Context. Determining the dust properties and UV attenuation of distant star-forming galaxies is of great interest for our understanding of galaxy formation and cosmic star formation in the early ...Universe. However, few direct measurements exist so far. Aims. To shed new light on these questions, we have targeted two recently discovered Lyman-break galaxies (LBGs) at z ≈ 6.8 and z = 7.508 to search for dust continuum and C ii λ158 μm line emission. Methods. The strongly lensed z ≈ 6.8 LBG A1703-zD1 behind the galaxy cluster Abell 1703 and the spectroscopically confirmed z = 7.508 LBG z8-GND-5296 in the GOODS-N field were observed with the Plateau de Bure Interferometer (PdBI) at 1.2 mm. These observations were combined with those of three z> 6.5 Lyα emitters (HCM6A, Himiko, and IOK-1), for which deep measurements were recently obtained with the PdBI and ALMA. Results. C ii is undetected in both galaxies, providing a deep upper limit of LC II< 2.8 × 107L⊙ for A1703-zD1, comparable to the nondetections of Himiko and IOK-1 with ALMA. Dust continuum emission from A1703-zD1 and z8-GND-5296 is not detected with an rms of 0.12 and 0.16 mJy/beam. From these nondetections and earlier multiwavelength observations we derive upper limits on their IR luminosity and star formation rate, dust mass, and UV attenuation. Thanks to strong gravitational lensing, the achieved limit for A1703-zD1 is similar to those achieved with ALMA, probing below the luminous infrared galaxy (LIRG) regime (LIR< 8.1 × 1010L⊙) and very low dust masses (Md< 1.6 × 107M⊙). We find that all five galaxies are compatible with the Calzetti IRX-β relation, their UV attenuation is compatible with several indirect estimates from other methods (the UV slope, extrapolation of the attenuation measured from the IR/UV ratio at lower redshift, and spectral energy distribution fits), and the dust-to-stellar mass ratio is compatible with that of galaxies from z = 0 to 3. From their stellar mass, the high-z galaxies studied here have an attenuation below the one expected from the mean relation of low-redshift (z ≲ 1.5) galaxies. Conclusions. More and deeper (sub-)mm data are clearly needed to directly determine the UV attenuation and dust content of the dominant population of high-z star-forming galaxies and to establish their dependence on stellar mass, redshift, and other properties more firmly.
To extend the molecular gas measurements to more typical star-forming galaxies (SFGs) with star formation rates SFR< 40 M⊙ yr-1 and stellar masses M∗< 2.5 × 1010M⊙ at z ~ 1.5−3, we have observed CO ...emission with the IRAM Plateau de Bure Interferometer and the IRAM 30 m telescope for five strongly lensed galaxies, selected from the Herschel Lensing Survey. These observations are combined with a compilation of CO measurements from the literature. From this, we infer the CO luminosity correction factors r2,1 = 0.81 ± 0.20 and r3,1 = 0.57 ± 0.15 for the J = 2 and J = 3 CO transitions, respectively, valid for SFGs at z> 1. The combined sample of CO-detected SFGs at z> 1 shows a large spread in star formation efficiency (SFE) with a dispersion of 0.33 dex, such that the SFE extends well beyond the low values of local spirals and overlaps the distribution of z> 1 submm galaxies. We find that the spread in SFE (or equivalently in molecular gas depletion timescale) is due to the variations of several physical parameters, primarily the specific star formation rate, and also stellar mass and redshift. The dependence of SFE on the offset from the main sequence and the compactness of the starburst is less clear. The possible increase of the molecular gas depletion timescale with stellar mass, now revealed by low M∗ SFGs at z> 1 and also observed at z = 0, contrasts with the generally acknowledged constant molecular gas depletion timescale and refutes the linearity of the Kennicutt-Schmidt relation. A net rise of the molecular gas fraction (fgas) is observed from z ~ 0.2 to z ~ 1.2, followed by a very mild increase toward higher redshifts, as found in earlier studies. At each redshift the molecular gas fraction shows a large dispersion, mainly due to the dependence of fgas on stellar mass, producing a gradient of increasing fgas with decreasing M∗. We provide the first measurement of the molecular gas fraction of z> 1 SFGs at the low-M∗ end between 109.4<M∗/M⊙< 109.9, reaching a mean ⟨ fgas ⟩ = 0.69 ± 0.18, which shows a clear fgas upturn at these lower stellar masses. Finally, we find evidence for a nonuniversal dust-to-gas ratio among high-redshift SFGs, high-redshift submm galaxies, local spirals, and local ultraluminous IR galaxies with near-solar metallicities, as inferred from a homogeneous analysis of their rest-frame 850 μm luminosity per unit gas mass. The SFGs with z> 1 show a trend for a lower Lν(850 μm) /Mgas mean by 0.33 dex compared to the other galaxy populations.
