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E. Vanzella

Publications and source records attributed to E. Vanzella.

At least 109 records · Page 6Linked to original sources
arXiv2018

Direct Lyman continuum and Lyman-alpha escape observed at redshift 4

We report on the serendipitous discovery of a z=4.0, M1500=-22.20 star-forming galaxy (Ion3) showing copious Lyman continuum (LyC) leakage (~60% escaping), a remarkable multiple peaked Lya emission, and significant Lya radiation directly emerging at the resonance frequency. This is the highest redshift confirmed LyC emitter in which the ionising and Lya radiation possibly share a common ionised cavity (with N_HI<10^17.2 cm^-2). Ion3 is spatially resolved, it shows clear stellar winds signatures like the P-Cygni NV1240 profile, and has blue ultraviolet continuum (β= -2.5 +/- 0.25, F_λ~ λ^β) with weak low-ionisation interstellar metal lines. Deep VLT/HAWKI Ks and Spitzer/IRAC 3.6um and 4.5um imaging show a clear photometric signature of the Halpha line with equivalent width of 1000A rest-frame emerging over a flat continuum (Ks-4.5um ~ 0). From the SED fitting we derive a stellar mass of 1.5x10^9 Msun, SFR of 140 Msun/yr 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 reionising sources at z>6.5 with JWST.

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arXiv2018

The VANDELS ESO public spectroscopic survey: observations and first data release

This paper describes the observations and the first data release (DR1) of the ESO public spectroscopic survey "VANDELS, a deep VIMOS survey of the CANDELS CDFS and UDS fields". VANDELS' main targets are star-forming galaxies at 2.4<z<5.5 and massive passive galaxies at 1<z<2.5. By adopting a strategy of ultra-long exposure times, from 20 to 80 hours per source, VANDELS is designed to be the deepest ever spectroscopic survey of the high-redshift Universe. Exploiting the red sensitivity of the VIMOS spectrograph, the survey has obtained ultra-deep spectra covering the wavelength 4800-10000 A with sufficient signal-to-noise to investigate the astrophysics of high-redshift galaxy evolution via detailed absorption line studies. The VANDELS-DR1 is the release of all spectra obtained during the first season of observations and includes data for galaxies for which the total (or half of the total) scheduled integration time was completed. The release contains 879 individual objects with a measured redshift and includes fully wavelength and flux-calibrated 1D spectra, the associated error spectra, sky spectra and wavelength-calibrated 2D spectra. We also provide a catalog with the essential galaxy parameters, including spectroscopic redshifts and redshift quality flags. In this paper we present the survey layout and observations, the data reduction and redshift measurement procedure and the general properties of the VANDELS-DR1 sample. We also discuss the spectroscopic redshift distribution, the accuracy of the photometric redshifts and we provide some examples of data products. All VANDELS-DR1 data are publicly available and can be retrieved from the ESO archive. Two further data releases are foreseen in the next 2 years with a final release scheduled for June 2020 which will include improved re-reduction of the entire spectroscopic data set. (abridged)

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arXiv2018

The 500 ks Chandra observation of the z = 6.31 QSO SDSS J1030+0524

We present the results from a $\sim500$ ks Chandra observation of the $z=6.31$ QSO SDSS J1030+0524. This is the deepest X-ray observation to date of a $z\sim6$ QSO. The QSO is detected with a total of 125 net counts in the full ($0.5-7$ keV) band and its spectrum can be modeled by a single power-law model with photon index of $Γ= 1.81 \pm 0.18$ and full band flux of $f=3.95\times 10^{-15}$ erg s$^{-1}$ cm$^{-2}$. When compared with the data obtained by XMM-Newton in 2003, our Chandra observation in 2017 shows a harder ($ΔΓ\approx -0.6$) spectrum and a 2.5 times fainter flux. Such a variation, in a timespan of $\sim2$ yrs rest-frame, is unexpected for such a luminous QSO powered by a $> 10^9 \: M_{\odot}$ black hole. The observed source hardening and weakening could be related to an intrinsic variation in the accretion rate. However, the limited photon statistics does not allow us to discriminate between an intrinsic luminosity and spectral change, and an absorption event produced by an intervening gas cloud along the line of sight. We also report the discovery of diffuse X-ray emission that extends for 30"x20" southward the QSO with a signal-to-noise ratio of $\sim$6, hardness ratio of $HR=0.03_{-0.25}^{+0.20}$, and soft band flux of $f_{0.5-2 \: keV}= 1.1_{-0.3}^{+0.3} \times 10^{-15}$ erg s$^{-1}$ cm$^{-2}$, that is not associated to a group or cluster of galaxies. We discuss two possible explanations for the extended emission, which may be either associated with the radio lobe of a nearby, foreground radio galaxy (at $z \approx 1-2$), or ascribed to the feedback from the QSO itself acting on its surrounding environment, as proposed by simulations of early black hole formation.

