Aims. Cosmic reionization is an important process occurring in the early epochs of the Universe. However, because of observational limitations due to the opacity of the intergalactic medium to Lyman ...continuum photons, the nature of ionizing sources is still not well constrained. While high-redshift star-forming galaxies are thought to be the main contributors to the ionizing background at z> 6, it is impossible to directly detect their ionizing emission. Therefore, looking at intermediate redshift analogues (z ~ 2−4) can provide useful hints about cosmic reionization. Methods. We investigate the physical properties of one of the best Lyman continuum emitter candidate at z = 3.212 found in the GOODS-S/CANDELS field with photometric coverage from the U to the MIPS 24 μm band and VIMOS/VLT and MOSFIRE/Keck spectroscopy. These observations allow us to derive physical properties such as stellar mass, star formation rate, age of the stellar population, dust attenuation, metallicity, and ionization parameter, and to determine how these parameters are related to the Lyman continuum emission. Results. Investigation of the UV spectrum confirms a direct spectroscopic detection of the Lyman continuum emission with S/N> 5. Non-zero Lyα flux at the systemic redshift and high Lyman-α escape fraction (fesc(Lyα) ≥ 0.78) suggest a low H i column density. The weak C and Si low-ionization absorption lines are also consistent with a low covering fraction along the line of sight. The subsolar abundances are consistent with a young and extreme starburst. The O iiiλλ4959,5007+Hβ equivalent width (EW) is one of the largest reported for a galaxy at z> 3 (EW( O iii λλ4959,5007 + Hβ) ≃ 1600 Å, rest-frame; 6700 Å observed-frame) and the near-infrared spectrum shows that this is mainly due to an extremely strong O iii emission. The large observed O iii/O ii ratio (>10) and high ionization parameter are consistent with prediction from photoionization models in the case of a density-bounded nebula scenario. Furthermore, the EW(O iiiλλ4959,5007+Hβ) is comparable to recent measurements reported at z ~ 7−9, in the reionization epoch. We also investigate the possibility of an AGN contribution to explain the ionizing emission but most of the AGN identification diagnostics suggest that stellar emission dominates instead. Conclusions. This source is currently the first high-z example of a Lyman continuum emitter exhibiting indirect and direct evidences of a Lyman continuum leakage and having physical properties consistent with theoretical expectation from Lyman continuum emission from a density-bounded nebula. A low H i column density, low covering fraction, compact star formation activity, and a possible interaction/merging of two systems may contribute to the Lyman continuum photon leakage.
ABSTRACT We present U336V606J125H160 follow-up Hubble Space Telescope (HST) observations of 16 z ∼ 3 candidate Lyman continuum (LyC) emitters in the HS1549+1919 field. With these data, we obtain high ...spatial-resolution photometric redshifts of all sub-arcsecond components of the LyC candidates in order to eliminate foreground contamination and identify robust candidates for leaking LyC emission. Of the 16 candidates, we find one object with a robust LyC detection that is not due to foreground contamination. This object (MD5) resolves into two components; we refer to the LyC-emitting component as MD5b. MD5b has an observed 1500 to 900 flux-density ratio of , compatible with predictions from stellar population synthesis models. Assuming minimal IGM absorption, this ratio corresponds to a relative (absolute) escape fraction of = 75%-100% ( = 14%-19%). The stellar population fit to MD5b indicates an age of 50 Myr, which is in the youngest 10% of the HST sample and the youngest third of typical z ∼ 3 Lyman break galaxies, and may be a contributing factor to its LyC detection. We obtain a revised, contamination-free estimate for the comoving specific ionizing emissivity at z = 2.85, indicating (with large uncertainties) that star-forming galaxies provide roughly the same contribution as QSOs to the ionizing background at this redshift. Our results show that foreground contamination prevents ground-based LyC studies from obtaining a full understanding of LyC emission from z ∼ 3 star-forming galaxies. Future progress in direct LyC searches is contingent upon the elimination of foreground contaminants through high spatial-resolution observations, and upon acquisition of sufficiently deep LyC imaging to probe ionizing radiation in high-redshift galaxies.
