arXiv subjects

S. Cristiani

Publications and source records attributed to S. Cristiani.

At least 127 records · Page 7Linked to original sources

arXiv2016

High-resolution spectroscopy of a young, low-metallicity optically-thin L=0.02L* star-forming galaxy at z=3.12

We present VLT/X-Shooter and MUSE spectroscopy of an faint F814W=28.60+/-0.33 (Muv=-17.0), low mass (~<10^7 Msun) and compact (Reff=62pc) freshly star-forming galaxy at z=3.1169 magnified (16x) by the Hubble Frontier Fields galaxy cluster Abell S1063. Gravitational lensing allows for a significant jump toward low-luminosity regimes, in moderately high resolution spectroscopy (R=lambda/dlambda ~ 3000-7400). We measured CIV1548,1550, HeII1640, OIII]1661,1666, CIII]1907,1909, Hbeta, [OIII]4959,5007, emission lines with FWHM< 50 km/s and (de-lensed) fluxes spanning the interval 1.0x10^-19 - 2.0x10^-18 erg/s/cm2 at S/N=4-30. The double peaked Lya emission with Delta_v(red-blue) = 280(+/-7)km/s and de-lensed fluxes 2.4_(blue)|8.5_(red)x10^-18 erg/s/cm2 (S/N=38_(blue)|110_(red)) indicate a low column density of neutral hydrogen gas consistent with a highly ionized interstellar medium as also inferred from the large [OIII]5007/[OII]3727>10 ratio. We detect CIV1548,1550 resonant doublet in emission, each component with FWHM ~< 45 km/s, and redshifted by +51(+/-10)km/s relative to the systemic redshift. We interpret this as nebular emission tracing an expanding optically-thin interstellar medium. Both CIV1548,1550 and HeII1640 suggest the presence of hot and massive stars (with a possible faint AGN). The ultraviolet slope is remarkably blue, beta =-2.95 +/- 0.20 (F_lambda=lambda^beta), consistent with a dust-free and young ~<20 Myr galaxy. Line ratios suggest an oxygen abundance 12+log(O/H)<7.8. We are witnessing an early episode of star-formation in which a relatively low NHI and negligible dust attenuation might favor a leakage of ionizing radiation. This galaxy currently represents a unique low-luminosity reference object for future studies of the reionization epoch with JWST.

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arXiv2016

Are LGRBs biased tracers of star formation? Clues from the host galaxies of the Swift/BAT6 complete sample of bright LGRBs. II: star formation rates and metallicities at z < 1

Long gamma-ray bursts (LGRBs) are associated with the deaths of massive stars and could thus be a potentially powerful tool to trace cosmic star formation. However, especially at low redshifts (z < 1.5) LGRBs seem to prefer particular types of environment. Our aim is to study the host galaxies of a complete sample of bright LGRBs to investigate the impact of the environment on GRB formation. We study host galaxy spectra of the Swift/BAT6 complete sample of 14 z < 1 bright LGRBs. We use the detected nebular emission lines to measure the dust extinction, star formation rate (SFR) and nebular metallicity (Z) of the hosts and supplement the data set with previously measured stellar masses M$_{\star}$. The distributions of the obtained properties and their interrelations (e.g. mass-metallicity and SFR-M$_{\star}$ relations) are compared to samples of field star-forming galaxies.We find that LGRB hosts at z < 1 have on average lower SFRs than if they were direct star-formation tracers. By directly comparing metallicity distributions of LGRB hosts and star-forming galaxies, we find a good match between the two populations up to Z $\sim 8.4-8.5$, after which the paucity of metal-rich LGRB hosts becomes apparent. The LGRB host galaxies of our complete sample are not inconsistent with the mass-metallicity relation at similar mean redshift and stellar masses. The cutoff against high metallicities (and high masses) can explain the low SFR values of LGRB hosts. We find a hint of increased incidence of starburst galaxies in the Swift/BAT6 z < 1 sample with respect to that of a field star-forming population. Given that the SFRs are low on average, the latter is ascribed to low stellar masses. Nevertheless the limits on the completeness and metallicity availability of current surveys, coupled with the limited number of LGRB host galaxies, prevent us from reaching more quantitative conclusions.