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arXiv2018

The contribution of faint AGNs to the ionizing background at z~4

Finding the sources responsible for the hydrogen reionization is one of the most pressing issues in cosmology. Bright QSOs are known to ionize their surrounding neighborhood, but they are too few to ensure the required HI ionizing background. A significant contribution by faint AGNs, however, could solve the problem, as recently advocated on the basis of a relatively large space density of faint active nuclei at z>4. We have carried out an exploratory spectroscopic program to measure the HI ionizing emission of 16 faint AGNs spanning a broad U-I color interval, with I~21-23 and 3.6 ~4. The LyC escape fraction has been detected with S/N ratio of ~10-120 and is between 44 and 100% for all the observed faint AGNs, with a mean value of 74% at 3.6<z<4.2 and -25.1<M1450<-23.3, in agreement with the value found in the literature for much brighter QSOs (M1450<~-26) at the same redshifts. The LyC escape fraction of our faint AGNs does not show any dependence on the absolute luminosities or on the observed U-I colors. Assuming that the LyC escape fraction remains close to ~75% down to M1450~-18, we find that the AGN population can provide between 16 and 73% (depending on the adopted luminosity function) of the whole ionizing UV background at z~4, measured through the Lyman forest. This contribution increases to 25-100% if other determinations of the ionizing UV background are adopted. Extrapolating these results to z~5-7, there are possible indications that bright QSOs and faint AGNs can provide a significant contribution to the reionization of the Universe, if their space density is high at M1450~-23.

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arXiv2018

Kinematics of the SN Refsdal host revealed by MUSE: a regularly rotating spiral galaxy at z~1.5

We use Multi Unit Spectroscopic Explorer (MUSE) observations of the galaxy cluster MACS J1149.5+2223 to explore the kinematics of the grand-design spiral galaxy Sp1149 hosting the SN Refsdal. Sp1149 lies at $z\simeq1.49$, has a stellar mass $M_*\simeq5\times10^9 \, \mathrm{M_\odot}$, a star-formation rate $\mathrm{SFR} \simeq1-6 \, \mathrm{M_\odot/yr}$ and represents a likely progenitor of a Milky-Way-like galaxy. All the four multiple images of Sp1149 in our data show strong OII-line emissions pointing to a clear rotation pattern. We take advantage of the gravitational lensing magnification effect ($\simeq 4 \times$) on the OII emission of the least distorted image to fit 3D kinematic models to the MUSE data-cube and derive the rotation curve and the velocity dispersion profile of Sp1149. We find that the rotation curve steeply rises, peaks at $R\simeq1$ kpc and then (initially) declines and flattens to an average $V_\mathrm{flat} = 128^{+29}_{-19}$ km/s. The shape of the rotation curve is well determined but the actual value of $V_\mathrm{flat}$ is quite uncertain because of the nearly face-on configuration of the galaxy. The intrinsic velocity dispersion due to gas turbulence is almost constant across the entire disc with an average of $27\pm5$ km/s. This value is consistent with $z=0$ measurements in the ionized gas component and a factor 2-4 lower than other estimates in different galaxies at similar redshifts. The average stellar-to-total mass fraction is of the order of one fifth. Our kinematic analysis returns the picture of a regular star-forming, mildly turbulent, rotation-dominated ($V / σ\simeq5$) spiral galaxy in a 4 Gyr old Universe.

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arXiv2018

Ly$α$-Lyman Continuum connection in 3.5 < z < 4.3 star-forming galaxies from the VUDS survey