Massive galaxies in the early Universe have been shown to be forming stars at surprisingly high rates. Prominent examples are dust-obscured galaxies which are luminous when observed at sub-millimetre ...wavelengths and which may be forming stars at a rate of 1,000 solar masses (M ) per year. These intense bursts of star formation are believed to be driven by mergers between gas-rich galaxies. Probing the properties of individual star-forming regions within these galaxies, however, is beyond the spatial resolution and sensitivity of even the largest telescopes at present. Here we report observations of the sub-millimetre galaxy SMMJ2135-0102 at redshift z = 2.3259, which has been gravitationally magnified by a factor of 32 by a massive foreground galaxy cluster lens. This magnification, when combined with high-resolution sub-millimetre imaging, resolves the star-forming regions at a linear scale of only 100 parsecs. We find that the luminosity densities of these star-forming regions are comparable to the dense cores of giant molecular clouds in the local Universe, but they are about a hundred times larger and 107 times more luminous. Although vigorously star-forming, the underlying physics of the star-formation processes at z 2 appears to be similar to that seen in local galaxies, although the energetics are unlike anything found in the present-day Universe.
Spectroscopic observations of H alpha and H beta emission lines of 128 star-forming galaxies in the redshift range 0.75 < or =, slant z < or =, slant 1.5 are presented. These data were taken with ...slitless spectroscopy using the G102 and G141 grisms of the Wide-Field-Camera 3 (WFC3) on board the Hubble Space Telescope as part of the WFC3 Infrared Spectroscopic Parallel survey. Interstellar dust extinction is measured from stacked spectra that cover the Balmer decrement (H alpha /H beta ). We present dust extinction as a function of H alpha luminosity (down to 3 x 10 super(41) erg s super(-1)), galaxy stellar mass (reaching 4 x 10 super(8) M sub(middot in circle)), and rest-frame H alpha equivalent width. The faintest galaxies are two times fainter in H alpha luminosity than galaxies previously studied at z ~ 1.5. An evolution is observed where galaxies of the same H alpha luminosity have lower extinction at higher redshifts, whereas no evolution is found within our error bars with stellar mass. The lower H alpha luminosity galaxies in our sample are found to be consistent with no dust extinction. We find an anti-correlation of the OIII lambda5007/H alpha flux ratio as a function of luminosity where galaxies with L sub(H alpha ) < 5 x 10 super(41) erg s super(-1) are brighter in OIII lambda5007 than H alpha . This trend is evident even after extinction correction, suggesting that the increased OIII lambda5007/H alpha ratio in low-luminosity galaxies is likely due to lower metallicity and/or higher ionization parameters.
We investigate the possibility of contamination by lower redshift interlopers in the measure of the ionizing radiation escaping from high-redshift galaxies. Taking advantage of the new ultradeep Very ...Large Telescope/Visible Multiobject Spectrograph U-band number counts in the Great Observatories Origins Deep Survey (GOODS)-South field, we calculate the expected probability of contamination by low-redshift interlopers as a function of the U magnitude and the image spatial resolution (point spread function). Assuming that ground-based imaging or spectroscopy cannot resolve objects lying within a 0.5-arcsec radius of each other, then each z≳ 3 galaxy has a 2.1 and 3.2 per cent chance of foreground contamination, adopting surface density U-band number counts down to 27.5 and 28.5, respectively. Those probabilities increase to 8.5 and 12.6 per cent, assuming 1.0-arcsec radius. If applied to the estimates reported in the literature at redshift ∼3 for which a Lyman continuum has been observed directly, the probability that at least one-third of them are affected by foreground contamination is larger than 50 per cent. From a Monte Carlo simulation, we estimate the median integrated contribution of foreground sources to the Lyman continuum flux (f900). Current estimations from stacked data are >2σ of the median integrated pollution by foreground sources. If the correction to the observed f900 flux is applied, the relative escape fraction decreases by a factor of ∼1.3 and 2, depending on the cases reported in literature. The spatial cross-correlation between the U-band ultradeep catalogue and a sample of galaxies at z≳ 3.4 in the GOODS-South field produces a number of U-band detected systems fully consistent with the expected superposition statistics. Indeed, each of them shows the presence of at least one offset contaminant in the Advanced Camera for Surveys images. An exemplary case of a foreground contamination in the Hubble Ultra Deep Field at redshift 3.797 by a foreground blue compact source (U= 28.63 ± 0.2) is reported; if observed with a low-resolution image (seeing larger than 0.5 arcsec) the polluting source would mimic an observed (f1500/f900)OBS∼ 38, erroneously ascribed to the source at higher redshift.