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arXiv2016

First observational support for overlapping reionized bubbles generated by a galaxy overdensity

We present the analysis of deep HST multi-band imaging of the BDF field specifically designed to identify faint companions around two of the few Ly-alpha emitting galaxies spectroscopically confirmed at z~7 (Vanzella et al. 2011). Although separated by only 4.4 proper Mpc these galaxies cannot generate HII regions large enough to explain visibility of their Ly-alpha line, thus requiring a population of fainter ionizing sources in their vicinity. We use deep HST and VLT-Hawk-I data to select z~7 Lyman break galaxies around the emitters. We select 6 new robust z~7 LBGs at Y~26.5-27.5 whose average spectral energy distribution is consistent with the objects being at the redshift of the close-by Ly-alpha emitters. The resulting number density of z~7 LBGs in the BDF field is a factor ~3-4 higher than expected in random pointings of the same size. We compare these findings with cosmological hydrodynamic plus radiative transfer simulations of a universe with a half neutral IGM: we find that indeed Ly-alpha emitter pairs are only found in completely ionized regions characterized by significant LBG overdensities. Our findings match the theoretical prediction that the first ionization fronts are generated within significant galaxy overdensities and support a scenario where faint, "normal" star-forming galaxies are responsible for reionization.

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arXiv2015

ALMA constraints on the faint millimetre source number counts and their contribution to the cosmic infrared background

We have analysed 18 ALMA continuum maps in Bands 6 and 7, with rms down to 7.8$μ$Jy, to derive differential number counts down to 60$μ$Jy and 100$μ$Jy at $λ=$1.3 mm and $λ=$1.1 mm, respectively. The area covered by the combined fields is $\rm 9.5\times10^{-4}deg^2$ at 1.1mm and $\rm 6.6\times10^{-4}deg^{2}$ at 1.3mm. We improved the source extraction method by requiring that the dimension of the detected sources be consistent with the beam size. This method enabled us to remove spurious detections that have plagued the purity of the catalogues in previous studies. We detected 50 faint sources with S/N$>$3.5 down to 60$μ$Jy, hence improving the statistics by a factor of four relative to previous studies. The inferred differential number counts are $\rm dN/d(Log_{10}S)=1\times10^5~deg^2$ at a 1.1 mm flux $S_{λ= 1.1~mm} = 130~μ$Jy, and $\rm dN/d(Log_{10}S)=1.1\times10^5~deg^2$ at a 1.3 mm flux $\rm S_{λ= 1.3~mm} = 60~μ$Jy. At the faintest flux limits, i.e. 30$μ$Jy and 40$μ$Jy, we obtain upper limits on the differential number counts of $\rm dN/d(Log_{10}S) < 7\times10^5~deg^2$ and $\rm dN/d(Log_{10}S)<3\times10^5~deg^2$, respectively. Our results provide a new lower limit to CIB intensity of 17.2${\rm Jy\ deg^{-2}}$ at 1.1mm and of 12.9${\rm Jy\ deg^{-2}}$ at 1.3mm. Moreover, the flattening of the integrated number counts at faint fluxes strongly suggests that we are probably close to the CIB intensity. Our data imply that galaxies with SFR$<40~M_{\odot}/yr$ certainly contribute less than 50% to the CIB while more than 50% of the CIB must be produced by galaxies with $\rm SFR>40~M_{\odot}/yr$. The differential number counts are in nice agreement with recent semi-analytical models of galaxy formation even as low as our faint fluxes. Consequently, this supports the galaxy evolutionary scenarios and assumptions made in these models.

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arXiv2015

The assembly of "normal" galaxies at z=7 probed by ALMA

We report new deep ALMA observations aimed at investigating the [CII]158um 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 ~ 5-15 Msun/yr these systems are much more representative of the high-z galaxy population than other systems targeted in the past by millimeter observations. For the galaxy with the deepest observation we detect [CII] emission at redshift z=7.107, fully consistent with the Lyalpha redshift, but spatially offset by 0.7" (4 kpc) from the optical emission. At the location of the optical emission, tracing both the Lyalpha line and the far-UV continuum, no [CII] emission is detected in any of the three galaxies, with 3sigma upper limits significantly lower than the [CII] emission observed in lower reshift galaxies. These results suggest that molecular clouds in the central parts of primordial galaxies are rapidly disrupted by stellar feedback. As a result, [CII] 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.

