ALBERT

Description: The large cross-section of the Lyα line makes it a sensitive probe of the ionization state of the intergalactic medium (IGM). Here, we present the most complete study to date of the IGM Lyα opacity, and its application to the redshift evolution of the ‘Lyα fraction’, i.e. the fraction of colour-selected galaxies with a detectable Lyα emission line. We use a tiered approach, which combines large-scale seminumeric simulations of reionization with moderate-scale hydrodynamic simulations of the ionized IGM. This allows us to simultaneously account for evolution in both: (i) the opacity from an incomplete (patchy) reionization, parametrized by the filling factor of ionized regions, Q H  ii ; and (ii) the opacity from self-shielded systems in the ionized IGM, parametrized by the average photoionization rate inside H  ii regions, 〈 12 〉 H  ii . In contrast to recent empirical models, attenuation from patchy reionization has a unimodal distribution along different sightlines, while attenuation from self-shielded systems is more bimodal. We quantify the average IGM transmission in our ( Q H  ii , 〈 12 〉 H  ii ) parameter space, which can easily be used to interpret new data sets. Our new, improved models highly disfavour an evolution in 〈 12 〉 H  ii as the sole driver of a large change in IGM opacity. Using current observations, we predict that the Lyα fraction cannot drop by more than a factor of 2 with IGM attenuation alone, even for H  ii filling factors as low as Q H  ii   0.1. Larger changes in the Lyα fraction could result from a co-evolution with galaxy properties. Marginalizing over 〈 12 〉 H  ii , we find that current observations constrain Q H  ii ( z   7) ≤ 0.6, at a 68 per cent confidence level (CL). However, all of our parameter space is consistent with observations at 95 per cent CL, highlighting the need for larger observational samples at z  ≥ 6.

Description: We use a sample of 1669 quasars ( r 〈 20.15, 3.6 〈 z 〈 4.0) from the Baryon Oscillation Spectroscopic Survey to study the intrinsic shape of their continuum and the Lyman continuum photon escape fraction ( f esc,q ), estimated as the ratio between the observed flux and the expected intrinsic flux (corrected for the intergalactic medium absorption) in the wavelength range 865–885 Å rest frame. Modelling the intrinsic quasar (QSO) continuum shape with a power law, F – , we find a median = 1.30 (with a dispersion of 0.38, no dependence on the redshift and a mild intrinsic luminosity dependence) and a mean f esc,q = 0.75 (independent of the QSO luminosity and/or redshift). The f esc,q distribution shows a peak around zero and a long tail of higher values, with a resulting dispersion of 0.7. If we assume for the QSO continuum a double power-law shape (also compatible with the data) with a break located at br = 1000 Å and a softening = 0.72 at wavelengths shorter than br , the mean f esc,q rises to 0.82. Combining our and f esc,q estimates with the observed evolution of the active galactic nucleus (AGN) luminosity function (LF), we compute the AGN contribution to the UV ionizing background (UVB) as a function of redshift. AGN brighter than one-tenth of the characteristic luminosity of the LF are able to produce most of it up to z ~ 3, if the present sample is representative of their properties. At higher redshifts, a contribution of the galaxy population is required. Assuming an escape fraction of Lyman continuum photons from galaxies between 5.5 and 7.6 per cent, independent of the galaxy luminosity and/or redshift, a remarkably good fit to the observational UVB data up to z ~ 6 is obtained. At lower redshift, the extrapolation of our empirical estimate agrees well with recent UVB observations, dispelling the so-called Photon Underproduction Crisis.

Description: We report the identification of extended Lyα nebulae at z ~= 3.3 in the Hubble Ultra Deep Field ( HUDF , ~=40 kpc x 80 kpc) and behind the Hubble Frontier Field galaxy cluster MACSJ0416 (~=40 kpc), spatially associated with groups of star-forming galaxies. VLT/MUSE integral field spectroscopy reveals a complex structure with a spatially varying double-peaked Lyα emission. Overall, the spectral profiles of the two Lyα nebulae are remarkably similar, both showing a prominent blue emission, more intense and slightly broader than the red peak. From the first nebula, located in the HUDF , no X-ray emission has been detected, disfavouring the possible presence of active galactic nuclei. Spectroscopic redshifts have been derived for 11 galaxies within 2 arcsec from the nebula and spanning the redshift range 1.037 〈 z 〈 5.97. The second nebula, behind MACSJ0416, shows three aligned star-forming galaxies plausibly associated with the emitting gas. In both systems, the associated galaxies reveal possible intense rest-frame-optical nebular emissions lines [O iii ] 4959, 5007+Hβ with equivalent widths as high as 1500 Å rest frame and star formation rates ranging from a few to tens of solar masses per year. A possible scenario is that of a group of young, star-forming galaxies emitting ionizing radiation that induces Lyα fluorescence, therefore revealing the kinematics of the surrounding gas. Also Lyα powered by star formation and/or cooling radiation may resemble the double-peaked spectral properties and the morphology observed here. If the intense blue emission is associated with inflowing gas, then we may be witnessing an early phase of galaxy or a proto-cluster (or group) formation.

