. We find that the average electron densities traced by the $[rm N{small II}]$ and $[rm O{small III}]$ lines are higher than the average values in local star-forming galaxies and ULIRGs, using the same tracers. From the $[rm N{small II}]/[rm C{small II}]$ and $[rm O{small I}]/[rm C{small II}]$ ratios, we find that the $[rm C{small II}]$ emission is likely dominated by the photo-dominated regions (PDR), instead of by ionised gas or large-scale shocks.
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 Spectrograph (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 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 report the detection of a significant excess in the surface density of far-infrared sources from the Herschel-Astrophysical Terahertz Large Area Survey (H-ATLAS) within ~1 Mpc of the centres of 66 optically-selected clusters of galaxies in the SDSS with <z>~0.25. From the analysis of the multiwavelength properties of their counterparts we conclude that the far-infrared emission is associated with dust-obscured star formation and/or active galactic nuclei within galaxies in the clusters themselves. The excess reaches a maximum at a radius of ~0.8 Mpc, where we find 1.0pm0.3 S_250um>34 mJy sources on average per cluster above what would be expected for random field locations. If the far-infrared emission is dominated by star formation (as opposed to AGN) then this corresponds to an average star formation rate of ~7 M_sun/yr per cluster in sources with L_IR>5d10 L_sun. Although lensed sources make a negligible contribution to the excess signal, a fraction of the sources around the clusters could be gravitationally lensed, and we have identified a sample of potential cases of cluster-lensed Herschel sources that could be targeted in follow-up studies.
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-SPIRE-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.
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