We report on the galaxy MACSJ0032-arc at zCO = 3.6314 discovered during the Herschel Lensing snapshot Survey of massive galaxy clusters, and strongly lensed by the cluster MACS J0032.1+1808. The ...successful detections of its rest-frame ultraviolet (UV), optical, far-infrared (FIR), millimeter, and radio continua, and of its CO emission enable us to characterize, for the first time at such a high redshift, the stellar, dust, and molecular gas properties of a compact star-forming galaxy with a size smaller than 2.5 kpc, a fairly low stellar mass of 4.8+ 0.5-1.0 × 109M⊙, and a moderate IR luminosity of 4.8+ 1.2-0.6 × 1011L⊙. By combining the stretching effect of the lens with the high angular resolution imaging of the CO(1–0) line emission and the radio continuum at 5 GHz, we find that the bulk of the molecular gas mass and star formation seems to be spatially decoupled from the rest-frame UV emission. About 90% of the total star formation rate is undetected at rest-frame UV wavelengths because of severe obscuration by dust, but is seen through the thermal FIR dust emission and the radio synchrotron radiation. The observed CO(4–3) and CO(6–5) lines demonstrate that high-J transitions, at least up to J = 6, remain excited in this galaxy, whose CO spectral line energy distribution resembles that of high-redshift submm galaxies, even though the IR luminosity of MACSJ0032-arc is ten times lower. This high CO excitation is possibly due to the compactness of the galaxy. We find evidence that this high CO excitation has to be considered in the balance when estimating the CO-to-H2 conversion factor. Indeed, the respective CO-to-H2 conversion factors as derived from the correlation with metallicity and the FIR dust continuum can only be reconciled if excitation is accounted for. The inferred depletion time of the molecular gas in MACSJ0032-arc supports the decrease in the gas depletion timescale of galaxies with redshift, although to a lesser degree than predicted by galaxy evolution models. Instead, the measured molecular gas fraction as high as 60–79% in MACSJ0032-arc favors the continued increase in the gas fraction of galaxies with redshift as expected, despite the plateau observed between z ~ 1.5 and z ~ 2.5.
We present photometric redshifts and spectral energy distribution (SED) classifications for a sample of 1542 optically identified sources detected with XMM in the COSMOS field. Our template fitting ...classifies 46 sources as stars and 464 as nonactive galaxies, while the remaining 1032 require templates with an active galactic nucleus (AGN) contribution. High accuracy in the derived photometric redshifts was accomplished as the result of (1) photometry in up to 30 bands with high-significance detections, (2) a new set of SED templates, including 18 hybrids covering the far-UV to mid-infrared, which have been constructed by the combination of AGNs and nonactive galaxies templates, and (3) multiepoch observations that have been used to correct for variability (most important for type 1 AGNs). The reliability of the photometric redshifts is evaluated using the subsample of 442 sources with measured spectroscopic redshifts. We achieved an accuracy of for i* AB < 22.5 ( for i* AB < 24.5). The high accuracies were accomplished for both type 2 (where the SED is often dominated by the host galaxy) and type 1 AGNs and QSOs out to z = 4.5. The number of outliers is a large improvement over previous photometric redshift estimates for X-ray-selected sources (4.0% and 4.8% outliers for i* AB < 22.5 and i* AB < 24.5, respectively). We show that the intermediate band photometry is vital to achieving accurate photometric redshifts for AGNs, whereas the broad SED coverage provided by mid-infrared (Spitzer/IRAC) bands is important to reduce the number of outliers for normal galaxies.