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arXiv2015

New observations of z~7 galaxies: evidence for a patchy reionization

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 redshift 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$α$ 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.

astro-ph.CO↗

arXiv2015

Faint AGNs at z>4 in the CANDELS GOODS-S field: looking for contributors to the reionization of the Universe

In order to derive the AGN contribution to the cosmological ionizing emissivity we have selected faint AGN candidates at $z>4$ in the CANDELS GOODS-South field which is one of the deepest fields with extensive multiwavelength coverage from Chandra, HST, Spitzer and various groundbased telescopes. We have adopted a relatively novel criterion. As a first step high redshift galaxies are selected in the NIR $H$ band down to very faint levels ($H\leq27$) using reliable photometric redshifts. This corresponds at $z>4$ to a selection criterion based on the galaxy rest-frame UV flux. AGN candidates are then picked up from this parent sample if they show X-ray fluxes above a threshold of $F_X\sim 1.5\times 10^{-17}$ cgs (0.5-2 keV). We have found 22 AGN candidates at $z>4$ and we have derived the first estimate of the UV luminosity function in the redshift interval $4<z<6.5$ and absolute magnitude interval $-22.5\lesssim M_{1450} \lesssim -18.5$ typical of local Seyfert galaxies. The faint end of the derived luminosity function is about two/four magnitudes fainter at $z\sim 4-6$ than that derived from previous UV surveys. We have then estimated ionizing emissivities and hydrogen photoionization rates in the same redshift interval under reasonable assumptions and after discussion of possible caveats, the most important being the large uncertainties involved in the estimate of photometric redshift for sources with featureless, almost power-law SEDs and/or low average escape fraction of ionizing photons from the AGN host galaxies. We argue that, under reasonable evaluations of possible biases, the probed AGN population can produce at $z=4-6.5$ photoionization rates consistent with that required to keep highly ionized the intergalactic medium observed in the Lyman-$α$ forest of high redshift QSO spectra, providing an important contribution to the cosmic reionization.

astro-ph.CO↗

arXiv2014

A 52 hours VLT/FORS2 spectrum of a bright z~7 HUDF galaxy: no Ly-alpha emission

We aim to determine the redshift of GDS-1408, the most solid z~7 galaxy candidate lying in the Hubble Ultra Deep Field. We have used all the VLT spectra of GDS-1408 collected by us and two other groups with FORS2 at VLT in the last five years, for a total integration time of 52hr. The combined spectrum is the deepest ever obtained of a galaxy in the Reionization epoch. We do not detect any emission line or continuum over the whole wavelength range, up to 10100A. Based on an accurate set of simulations, we are able to put a stringent upper limit of f(Lya)<3x10^(-18) erg/s/cm2 at 3-9 sigma in the explored wavelength range, corresponding to a rest-frame equivalent width EW<9A. Combining this limit with the SED modelling we refine the redshift to be z=6.82+/- 0.1 (1-sigma). The same SED fitting indicates that GDS-1408 is relatively extinct (A1600~1) with a dust corrected star formation rate of ~ 20 Msol/yr. The comparison between the un-attenuated equivalent width predicted by the case-B recombination theory and the observed upper limit, provides a limit on the effective Lya escape fraction of f_(esc)^(eff)(Lya)<8%. Even though we cannot rule out a major contribution of the inter/circum galactic medium in damping the line, a plausible interpretation is that GDS-1408 is moderately evolved and contains sufficient gas and dust to attenuate the Lya emission, before it reaches the intergalactic medium. The redshift confirmation of even the best z~7 candidates is very hard to achieve (unless the Lya or unusually strong rest-UV nebular emission lines are present) with the current generation of 8-10m class telescopes. We show that both JWST and E-ELT will be necessary to make decisive progresses. Currently, the increased redshift accuracy obtained with this kind of analysis makes ALMA an interesting option for the redshift confirmation.

astro-ph.GA↗

arXiv2014

Characterizing faint galaxies in the reionization epoch: LBT confirms two L<0.2L* sources at z=6.4 behind the CLASH/Frontier Fields cluster MACS0717.5+3745