To identify the galaxies responsible for the reionization of the Universe, we must rely on the investigation of the Lyman Continuum (LyC) properties of z<5 star-forming galaxies, where we can still directly observe their ionizing radiation. We selected a sample of 201 star-forming galaxies from the Vimos Ultra Deep Survey at 3.5 70Å, Lya(ext)<5.7kpc, rUV<0.3kpc and |Dv Lya|<200km/s, stacked all the galaxies in each subsample and measured the flux density ratio fnu(895)/fnu(1470), that we consider to be a proxy for LyC emission. We compared these ratios to those obtained for the complementary samples. We find that the stacks of galaxies which are UV compact (rUV<0.3kpc) and have bright Lya emission (EWLya>70Å), have much higher LyC fluxes compared to the rest of the galaxy population in agreement with theoretical studies and previous observational works. We also find that galaxies with low Lya(ext) have the highest LyC flux: this new correlation seems even stronger than the correlations with high EWLya and small rUV. These results assume that the stacks from all the subsamples present the same statistical contamination from lower redshift interlopers. If we subtract a statistical contamination from low redshift interlopers obtained with dedicated Monte Carlo simulations, from the flux density ratios (fnu(895)/fnu(1470)) of the significant subsamples we find that these samples contain real LyC leaking flux with a very high probability, but the true average escape fractions remain uncertain. Our work indicates that galaxies with very high EWLya, small Lya(ext) and small rUV are very likely the best candidates to show LyC radiation at z=4 and could therefore be the galaxies that contributed more to reionization.

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arXiv2017

The VIMOS Ultra Deep Survey: Nature, ISM properties, and ionizing spectra of CIII]1909 emitters at z=2-4

Context: Ultraviolet (UV) emission-line spectra are used to spectroscopically confirm high-z galaxies and increasingly also to determine their physical properties. Aims: To interpret the observed UV spectra of distant galaxies in terms of the dominant radiation field and the physical condition of the interstellar medium. Methods: We construct a large grid of photoionization models and derive new spectral UV line diagnostics using equivalent widths (EWs) of CIII]1909, CIV1549 and the line ratios of CIII], CIV, and HeII1640 recombination lines. We apply these diagnostics to a sample of 450 CIII]-emitting galaxies at z=2-4 previously identified in the VIMOS Ultra Deep Survey. Results: We show that the average star-forming galaxy (EW(CIII])~2A) is well described by stellar photoionization from single and binary stars. The inferred metallicity and ionization parameter is typically Z=0.3-0.5Zsun and logU=-2.7 to -3, in agreement with earlier works at similar redshifts. The models also indicate an average age of 50-200Myr since the beginning of the current star-formation, and an ionizing photon production rate, xi_ion, of log(xi_ion/[Hz/erg])~25.3-25.4. Among the sources with EW(CIII])=10-20A, ~30% are likely dominated by AGNs. Their derived metallicity is low, Z=0.02-0.2Zsun, and the ionization parameter higher (logU=-1.7). To explain the average UV observations of the strongest but rarest CIII] emitters (EW(CIII])>20A), we find that stellar photoionization is clearly insufficient. A radiation field consisting of a mix of a young stellar population (log(xi_ion/[Hz/erg])~25.7) plus an AGN component is required. Furthermore an enhanced C/O abundance ratio is needed for metallicities Z=0.1-0.2Zsun and logU=-1.7 to -1.5. Conclusions: The UV diagnostics we propose should serve as an important basis for the interpretation of observations of high-redshift galaxies. [abridged]

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arXiv2017

A refined mass distribution of the cluster MACS J0416.1$-$2403 from a new large set of spectroscopic multiply lensed sources

We report the spectroscopic confirmation of 22 new multiply lensed sources behind the Hubble Frontier Field (HFF) galaxy cluster MACS~J0416.1$-$2403 (MACS 0416), using archival data from the Multi Unit Spectroscopic Explorer (MUSE) on the VLT. Combining with previous spectroscopic measurements of 15 other multiply imaged sources, we obtain a sample of 102 secure multiple images with measured redshifts, the largest to date in a single strong lensing system. The newly confirmed sources are largely low-luminosity Lyman-$α$ emitters with redshift in the range [3.08-6.15]. With such a large number of secure constraints, and a significantly improved sample of galaxy members in the cluster core, we have improved our previous strong lensing model and obtained a robust determination of the projected total mass distribution of MACS 0416. We find evidence of three cored dark-matter halos, adding to the known complexity of this merging system. The total mass density profile, as well as the sub-halo population, are found in good agreement with previous works. We update and make public the redshift catalog of MACS 0416 from our previous spectroscopic campaign with the new MUSE redshifts. We also release lensing maps (convergence, shear, magnification) in the standard HFF format.

