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. The intergalactic medium prescription of Inoue et al.(2014) and galaxy synthes
is models of Bruzual&Charlot (2003) have been used to properly treat the ultraviolet stellar emission, the stochasticity of the intergalactic transmission and mean free path in the ionizing regime. Color tracks are computed by turning on/off the escape fraction of ionizing radiation. At variance with recent studies, a careful treatment of IGM transmission leads to no significant effects on the high-redshift broad-band color selection. The decreasing mean free path of ionizing photons with increasing redshift further diminishes the contribution of the LyC to broad-band colors. We also demonstrate that prominent LyC sources can be selected under suitable conditions by calculating the probability of a null escaping ionizing radiation. The method is applied to a sample of galaxies extracted from the GOODS-S field. A known LyC source at z=3.795 is 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: Classical broad-band color selection of 2.5<z<4.5 galaxies does not prevent the inclusion of LyC emitters in the selected samples. Large fesc in relatively bright galaxies (L>0.1L*) could be favored by the presence of a faint AGN 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 new results from our search for z~7 galaxies from deep spectroscopic observations of candidate z-dropouts in the CANDELS fields. Despite the extremely low flux limits achieved by our sensitive observations, only 2 galaxies have robust reds
hift identifications, one from its Lyalpha emission line at z=6.65, the other from its Lyman-break, i.e. the continuum discontinuity at the Lyalpha wavelength consistent with a redshift 6.42, but with no emission line. In addition, for 23 galaxies we present deep limits in the Lyalpha EW derived from the non detections in ultra-deep observations. Using this new data as well as previous samples, we assemble a total of 68 candidate z~7 galaxies with deep spectroscopic observations, of which 12 have a line detection. With this much enlarged sample we can place solid constraints on the declining fraction of Ly$alpha$ emission in z~7 Lyman break galaxies compared to z~6, both for bright and faint galaxies. Applying a simple analytical model, we show that the present data favor a patchy reionization process rather than a smooth one.
We present VIsible Multi-Object Spectrograph (VIMOS) observations of a z 6 galaxy quintuply imaged by the Frontier Fields galaxy cluster RXC J2248.7-4431 (z=0.348). This sub-L^*, high-z galaxy has been recently discovered by Monna et al. (2013) using
dropout techniques with the 16-band HST photometry acquired as part of the Cluster Lensing And Supernova survey with Hubble (CLASH). Obtained as part of the CLASH-VLT survey, the VIMOS medium-resolution spectra of this source show a very faint continuum between ~8700A and ~9300A and a prominent emission line at 8643A, which can be readily identified with Lyman-alpha at z=6.110. The emission line exhibits an asymmetric profile, with a more pronounced red wing. The rest-frame equivalent width of the line is EW=79+-10A. After correcting for magnification, the star-formation rate (SFR) estimated from the Lya line is SFR(Lya)=11 M_{sol}/yr and that estimated from the UV data is SFR(UV)=3 M_{sol}/yr. We estimate that the effective radius of the source is R_e<~0.4 kpc, which implies a star formation surface mass density Sigma_{SFR}>6 M_{sol}/yr/kpc^2 and, using the Kennicutt-Schmidt relation, a gas surface mass density Sigma_{gas}>10^3 M_{sol}/pc^2. Our results support the idea that this magnified, distant galaxy is a young and compact object with 0.4 L^* at z=6, with comparable amount of mass in gas and stars. Future follow-up observations with ALMA will provide valuable insight into the SFR and molecular gas content of this source. In the spirit of the Frontier Fields initiative, we also publish the redshifts of several multiply imaged sources and other background objects which will help improving the strong lensing model of this galaxy cluster.
We present the morphological catalog of galaxies in nearby clusters of the WINGS survey (Fasano et al. 2006). The catalog contains a total number of 39923 galaxies, for which we provide the automatic estimates of the morphological type applying the p
urposely devised tool MORPHOT to the V-band WINGS imaging. For ~3000 galaxies we also provide visual estimates of the morphological types. A substantial part of the paper is devoted to the description of the MORPHOT tool, whose application is limited, at least for the moment, to the WINGS imaging only. The approach of the tool to the automation of morphological classification is a non parametric and fully empiri- cal one. In particular, MORPHOT exploits 21 morphological diagnostics, directly and easily computable from the galaxy image, to provide two independent classifications: one based on a Maximum Likelihood (ML), semi-analytical technique, the other one on a Neural Network (NN) machine. A suitably selected sample of ~1000 visually clas- sified WINGS galaxies is used to calibrate the diagnostics for the ML estimator and as a training set in the NN machine. The final morphological estimator combines the two techniques and proves to be effective both when applied to an additional test sample of ~1000 visually classified WINGS galaxies and when compared with small samples of SDSS galaxies visually classified by Fukugita et al. (2007) and Nair et al. (2010). Finally, besides the galaxy morphology distribution (corrected for field contamination) in the WINGS clusters, we present the ellipticity ({epsilon}), color (B-V) and Sersic index (n) distributions for different morphological types, as well as the morphological fractions as a function of the clustercentric distance (in units of R200).