We report the LBT/MODS1 spectroscopic confirmation of two images of faint Lyman alpha emitters at $z=6.4$ behind the Frontier Fields galaxy cluster MACSJ0717.5+3745. A wide range of lens models suggests that the two images are highly magnified, with a strong lower limit of mu>5. These are the faintest z>6 candidates spectroscopically confirmed to date. These may be also multiple images of the same z=6.4 source as supported by their similar intrinsic properties, but the lens models are inconclusive regarding this interpretation. To be cautious, we derive the physical properties of each image individually. Thanks to the high magnification, the observed near-infrared (restframe ultraviolet) part of the spectral energy distributions and Ly-alpha lines are well detected with S/N(m_1500)>~10 and S/N(Ly-alpha)~10-15. Adopting mu>5, the absolute magnitudes, M_1500, and Ly-alpha fluxes, are fainter than -18.7 and 2.8x10^(-18)erg/s/cm2, respectively. We find a very steep ultraviolet spectral slope beta=-3.0+/-0.5 (F_lambda=lambda^(beta)), implying that these are very young, dust-free and low metallicity objects, made of standard stellar populations or even extremely metal poor stars (age<~30Myr, E(B-V)=0 and metallicity 0.0-0.2 Z/Zsolar). The objects are compact (< 1 kpc^(2)), and with a stellar mass M* < 10^(8) M_solar. The very steep beta, the presence of the Ly-alpha line and the intrinsic FWHM (<300 km/s) of these newborn objects do not exclude a possible leakage of ionizing radiation. We discuss the possibility that such faint galaxies may resemble those responsible for cosmic reionization.

astro-ph.CO↗

arXiv2014

ESPRESSO: The next European exoplanet hunter

The acronym ESPRESSO stems for Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations; this instrument will be the next VLT high resolution spectrograph. The spectrograph will be installed at the Combined-Coudé Laboratory of the VLT and linked to the four 8.2 m Unit Telescopes (UT) through four optical Coudé trains. ESPRESSO will combine efficiency and extreme spectroscopic precision. ESPRESSO is foreseen to achieve a gain of two magnitudes with respect to its predecessor HARPS, and to improve the instrumental radial-velocity precision to reach the 10 cm/s level. It can be operated either with a single UT or with up to four UTs, enabling an additional gain in the latter mode. The incoherent combination of four telescopes and the extreme precision requirements called for many innovative design solutions while ensuring the technical heritage of the successful HARPS experience. ESPRESSO will allow to explore new frontiers in most domains of astrophysics that require precision and sensitivity. The main scientific drivers are the search and characterization of rocky exoplanets in the habitable zone of quiet, nearby G to M-dwarfs and the analysis of the variability of fundamental physical constants. The project passed the final design review in May 2013 and entered the manufacturing phase. ESPRESSO will be installed at the Paranal Observatory in 2016 and its operation is planned to start by the end of the same year.

astro-ph.IM↗

arXiv2013

A Community Science Case for E-ELT HIRES

Building on the experience of the high-resolution community with the suite of VLT high-resolution spectrographs, which has been tremendously successful, we outline here the (science) case for a high-fidelity, high-resolution spectrograph with wide wavelength coverage at the E-ELT. Flagship science drivers include: the study of exo-planetary atmospheres with the prospect of the detection of signatures of life on rocky planets; the chemical composition of planetary debris on the surface of white dwarfs; the spectroscopic study of protoplanetary and proto-stellar disks; the extension of Galactic archaeology to the Local Group and beyond; spectroscopic studies of the evolution of galaxies with samples that, unlike now, are no longer restricted to strongly star forming and/or very massive galaxies; the unraveling of the complex roles of stellar and AGN feedback; the study of the chemical signatures imprinted by population III stars on the IGM during the epoch of reionization; the exciting possibility of paradigm-changing contributions to fundamental physics. The requirements of these science cases can be met by a stable instrument with a spectral resolution of R~100,000 and broad, simultaneous spectral coverage extending from 370nm to 2500nm. Most science cases do not require spatially resolved information, and can be pursued in seeing-limited mode, although some of them would benefit by the E-ELT diffraction limited resolution. Some multiplexing would also be beneficial for some of the science cases. (Abridged)

astro-ph.IM↗

arXiv2013

Metals in the IGM approaching the re-ionization epoch: results from X-shooter at the VLT