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arXiv2017

A VLT/FORS2 view at $z\sim6$: Lyman-$α$ emitter fraction and galaxy physical properties at the edge of the epoch of cosmic reionization

The fraction of Lyman-$α$ emitters among the galaxy population has been found to increase from $z\sim0$ to $z\sim6$ and drop dramatically at $z>6$. This drop has been interpreted as an effect of an increasingly neutral intergalactic medium with increasing redshift, while a LyC escape fraction evolving with redshift. We report the result of a large VLT/FORS2 program aiming to confirm spectroscopically a large galaxy sample at $z\geq6$ that has been selected in several independent fields through the Lyman Break technique. Combining those data with archival data, we create a large and homogeneous sample of $z\sim6$ galaxies ($N=127$), complete in terms of Ly$α$ detection at $>95\%$ for EW(Ly$α)\geq25Å$. We use this sample to derive a new measurement of the LAE fraction at $z\sim6$ and derive the physical properties of these galaxies through spectral energy distribution fitting. We find a median LAE fraction at $z\sim6$ lower than in previous studies. The main difference between LAEs and non-LAEs is that the latter are significantly dustier. Using predictions of our SED fitting code accounting for nebular emission, we find an effective Ly$α$ escape fraction $f^{eff}_{esc}(Lyα)=0.23^{+0.36}_{-0.17}$ remarkably consistent with the value derived by comparing UV luminosity function with Ly$α$ luminosity function. We conclude that the drop in the LAE fraction from $z\sim6$ to $z>6$ is less dramatic than previously found and the effect of an increasing IGM neutral fraction is possibly observed at $5<z<6$. Based on our derived $f^{eff}_{esc}(Lyα)$, we find that the IGM has a relatively small impact on Ly$α$ photon visibility at $z\sim6$, with a lower limit for the IGM transmission to \lya\ photons, $T_{IGM}\gtrsim0.20$, likely due to the presence of outflows. [abdridged]

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arXiv2017

Mass distribution in the core of MACS J1206: robust modeling from an exceptionally large sample of central multiple images

We present a new strong lensing analysis of the galaxy cluster MACS J1206.2-0847 (MACS 1206), at z=0.44, using deep spectroscopy from CLASH-VLT and VLT/MUSE archival data in combination with imaging from the Cluster Lensing and Supernova survey with Hubble. MUSE observations enable the spectroscopic identification of 23 new multiply imaged sources, extending the previous compilations by a factor of approximately five. In total, we use the positional measurements of 82 spectroscopic multiple images belonging to 27 families at z=1.0-6.1 to reconstruct the projected total mass distribution of MACS 1206. Remarkably, 11 multiple images are found within 50 kpc of the brightest cluster galaxy, making this an unprecedented set of constraints for the innermost projected mass distribution of a galaxy cluster. We thus find that, although dynamically relaxed, the smooth matter component (dark matter plus hot gas) of MACS 1206 shows a significant asymmetry, which closely follows the asymmetric distribution of the stellar component (galaxy members and intracluster light). We determine the value of the innermost logarithmic slope of the projected total mass density profile and find it to be close to the canonical Navarro-Frenk-White value. We demonstrate that this quantity is very robust against different parametrizations of the diffuse mass component; however, this is not the case when only one central image is used in the mass reconstruction. We also show that the mass density profile from our new strong lensing model is in very good agreement with dynamical and X-ray measurements at larger radii, where they overlap.

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arXiv2017

AGN-enhanced outflows of low-ionization gas in star-forming galaxies at 1.7<z<4.6

Fast and energetic winds are invoked by galaxy formation models as essential processes in the evolution of galaxies. These outflows can be powered either by star-formation and/or AGN activity, but the relative dominance of the two mechanisms is still under debate. We use spectroscopic stacking analysis to study the properties of the low-ionization phase of the outflow in a sample of 1330 star-forming galaxies (SFGs) and 79 X-ray detected (42<log(L_X)<45 erg/s) Type 2 AGN at 1.7<z<4.6 selected from a compilation of deep optical spectroscopic surveys, mostly zCOSMOS-Deep and VUDS. We measure mean velocity offsets of -150 km/s in the SFGs while in the AGN sample the velocity is much higher (-950 km/s), suggesting that the AGN is boosting the outflow up to velocities that could not be reached only with the star- formation contribution. The sample of X-ray AGN has on average a lower SFR than non-AGN SFGs of similar mass: this, combined with the enhanced outflow velocity in AGN hosts, is consistent with AGN feedback in action. We further divide our sample of AGN into two X-ray luminosity bins: we measure the same velocity offsets in both stacked spectra, at odds with results reported for the highly ionized phase in local AGN, suggesting that the two phases of the outflow may be mixed only up to relatively low velocities, while the highest velocities can be reached only by the highly ionized phase.