Context. Multi-wavelength, optical to IR/submm observations of strongly lensed galaxies identified by the Herschel Lensing Survey are used to determine the physical properties of high-redshift ...star-forming galaxies close to or below the detection limits of blank fields. Aims. We aim to constrain theIR stellar and dust content, and to determine star formation rates and histories, dust attenuation and extinction laws, and other related properties. Methods. We studied a sample of seven galaxies with spectroscopic redshifts z ~ 1.5−3 that have been detected with precision thanks to gravitational lensing, and whose spectral energy distribution (SED) has been determined from the rest-frame UV to the IR/mm domain. For comparison, our sample includes two previously well-studied lensed galaxies, MS1512-cB58 and the Cosmic Eye, for which we also provide updated Herschel measurements. We performed SED fits of the full photometry of each object, and of the optical and infrared parts separately, exploring various star formation histories, using different extinction laws, and exploring the effects of nebular emission. The IR luminosity, in particular, is predicted consistently from the stellar population model. The IR observations and emission line measurements, where available, are used as a posteriori constraints on the models. We also explored energy conserving models, that we created by using the observed IR/UV ratio to estimate the extinction. Results. Among the models we have tested, models with exponentially declining star-forming histories including nebular emission and assuming the Calzetti attenuation law best fit most of the observables. Models assuming constant or rising star formation histories predict in most cases too much IR luminosity. The SMC extinction law underpredicts the IR luminosity in most cases, except for two out of seven galaxies, where we cannot distinguish between different extinction laws. Our sample has a median lensing-corrected IR luminosity ~3 × 1011L⊙, stellar masses between 2 × 109M⊙ and 2 × 1011M⊙, and IR/UV luminosity ratios spanning a wide range. The dust masses of our galaxies are in the range 2−17 × 107M⊙, extending previous studies at the same redshift down to lower masses. We do not find any particular trend of the dust temperature Tdust with LIR, suggesting an overall warmer dust regime at our redshift regardless of IR luminosity. Conclusions. Gravitational lensing enables us to study the detailed physical properties of individual IR-detected z ~ 1.5−3 galaxies up to a factor of ~10 fainter than achieved with deep blank field observations. We have in particular demonstrated that multi-wavelength observations combining stellar and dust emission can constrain star formation histories and extinction laws of star-forming galaxies, as proposed in an earlier paper. Fixing the extinction based on the IR/UV observations successfully breaks the age-extinction degeneracy often encountered in obscured galaxies.
Aims. Our objectives are to determine the properties of the interstellar medium (ISM) and of star formation in typical star-forming galaxies at high redshift. Methods. Following up on our previous ...multi-wavelength observations with HST, Spitzer, Herschel, and the Plateau de Bure Interferometer (PdBI), we have studied a strongly lensed z = 2.013 galaxy, the arc behind the galaxy cluster MACS J0451+0006, with ALMA to measure the C ii 158 μm emission line, one of the main coolants of the ISM. Results. Emission of the C ii line from the southern part of this galaxy is detected at 10σ. Taking strong gravitational lensing into account, which provides a magnification of μ = 49, the intrinsic lensing-corrected C ii luminosity is LCII = 1.2 × 108L⊙. The observed ratio of C ii-to-IR emission, LCII/LFIR ≈ (1.2−2.4) × 10-3, is found to be similar to that in nearby galaxies. The same also holds for the observed ratio LCII/LCO= 2.3 × 103, which is comparable to that of star-forming galaxies and active galaxy nuclei (AGN) at low redshift. Conclusions. We utilize strong gravitational lensing to extend diagnostic studies of the cold ISM to an order of magnitude lower luminosity (LIR ~ (1.1−1.3) × 1011L⊙) and SFR than previous work at high redshift. While larger samples are needed, our results provide evidence that the cold ISM of typical high-redshift galaxies has physical characteristics similar to normal star-forming galaxies in the local Universe.
The "Cosmic Evolution Survey" (COSMOS) enables the study of the spectral energy distributions (SEDs) of active galactic nuclei (AGNs) because of the deep coverage and rich sampling of frequencies ...from X-ray to radio. Here we present an SED catalog of 413 X-ray (XMM-Newton)-selected type 1 (emission line FWHM > 2000 km s super(-1)) AGNs with Magellan, SDSS, or VLT spectrum. The SEDs are corrected for Galactic extinction, broad emission line contributions, constrained variability, and host galaxy contribution. We present the mean SED and the dispersion SEDs after the above corrections in the rest-frame 1.4 GHz to 40 keV, and show examples of the variety of SEDs encountered. In the near-infrared to optical (rest frame ~8 mum-4000 A), the photometry is complete for the whole sample and the mean SED is derived from detections only. Reddening and host galaxy contamination could account for a large fraction of the observed SED variety. The SEDs are all available online.