The first statistically significant detection of the cosmic gamma -ray horizon (CGRH) that is independent of any extragalactic background light (EBL) model is presented. The CGRH is a fundamental ...quantity in cosmology. It gives an estimate of the opacity of the universe to very high energy (VHE) gamma -ray photons due to photon-photon pair production with the EBL. The only estimations of the CGRH to date are predictions from EBL models and lower limits from gamma -ray observations of cosmological blazars and gamma -ray bursts. Here, we present homogeneous synchrotron/synchrotron self-Compton (SSC) models of the spectral energy distributions of 15 blazars based on (almost) simultaneous observations from radio up to the highest energy gamma -rays taken with the Fermi satellite. These synchrotron/SSC models predict the unattenuated VHE fluxes, which are compared with the observations by imaging atmospheric Cherenkov telescopes. This comparison provides an estimate of the optical depth of the EBL, which allows us a derivation of the CGRH through a maximum likelihood analysis that is EBL-model independent. We find that the observed CGRH is compatible with the current knowledge of the EBL.
We present the WFC3 Infrared Spectroscopic Parallel (WISP) Survey. WISP is obtaining slitless, near-infrared grism spectroscopy of ~90 independent, high-latitude fields by observing in the ...pure-parallel mode with the Wide Field Camera Three on the Hubble Space Telescope for a total of ~250 orbits. Spectra are obtained with the G 102 ( Delta *l = 0.8-1.17 Delta *mm, R ~210) and G 141 grisms ( Delta *l = 1.11-1.67 Delta *mm, R ~130), together with direct imaging in the J and H bands (F110W and F140W, respectively). In the present paper, we present the first results from 19 WISP fields, covering approximately 63 arcmin2. For typical exposure times (~6400 s in G 102 and ~2700 s in G 141), we reach 5 Delta *s detection limits for emission lines of f ~ 5 X 10--17 erg s--1 cm--2 for compact objects. Typical direct imaging 5 Delta *s limits are 26.3 and 26.1 mag. (AB) in F110W and F140W, respectively. Restricting ourselves to the lines measured with the highest confidence, we present a list of 328 emission lines, in 229 objects, in a redshift range 0.3 < z < 3. The single-line emitters are likely to be a mix of H Delta *a and O III5007,4959 A, with H Delta *a predominating. The overall surface density of high-confidence emission-line objects in our sample is approximately 4 per arcmin2. These first fields show high equivalent width sources, active galactic nucleus, and post-starburst galaxies. The median observed star formation rate (SFR) of our H Delta *a-selected sample is 4 M yr--1. At intermediate redshifts, we detect emission lines in galaxies as faint as H 140 ~ 25, or MR < --19, and are sensitive to SFRs down to less than 1 M yr--1. The slitless grisms on WFC3 provide a unique opportunity to study the spectral properties of galaxies much fainter than L* at the peak of the galaxy assembly epoch.