We present the results of observations taken with the X-shooter spectrograph devoted to the study of quasars at z ~ 6. This paper focuses on the properties of metals at high redshift traced, in particular, by the C IV doublet absorption systems. Six objects were observed with resolutions ~27 and 34 km/s in the visual, and 37.5 and 53.5 km/s in the near infrared. We detected 102 C IV lines in the range: 4.35 < z < 6.2 of which 27 are above z ~ 5. Thanks to the characteristics of resolution and spectral coverage of X-shooter, we could also detect 25 Si IV doublets associated with the C IV at z>5. The column density distribution function of the C IV line sample is observed to evolve in redshift for z>~ 5.3, with respect to the normalization defined by low redshift (1.5 < z <4) C IV lines. This behaviour is reflected in the redshift evolution of the C IV cosmic mass density, Omega_CIV, of lines with column density in the range 13.4 < log N(C IV) < 15, which is consistent with a drop of a factor of ~2 for z>~ 5.3. Considering only the stronger C IV lines (13.8 < log N(CIV) < 15), Omega_CIV gently rises by a factor of ~10 between z ~ 6.2 and z~ 1.5 with a possible flattening toward z ~ 0. The increase is well fitted by a power law: Omega_CIV = (2+-1)x10^{-8} [(1+z)/4]^{-3.1+-0.1}. An insight into the properties of the C IV absorbers and their evolution with redshift is obtained by comparing the observed column densities of associated C IV, Si IV and C II absorptions with the output of a set of CLOUDY photo-ionization models. As already claimed by cosmological simulations, we find that C IV is a good tracer of the metallicity in the low-density intergalacitc medium gas at z ~ 5-6 while at z ~ 3 it arises in gas with over density delta ~100.

astro-ph.CO↗

arXiv2012

The Lyman alpha forest flux probability distribution at z>3

We present a measurement of the Lyman alpha flux probability distribution function (PDF) measured from a set of eight high resolution quasar spectra with emission redshifts at 3.3 < z < 3.8. We carefully study the effect of metal absorption lines on the shape of the PDF. Metals have a larger impact on the PDF measurements at lower redshift, where there are fewer Lyman alpha absorption lines. This may be explained by an increase in the number of metal lines which are blended with Lyman alpha absorption lines toward higher redshift, but may also be due to the presence of fewer metals in the intergalactic medium with increasing lookback time. We also provide a new measurement of the redshift evolution of the effective optical depth, tau_eff, at 2.8 < z < 3.6, and find no evidence for a deviation from a power law evolution in the log(tau_eff)-log(1+z) plane. The flux PDF measurements are furthermore of interest for studies of the thermal state of the intergalactic medium (IGM) at z ~ 3 . By comparing the PDF to state-of-the-art cosmological hydrodynamical simulations, we place constraints on the temperature of the IGM and compare our results with previous measurements of the PDF at lower redshift. At redshift z=3, our new PDF measurements are consistent with an isothermal temperature-density relation, T=T_0 Delta^{gamma-1}, with a temperature at the mean density of T_0 = 19250 +/- 4800 K and a slope gamma=0.90+/-0.21 (1 sigma uncertainties). In comparison, joint constraints with previous PDF measurements at z<3 favour an inverted (gamma<1) temperature-density relation with T_0=17900 +/- 3500 K and gamma=0.70 +/- 0.12, in broad agreement with previous analyses.

astro-ph.CO↗

arXiv2012

The blue UV slopes of z~4 Lyman break galaxies: implications for the corrected star formation rate density

We plan to analyse dust extinction in Lyman Break Galaxies (LBGs) by introducing a new and more reliable approach to their selection and to the characterization of their distribution of UV slopes beta, using deep IR images from HST. We exploit deep WFC3 IR observations of the ERS and HUDF fields over GOODS-South, combined with HST-ACS optical data, to select z~4 LBGs through a new (B-V) vs. (V-H) colour diagram. The UV slope of the selected galaxies is robustly determined by a linear fit over their observed I, Z, Y, J magnitudes, coherently with the original definition of beta. The same fit is used to determine their rest-frame UV magnitudes M1600 through a simple interpolation. We estimate the effect of observational uncertainties with detailed simulations that we also exploit, under a parametric maximum-likelihood approach, to constrain the probability density function of UV slopes PDF(beta) as a function of rest-frame magnitude. We find 142 and 25 robust LBGs in the ERS and HUDF fields respectively, limiting our sample to S/N(H)>10 objects. Our newly defined criteria improve the selection of z~4 LBGs and allow us to exclude red interlopers at lower redshift, especially z~3-3.5 objects. We find that z~4 LBGs are characterized by blue UV slopes, suggesting a low dust extinction: all L \simeq-2.1, while brighter objects only are slightly redder ( -1.9). We find an intrinsic dispersion ~ 0.3 for PDF(beta) at all magnitudes. The SFRD at z~4 corrected according to these estimates turns out to be lower than previously found: log(SFRD)\simeq-1.09 M_sun/yr/Mpc^3. Finally, we discuss how the UV slope of z~4 galaxies changes as a function of the dust-corrected UV magnitude (i.e. SFR) showing that most galaxies with a high SFR (> 80 M_sun/yr) are highly extincted objects. [Abridged]