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arXiv2017

The VIMOS Ultra Deep Survey: The role of HI kinematics and HI column density on the escape of Lyalpha photons in star-forming galaxies at 2<z<4

We selected a sample of 76 Lya emitting galaxies from the VIMOS Ultra Deep Survey (VUDS) at 2 300km/sec. An ISM with this value of NHI would favour a large amount of scattering events, especially when the medium is static, so it can explain large values of Ext(Lya-C) and small EW(Lya). On the contrary, an ISM with a lower NHI, but large velocity outflows would lead to a Lya spatial profile peaked at the galaxy center (i.e. low values of Ext(Lya-C)) and to a large EW(Lya), as we see in our data. Our results and their interpretation via radiative-transfer models tell us that it is possible to use Lya to study the properties of the HI gas.

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arXiv2017

The primordial environment of supermassive black holes (II): deep Y and J band images around the z=6.3 quasar SDSS J1030+0524

Many cosmological studies predict that early supermassive black holes (SMBHs) can only form in the most massive dark matter halos embedded within large scale structures marked by galaxy over-densities that may extend up to 10 physical Mpc. This scenario, however, has not been confirmed observationally, as the search for galaxy over-densities around high-z quasars has returned conflicting results. The field around the z=6.28 quasar SDSSJ1030+0524 (J1030) is unique for multi-band coverage and represents an excellent data legacy for studying the environment around a primordial SMBH. In this paper we present wide-area (25x25 arcmin), Y- and J-band imaging of the J1030 field obtained with the near infrared camera WIRCam at the Canada-France-Hawaii Telescope (CFHT). We built source catalogues in the Y- and J-band, and matched those with our photometric catalogue in the r, z, i bands presented in Morselli et al. (2014). We used these new infrared data together with H and K and Spitzer/IRAC data to refine our selection of Lyman Break Galaxies (LBGs), extending our selection criteria to galaxies in the range 25.2 =1.3. We found a significant asymmetry in the distribution of the high-z galaxies in J1030, supporting the existence of a coherent large-scale structure around the quasar. We compared our results with those of Bowler et al. (2015), who adopted similar LBGs selection criteria, and estimated an over-density of galaxies in the field of delta = 2.4, which is significant at >4 sigma. The over-density value and its significance are higher than those found in Morselli et al. (2014), and we interpret this as evidence of an improved LBG selection.

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arXiv2017

Magnifying the early episodes of star formation: super star clusters at cosmological distances

We study the spectrophotometric properties of a highly magnified (μ~40-70) pair of stellar systems identified at z=3.2222 behind the Hubble Frontier Field galaxy cluster MACS~J0416. Five multiple images (out of six) have been spectroscopically confirmed by means of VLT/MUSE and VLT/X-Shooter observations. Each image includes two faint (m_uv~30.6), young (<100 Myr), low-mass (<10^7 Msun), low-metallicity (12+Log(O/H)~7.7, or 1/10 solar) and compact (30 pc effective radius) stellar systems separated by ~300pc, after correcting for lensing amplification. We measured several rest-frame ultraviolet and optical narrow (σ_v <~ 25 km/s) high-ionization lines. These features may be the signature of very hot (T>50000 K) stars within dense stellar clusters, whose dynamical mass is likely dominated by the stellar component. Remarkably, the ultraviolet metal lines are not accompanied by Lya emission (e.g., CIV / Lya > 15), despite the fact that the Lya line flux is expected to be 150 times brighter (inferred from the Hbeta flux). A spatially-offset, strongly-magnified (μ>50) Lya emission with a spatial extent <~7.6 kpc^2 is instead identified 2 kpc away from the system. The origin of such a faint emission can be the result of fluorescent Lya induced by a transverse leakage of ionizing radiation emerging from the stellar systems and/or can be associated to an underlying and barely detected object (with m_uv > 34 de-lensed). This is the first confirmed metal-line emitter at such low-luminosity and redshift without Lya emission, suggesting that, at least in some cases, a non-uniform covering factor of the neutral gas might hamper the Lya detection.