Description: By combining cosmological simulations with Frontier Field (FF) project lens models, we find that, in the most optimistic case, galaxies as faint as m 33–34 (AB magnitude at 1.6 μm) can be detected in the Frontier Fields. Such faint galaxies are hosted by dark matter haloes of mass ~10 9 M and dominate the ionizing photon budget over currently observed bright galaxies, thus allowing for the first time the investigation of the dominant reionization sources. In addition, the observed number of these galaxies can be used to constrain the role of feedback in suppressing star formation in small haloes: for example, if galaxy formation is suppressed in haloes with circular velocity v c 〈 50 km s –1 , galaxies fainter than m  = 31 should not be detected in the FFs.

Description: We study the observed cosmic ionizing background as a constraint on the nature of the sources responsible for the reionization of the Universe. In earlier work, we showed that extrapolations of the ultraviolet (UV) luminosity function (LF) of Lyman-break galaxies (LBGs) at fixed Lyman continuum photon escape fraction ( f esc ) are not able to reproduce the redshift evolution of this background. Here, we employ extrapolations of the high- z LFs to describe the contribution of LBGs to the ionizing photon rate, taking into account the smoothing of the baryonic perturbations, due to the background itself (i.e. the filtering mass), as well as a possible sharp increase of f esc in dwarf galaxies. Under the hypothesis of a dominant contribution of LBGs to cosmic reionization, our results suggest that sources fainter than the current observational limits should be characterized by f esc values of the order of ~0.1–0.3 (larger than the current estimates for bright galaxies) to account for a z 6 reionization and the measured evolution of cosmic ionizing background, at the same time . The contribution to the background from quasars turns out to be relevant at z 3. Overall, our results support the case for dedicated observations of faint galaxies in the rest-frame UV, in order to better determine their physical properties. Observed f esc values outside our proposed range bear relevant consequences on the nature of the astrophysical sources responsible for cosmic reionization and/or its build-up process.

Description: We report new deep observations obtained with the Atacama Large Millimetre Array (ALMA) aimed at investigating the [C ii ]158 μm line and continuum emission in three spectroscopically confirmed Lyman break galaxies at 6.8 〈  z  ≤ 7.1, i.e. well within the re-ionization epoch. With star formation rates of SFR ~ 5–15M yr – 1 these systems are much more representative of the high- z galaxy population than other systems targeted in the past by millimetre observations. For the galaxy with the deepest observation we detect [C ii ] emission at redshift z  = 7.107, fully consistent with the Ly α redshift, but spatially offset by 0.7 arcsec (4 kpc) from the optical emission. At the location of the optical emission, tracing both the Ly α line and the far-UV continuum, no [C ii ] emission is detected in any of the three galaxies, with 3 upper limits significantly lower than the [C ii ] emission observed in lower redshift galaxies. These results suggest that molecular clouds in the central parts of primordial galaxies are rapidly disrupted by stellar feedback. As a result, [C ii ] emission mostly arises from more external accreting/satellite clumps of neutral gas. These findings are in agreement with recent models of galaxy formation. Thermal far-infrared continuum is not detected in any of the three galaxies. However, the upper limits on the infrared-to-UV emission ratio do not exceed those derived in metal- and dust-poor galaxies.