Aims. We aim to investigate the effect of the escaping ionizing radiation on the color selection of high-redshift galaxies and identify candidate Lyman-continuum (LyC) emitters. Methods. We used the ...intergalactic medium (IGM) prescription of Inoue et al. (2014, MNRAS, 442, 1805) and galaxy synthesis models of Bruzual & Charlot (2003, MNRAS, 344, 1000) to properly treat the ultraviolet stellar emission and the stochasticity of the intergalactic transmission and mean free path in the ionizing regime. Color tracks were computed by turning the escape fraction fesc of ionizing radiation on or off. Results. At variance with recent studies, a careful treatment of IGM transmission leads to no significant effects on the high-redshift broad-band color selection, even adopting the most extreme ionizing emission model (with an age of 1 Myr, zero dust, and metallicity Z/Z⊙ = 0.02). The decreasing mean free path of ionizing photons with increasing redshift further diminishes the contribution of the LyC to broad-band colors. We demonstrate that prominent LyC sources can be selected under suitable conditions by calculating the probability of a null escaping ionizing radiation. This was performed by running ad hoc Monte Carlo simulations anchored to the observed photometry, exploring the stochasticity of the IGM, and comparing the simulated and observed colors that encompass the Lyman edge. The method was applied to a sample of galaxies extracted from the GOODS-S field. A known LyC source at z = 3.795 was successfully recovered as a LyC-emitter candidate, and another convincing candidate at z = 3.212 is reported. A detailed analysis of the two sources (including their variability and morphology) suggests a possible mixture of stellar and non-stellar (AGN) contribution in the ultraviolet. Conclusions. The classical broad-band color selection of 2.5 < z < 4.5 galaxies does not prevent the inclusion of LyC emitters in the selected samples. High fesc in relatively bright galaxies (L > 0.1L⋆) could be favored by the presence of a faint active galactic nucleus (AGN) that is not easily detected at any wavelength. A hybrid stellar and non-stellar (AGN) ionizing emission could coexist in these systems and explain the tensions found among the UV excess and the stellar population synthesis models reported in literature.
We present a strong-lensing analysis of MACSJ0717.5+3745 (hereafter MACS?J0717), based on the full depth of the Hubble Frontier Field (HFF) observations, which brings the number of multiply imaged ...systems to 61, ten of which have been spectroscopically confirmed. The total number of images comprised in these systems rises to 165, compared to 48 images in 16 systems before the HFF observations. Our analysis uses a parametric mass reconstruction technique, as implemented in the Lenstool software, and the subset of the 132 most secure multiple images to constrain a mass distribution composed of four large-scale mass components (spatially aligned with the four main light concentrations) and a multitude of galaxy-scale perturbers. We find a superposition of cored isothermal mass components to provide a good fit to the observational constraints, resulting in a very shallow mass distribution for the smooth (large-scale) component. This uncertainty decreases the area of the image plane where we can reliably study the high-redshift Universe by 50 to 70%.
We investigate the high-redshift quasar luminosity function (QLF) down to an apparent magnitude of I sub(AB) = 25 in the Cosmic Evolution Survey (COSMOS). Careful analysis of the extensive COSMOS ...photometry and imaging data allows us to identify and remove stellar and low-redshift contaminants, enabling a selection that is nearly complete for type-1 quasars at the redshifts of interest. We find 155 likely quasars at z > 3.1, 39 of which have prior spectroscopic confirmation. We present our sample in detail and use these confirmed and likely quasars to compute the rest-frame UV QLF in the redshift bins 3.1 < z < 3.5 and 3.5 < z < 5. The space density of faint quasars decreases by roughly a factor of four from z ~ 3.2 to z ~ 4, with faint-end slopes of beta ~ -1.7 at both redshifts. The decline in space density of faint optical quasars at z > 3 is similar to what has been found for more luminous optical and X-ray quasars. We compare the rest-frame UV luminosity functions found here with the X-ray luminosity function at z > 3, and find that they evolve similarly between z ~ 3.2 and z ~ 4; however, the different normalizations imply that roughly 75% of X-ray bright active galactic nuclei (AGNs) at z ~ 3-4 are optically obscured. This fraction is higher than found at lower redshift and may imply that the obscured, type-2 fraction continues to increase with redshift at least to z ~ 4. Finally, the implications of the results derived here for the contribution of quasars to cosmic reionization are discussed.
![An extreme [O III] emitter ...](http://api.summon.serialssolutions.com/2.0.0/image/issn/XR4PS5YY4K/0004-6361/small "An extreme [O III] emitter at z = 3.2: a low metallicity Lyman continuum source")
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