We present the final results from our ultra-deep spectroscopic campaign with FORS2 at the ESO/VLT for the confirmation of z~7 z--band dropout candidates selected from our VLT/Hawk-I imaging survey over three independent fields. In particular we repor
t on two newly discovered galaxies at redshift ~6.7 in the NTT deep field: both galaxies show a Ly-alpha emission line with rest-frame EWs of the order 15-20 A and luminosities of 2-4 X 10^{42} erg/s. We also present the results of ultra-deep observations of a sample of i-dropout galaxies, from which we set a solid upper limit on the fraction of interlopers. Out of the 20 z-dropouts observed we confirm 5 galaxies at 6.7 < z < 7.1. This is systematically below the expectations drawn on the basis of lower redshift observations: in particular there is a significant lack of objects with intermediate Ly-alpha EWs (between 20 and 55 A). We conclude that the trend for the fraction of Ly-alpha emission in LBGs that is constantly increasing from z~3 to z~6 is most probably reversed from z~6 to z~7. Explaining the observed rapid change in the LAE fraction among the drop-out population with reionization requires a fast evolution of the neutral fraction of hydrogen in the Universe. Assuming that the Universe is completely ionized at z=6 and adopting the semi-analytical models of Dijkstra et al. (2011), we find that our data require a change of the neutral hydrogen fraction of the order Delta chi_{HI} ~ 0.6 in a time Delta z ~ 1, provided that the escape fraction does not increase dramatically over the same redshift interval.
We present ultradeep optical spectroscopy obtained with FORS2 on VLT of seven Lyman-break galaxy (LBG) candidates at z>6.5 selected in the GOODS-S field from Hawk-I/VLT and WFC3/HST imaging. For one galaxy we detect a low significance emission line (
S/N< 7), located at 9691.5 +/- 0.5A and with flux 3.4 x 10^(-18)erg/cm^2/s. If identified as Lyman alpha, it places the LBG at redshift z=6.972+/- 0.002, with a rest-frame equivalent width EW}=13A. Using Monte Carlo simulations and conservative EW distribution functions at 2<z<6, we estimate that the probability of observing no galaxies in our data with S/N>10 is ~ 2%, and that of observing only one galaxy out of seven with S/N=5 is ~4%, but these can be as small as ~1E-3, depending on the details of the EW distribution. We conclude that either a significant fraction of the candidates is not at high redshift or that some physical mechanism quenches the Lyman alpha emission emerging from the galaxies at z>6.5, abruptly reversing the trend of the increasing fraction of strong emitters with increasing redshift observed up to z~ 6.5. We discuss the possibility that an increasingly neutral intergalactic medium is responsible for such quenching.
The redshifts of all cosmologically distant sources are expected to experience a small, systematic drift as a function of time due to the evolution of the Universes expansion rate. A measurement of this effect would represent a direct and entirely mo
del-independent determination of the expansion history of the Universe over a redshift range that is inaccessible to other methods. Here we investigate the impact of the next generation of Extremely Large Telescopes on the feasibility of detecting and characterising the cosmological redshift drift. We consider the Lyman alpha forest in the redshift range 2 < z < 5 and other absorption lines in the spectra of high redshift QSOs as the most suitable targets for a redshift drift experiment. Assuming photon-noise limited observations and using extensive Monte Carlo simulations we determine the accuracy to which the redshift drift can be measured from the Ly alpha forest as a function of signal-to-noise and redshift. Based on this relation and using the brightness and redshift distributions of known QSOs we find that a 42-m telescope is capable of unambiguously detecting the redshift drift over a period of ~20 yr using 4000 h of observing time. Such an experiment would provide independent evidence for the existence of dark energy without assuming spatial flatness, using any other cosmological constraints or making any other astrophysical assumption.
Aims. We present the full data set of the spectroscopic campaign of the ESO/GOODS program in the GOODS-South field, obtained with the FORS2 spectrograph at the ESO/VLT. Method. Objects were selected as candidates for VLT/FORS2 observations primar
ily based on the expectation that the detection and measurement of their spectral features would benefit from the high throughput and spectral resolution of FORS2. The reliability of the redshift estimates is assessed using the redshift-magnitude and color-redshift diagrams, and comparing the results with public data. Results. Including the third part of the spectroscopic campaign (12 masks) to the previous work (26 masks, Vanzella et al. 2005, 2006), 1715 spectra of 1225 individual targets have been analyzed. The actual spectroscopic catalog provides 887 redshift determinations. The typical redshift uncertainty is estimated to be sigma(z) ~ 0.001. Galaxies have been selected adopting different color criteria and using photometric redshifts. The resulting redshift distribution typically spans two domains: from z=0.5 to 2 and z=3.5 to 6.3. The reduced spectra and the derived redshifts are released to the community through the ESO web page http://www.eso.org/science/goods/