Description: We report on the serendipitous discovery of a z  = 4.0, M 1500  = −22.20 star-forming galaxy ( Ion3 ) showing copious Lyman continuum (LyC) leakage (∼60 per cent escaping), a remarkable multiple peaked Ly α emission, and significant Ly α radiation directly emerging at the resonance frequency. This is the highest redshift confirmed LyC emitter in which the ionizing and Ly α radiation possibly share a common ionized channel (with N H I 〈 10 17.2  cm −2 ). Ion3 is spatially resolved, it shows clear stellar winds signatures like the P-Cygni N  v λ1240 profile, and has blue ultraviolet continuum (β = −2.5 ± 0.25, F λ  ∼ λ β ) with weak low-ionization interstellar metal lines. Deep VLT/HAWKI Ks and Spitzer/IRAC 3.6 and 4.5μm imaging show a clear photometric signature of the H α line with equivalent width of 1000 Å rest-frame emerging over a flat continuum (Ks − 4.5μm ≃ 0). From the SED fitting, we derive a stellar mass of 1.5 × 10 9  M ⊙ , SFR of 140 M ⊙  yr −1 and age of ∼10 Myr, with a low dust extinction, E ( B  −  V ) ≲ 0.1, placing the source in the starburst region of the SFR−M* plane. Ion3 shows similar properties of another LyC emitter previously discovered ( z  = 3.21, Ion2 , Vanzella et al. 2016 ). Ion3 (and Ion2 ) represents ideal high-redshift reference cases to guide the search for reionizing sources at z  〉 6.5 with JWST.

Description: In this work, we investigate the abundance and distribution of metals in the intergalactic medium (IGM) at 〈 z 〉 ~= 2.8 through the analysis of an ultra-high signal-to-noise ratio UVES spectrum of the quasar HE0940-1050. In the C iv forest, our deep spectrum is sensitive at 3 to lines with column density down to log N CIV ~= 11.4 and in 60 per cent of the considered redshift range down to ~=11.1. In our sample, all H i lines with log N HI ≥ 14.8 show an associated C iv absorption. In the range 14.0 ≤ log N HI 〈 14.8, 43 per cent of H i lines has an associated C iv absorption. At log N HI 〈 14.0, the detection rates drop to 〈10 per cent, possibly due to our sensitivity limits and not to an actual variation of the gas abundance properties. In the range log N HI ≥ 14, we observe a fraction of H i lines with detected C iv a factor of 2 larger than the fraction of H i lines lying in the circumgalactic medium (CGM) of relatively bright Lyman-break galaxies hosted by dark matter haloes with 〈 M 〉 ~ 10 12 M . The comparison of our results with the output of a grid of photoionization models and of two cosmological simulations implies that the volume filling factor of the IGM gas enriched to a metallicity $\log Z/Z_{{\odot }} {\gtrsim }-3$ should be of the order of ~10–13 per cent. In conclusion, our results favour a scenario in which metals are found also outside the CGM of bright star-forming galaxies, possibly due to pollution by lower mass objects and/or to an early enrichment by the first sources.

Description: VANDELS is a uniquely deep spectroscopic survey of high-redshift galaxies with the VIMOS spectrograph on ESO’s Very Large Telescope (VLT). The survey has obtained ultradeep optical (0.48 〈 λ 〈 1.0 |$mu $|m) spectroscopy of ≃2100 galaxies within the redshift interval 1.0 ≤ z ≤ 7.0, over a total area of ≃0.2 deg2 centred on the CANDELS Ultra Deep Survey and Chandra Deep Field South fields. Based on accurate photometric redshift pre-selection, 85 per cent of the galaxies targeted by VANDELS were selected to be at z ≥ 3. Exploiting the red sensitivity of the refurbished VIMOS spectrograph, the fundamental aim of the survey is to provide the high-signal-to-noise ratio spectra necessary to measure key physical properties such as stellar population ages, masses, metallicities, and outflow velocities from detailed absorption-line studies. Using integration times calculated to produce an approximately constant signal-to-noise ratio (20 〈 tint〈 80 h), the VANDELS survey targeted: (a) bright star-forming galaxies at 2.4 ≤ z ≤ 5.5, (b) massive quiescent galaxies at 1.0 ≤ z ≤ 2.5, (c) fainter star-forming galaxies at 3.0 ≤ z ≤ 7.0, and (d) X-ray/Spitzer-selected active galactic nuclei and Herschel-detected galaxies. By targeting two extragalactic survey fields with superb multiwavelength imaging data, VANDELS will produce a unique legacy data set for exploring the physics underpinning high-redshift galaxy evolution. In this paper, we provide an overview of the VANDELS survey designed to support the science exploitation of the first ESO public data release, focusing on the scientific motivation, survey design, and target selection.