We present CO observations of nine ultra-luminous infrared galaxies (ULIRGs) at z ~ 2 with f ν(24 μm) gsim 1 mJy, previously confirmed with the mid-IR spectra in the Spitzer First Look Survey. All ...targets are required to have accurate redshifts from Keck/GEMINI near-IR spectra. Using the Plateau de Bure millimeter-wave Interferometer at the Institute for Radioastronomy at Millimeter Wavelengths, we detect CO J(3-2) (seven objects) or J(2-1) (one object) line emission from eight sources with integrated intensities Ic ~ 5σ-9σ. The CO-detected sources have a variety of mid-IR spectra, including strong polycyclic aromatic hydrocarbon, deep silicate absorption, and power-law continuum, implying that these molecular gas-rich objects at z ~ 2 could be either starbursts or dust-obscured active galactic nuclei (AGNs). The measured line luminosity L'CO is (1.28-3.77) × 1010 K km/s pc2. The averaged molecular gas mass M_H_2 is 1.7 × 1010 M sun, assuming CO-to-H2 conversion factor of 0.8 M sun (K km/s pc2)-1. Three sources (33%)—MIPS506, MIPS16144, and MIPS8342—have double peak velocity profiles. The CO double peaks in MIPS506 and MIPS16144 show spatial separations of 45 kpc and 10.9 kpc, allowing the estimates of the dynamical masses of 3.2 × 1011 sin-2(i) M sun and 5.4 × 1011 sin-2(i) M sun, respectively. The implied gas fraction, M gas/M dyn, is 3% and 4%, assuming an average inclination angle. Finally, the analysis of the Hubble Space Telescope/NIC2 images, mid-IR spectra, and IR spectral energy distribution revealed that most of our sources are mergers, containing dust-obscured AGNs dominating the luminosities at (3-6) μm. Together, these results provide some evidence suggesting submillimeter galaxies, bright 24 μm, z ~ 2 ULIRGs, and QSOs could represent three different stages of a single evolutionary sequence, however, a complete physical model would require much more data, especially high spatial resolution spectroscopy. Based on observations obtained at the Institute for Radioastronomy at Millimeter Wavelengths (IRAM) Plateau de Bure Interferometer (PdBI). IRAM is funded by the Centre National de la Recherche Scientifique (France), the Max-Planck Gesellschaft (Germany), and the Instituto Geografico Nacional (Spain).
We present a study of the morphological nature of redshift z similar to 0.7 star-forming galaxies using a combination of HST ACS, GALEX, and ground-based images of the COSMOS field. Our sample ...consists of 8146 galaxies, 5777 of which are detected in the GALEX near-ultraviolet band (2310 AA or similar to 1360 AA rest frame) down to a limiting magnitude of 25.5 (AB), and all of which have a brightness of F814W(WST) < 23 mag and photometric redshifts in the range 0.65 < z < 0.8. We make use of the UV to estimate star formation rates, correcting for the effect of dust using the UV slope, and of the ground-based multiband data to calculate masses. For all galaxies in our sample, we compute, from the ACS F814W images, their concentration (C), asymmetry (A), and dumpiness (S), as well as their Gini coefficient (G) and the second moment of the brightest 20% of their light (M20). We observe a bimodality in the galaxy population in asymmetry and in clumpiness, although the separation is most evident when either of those parameters is combined with a concentration-like parameter (C, G, or M sub(20)). We further show that this morphological bimodality has a strong correspondence with the FUV-g color bimodality and conclude that UV-optical color predominantly evolves concurrently with morphology. We observe many of the most star-forming galaxies to have morphologies approaching that of early-type galaxies, and we interpret this as evidence that strong starburst events are linked to bulge growth and constitute a process through which galaxies can be brought from the blue to the red sequence while simultaneously modifying their morphology accordingly. We conclude that the red sequence has continued growing at z unk 0.7. We also observe z similar to 0.7 galaxies to have physical properties similar to that of local galaxies, except for higher star formation rates. Whence we infer that the dimming of star-forming galaxies is responsible for most of the evolution in star formation rate density since that redshift, although our data are also consistent with a mild number evolution.
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