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.
Determining the AGN content in structures of different mass/velocity dispersion and comparing them to higher mass/lower redshift analogs is important to understand how the AGN formation process is related to environmental properties. We use our well-
tested cluster finding algorithm to identify structures in the GOODS North and South fields, exploiting the available spectroscopic redshifts and accurate photometric redshifts. We identify 9 structures in GOODS-south (presented in a previous paper) and 8 new structures in GOODS-north. We only consider structures where at least 2/3 of the members brighter than M_R=-20 have a spectroscopic redshift. For those group members that coincide with X-ray sources in the 4 and 2 Msec Chandra source catalogs respectively, we determine if the X-ray emission originates from AGN activity or it is related to the galaxies star-formation activity. We find that the fraction of AGN with Log L_H > 42 erg/s in galaxies with M_R < -20 is on average 6.3+-1.3%, much higher than in lower redshift groups of similar mass and more than double the fraction found in massive clusters at a similarly high redshift. We then explore the spatial distribution of AGN in the structures and find that they preferentially populate the outer regions. The colors of AGN host galaxies in structures tend to be confined to the green valley, thus avoiding the blue cloud and, partially, also the red-sequence, contrary to what happens in the field. We finally compare our results to the predictions of two sets of semi analytic models to investigate the evolution of AGN and evaluate potential triggering and fueling mechanisms. The outcome of this comparison attests the importance of galaxy encounters, not necessarily leading to mergers, as an efficient AGN triggering mechanism. (abridged)
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.
[Abridged] We use the large public spectroscopic database available in the GOODS-South field to estimate the dynamical mass and the virialization status of cluster ClG 0332-2747 at z=0.734. Cluster members selected from their photometric redshift are
used with spectroscopic ones to analyse the galaxy population of the cluster. In the newly released Chandra 4Ms observations we detect a faint extended X-ray emission associated to the cluster. Finally, we compare the optical and X-ray properties of ClG 0332-2747 with the predictions of a well tested semianalytic model. We estimate the velocity dispersion and the virial mass considering all 44 spectroscopic members, or 20 red-sequence members only. We obtain sigma_v=634 +/- 105 Km/s, M_200=3.07 ^{+1.57}_{-1.16}~10^{14} M_sun in the former case, and slightly lower values in the latter case. The cluster appears to have reached the virial equilibrium: it shows a perfectly Gaussian velocity distribution and no evidence for substructures. ClG 0332-2747 contains a high fraction of bright red galaxies, and is dominated by a very massive (1.1 x 10^{12} M_sun) old brightest cluster galaxy (BCG), suggesting that it formed at an early epoch. We detect a faint extended X-ray source centered on the BCG, with a total X-ray luminosity of L_X ~ 2 x 10^{42} erg s^-1 (0.1-2.4 keV). This L_X is lower by a factor of ~10-20 than expected according to the M-L_X relation. We provide a possible explanation of this discrepancy as due to the effects of AGN feedback on the ICM: the semianalytic model reproduces the M-L_X relation measured from X-ray bright clusters, and it predicts a high scatter at low masses due to heating and expulsion of the cluster gas. Interestingly, in the model clusters with an evolved galaxy population like ClG 0332-2747 present the largest scatter in X-ray luminosity.
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.
A significant fraction of high redshift starburst galaxies presents strong Ly alpha emission. Understanding the nature of these galaxies is important to assess the role they played in the early Universe and to shed light on the relation between the n
arrow band selected Lyalpha emitters and the Lyman break galaxies: are the Lyalpha emitters a subset of the general LBG population? or do they represent the youngest galaxies in their early phases of formation? We studied a sample of UV continuum selected galaxies from z~2.5 to z~6 (U, B, V and i-dropouts) from the GOODS-South survey, that have been observed spectroscopically. Using the GOODS-MUSIC catalog we investigated their physical properties, such as total masses, ages, SFRs, extinction etc as determined from a spectrophotometric fit to the multi-wavelength (U band to mid-IR) SEDs, and their dependence on the emission line characteristics. In particular we determined the nature of the LBGs with Lyalpha in emission and compared them to the properties of narrow band selected Lyalpha emitters. For U and B-dropouts we also compared the properties of LBGs with and without the Lyalpha emission line.