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arXiv2017

Extended ionised and clumpy gas in a normal galaxy at z=7.1 revealed by ALMA

We present new ALMA observations of the [OIII]88$μ$m line and high angular resolution observations of the [CII]158$μ$m line in a normal star forming galaxy at z$=$7.1. Previous [CII] observations of this galaxy had detected [CII] emission consistent with the Ly$α$ redshift but spatially slightly offset relative to the optical (UV-rest frame) emission. The new [CII] observations reveal that the [CII] emission is partly clumpy and partly diffuse on scales larger than about 1kpc. [OIII] emission is also detected at high significance, offset relative to the optical counterpart in the same direction as the [CII] clumps, but mostly not overlapping with the bulk of the [CII] emission. The offset between different emission components (optical/UV and different far-IR tracers) is similar to what observed in much more powerful starbursts at high redshift. We show that the [OIII] emitting clump cannot be explained in terms of diffuse gas excited by the UV radiation emitted by the optical galaxy, but it requires excitation by in-situ (slightly dust obscured) star formation, at a rate of about 7 M$_{\odot}$/yr. Within 20 kpc from the optical galaxy the ALMA data reveal two additional [OIII] emitting systems, which must be star forming companions. We discuss that the complex properties revealed by ALMA in the z$\sim$7.1 galaxy are consistent with expectations by recent models and cosmological simulations, in which differential dust extinction, differential excitation and different metal enrichment levels, associated with different subsystems assembling a galaxy, are responsible for the different appearance of the system when observed with different tracers.

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arXiv2017

Optical Line Emission from z$\sim$6.8 Sources with Deep Constraints on Ly$α$ Visibility

We analyze a sample of $z$-dropout galaxies in the CANDELS GOODS South and UDS fields that have been targeted by a dedicated spectroscopic campaign aimed at detecting their Ly$α$ line. Deep IRAC observations at 3.6 and 4.5 $μ$m are used to determine the strength of optical emission lines affecting these bands at z$\sim$6.5-6.9 in order to i) investigate possible physical differences between Ly$α$ emitting and non-emitting sources; ii) constrain the escape fraction of ionizing photons; iii) provide an estimate of the specific star-formation rate at high redshifts. We find evidence of strong [OIII]+H$β$ emission in the average (stacked) SEDs of galaxies both with and without Ly$α$ emission. The blue IRAC [3.6]-[4.5] color of the stack with detected Ly$α$ line can be converted into a rest-frame equivalent width EW([OIII]+H$β$)=1500$^{+530}_{-440}Å$ assuming a flat intrinsic stellar continuum. This strong optical line emission enables a first estimate of f$_{esc}\lesssim$20% on the escape fraction of ionizing photons from Ly$α$ detected objects. The objects with no Ly$α$ line show less extreme EW([OIII]+H$β$)=520$^{+170}_{-150}Å$ suggesting different physical conditions of the HII regions with respect to Ly$α$-emitting ones, or a larger f$_{esc}$. The latter case is consistent with a combined evolution of f$_{esc}$ and the neutral hydrogen fraction as an explanation of the lack of bright Ly$α$ emission at z$>$6. A lower limit on the specific star formation rate, SSFR$>$9.1$Gyr^{-1}$ for $M_{star}=2 \times 10^9 M_{\odot}$ galaxies at these redshifts can be derived from the spectroscopically confirmed sample.

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arXiv2017

The Lyman Continuum escape fraction of faint galaxies at z~3.3 in the CANDELS/GOODS-North, EGS, and COSMOS fields with LBC

The reionization of the Universe is one of the most important topics of present day astrophysical research. The most plausible candidates for the reionization process are star-forming galaxies, which according to the predictions of the majority of the theoretical and semi-analytical models should dominate the HI ionizing background at z~3. We aim at measuring the Lyman continuum escape fraction, which is one of the key parameters to compute the contribution of star-forming galaxies to the UV background. We have used ultra-deep U-band imaging (U=30.2mag at 1sigma) by LBC/LBT in the CANDELS/GOODS-North field, as well as deep imaging in COSMOS and EGS fields, in order to estimate the Lyman continuum escape fraction of 69 star-forming galaxies with secure spectroscopic redshifts at 3.27 L*), while for the faint population (L=0.2L*) the limit to the escape fraction is ~10%. We have computed the contribution of star-forming galaxies to the observed UV background at z~3 and we have found that it is not enough to keep the Universe ionized at these redshifts, unless their escape fraction increases significantly (>10%) at low luminosities (M1500>-19). We compare our results on the Lyman continuum escape fraction of high-z galaxies with recent estimates in the literature and discuss future prospects to shed light on the end of the Dark Ages. In the future, strong gravitational lensing will be fundamental to measure the Lyman continuum escape fraction down to faint magnitudes (M1500~-16) which are inaccessible with the present instrumentation on blank fields.

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