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arXiv2011

Spectroscopic confirmation of z~7 LBGs: probing the earliest galaxies and the epoch of reionization

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 report 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.

astro-ph.CO↗

arXiv2011

Spectroscopic confirmation of two Lyman break galaxies at redshift beyond 7

We report the spectroscopic confirmation of two Lyman break galaxies at redshift > 7. The galaxies were observed as part of an 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. Both galaxies show a prominent emission line at 9735A and 9858A respectively: the lines have fluxes of ~ 1.6-1.2 x 10^(-17) erg/s/cm2 and exhibit a sharp decline on the blue side and a tail on the red side. The asymmetry is quantitatively comparable to the observed asymmetry in z ~ 6 Ly-alpha lines, where absorption by neutral hydrogen in the IGM truncates the blue side of the emission line profile. We carefully evaluate the possibility that the galaxies are instead at lower redshift and we are observing either [OII], [OIII] or H-alpha emission: however from the spectroscopic and the photometric data we conclude that there are no other plausible identifications, except for Ly-alpha at redshift >7, implying that these are two of the most robust redshift determination for galaxies in the reionization epoch. Based on their redshifts and broad--band photometry, we derive limits on the star formation rate and on the ultraviolet spectral slopes of the two galaxies. We argue that these two galaxies alone are unlikely to have ionized the IGM in their surroundings.

astro-ph.CO↗

arXiv2011

Optical-NIR spectra of quasars close to reionization (z~ 6)

X-shooter, with its characteristics of resolution, spectral coverage and efficiency, provides a unique opportunity to obtain spectra of the highest-redshift quasars (z ~ 6) that will allow us to carry out successful investigations on key cosmological issues, from the details of the re-ionization process, to the evolution of the first galaxies and AGNs. In this paper, we present the spectra of three z ~ 6 quasars: one obtained during the commissioning of X-shooter and two in the context of our ongoing GTO programme. Combining this sample with data in the literature, we update the value of the C IV cosmic mass density in the range 4.5 < z < 5, confirming the constant trend with redshift between 2.5 and 5.

astro-ph.CO↗

arXiv2010

Cosmic evolution of the CIV in high-resolution hydrodynamic simulations

We investigate the properties of triply ionized Carbon (CIV) in the Intergalactic Medium using a set of high-resolution and large box-size cosmological hydrodynamic simulations of a $Λ$CDM model. We rely on a modification of the GADGET-2 code that self-consistently follows the metal enrichment mechanism by means of a detailed chemical evolution model. We focus on several numerical implementations of galactic feedback: galactic winds in the energy driven and momentum driven prescriptions and Active Galactic Nuclei (AGN) powered by gas accretion onto massive black holes. We extract mock IGM transmission spectra in neutral hydrogen (HI) and CIV and perform Voigt profile fitting. The results are then compared with high-resolution quasar (QSO) spectra obtained with the UVES spectrograph at the VLT and the HIRES spectrograph at Keck. We find that feedback has little impact on statistics related to the neutral hydrogen, while CIV is more affected by galactic winds and/or AGN feedback. When the same analysis is performed over observed and simulated CIV lines, we find reasonables good agreement between data and simulations over the column density range $N_{\rm CIV}=10^{12.5-15}$ cm$^{-2}$. Also the CIV line-widths distribution appears to be in agreement with the observed values, while the HI Doppler parameters, $b_{\rm HI}$, are in general too large showing that the diffuse cosmic web is heated more than what is inferred by observations. The simulation without feedback fails in reproducing the CIV systems at high column densities at all redshift, while the AGN feedback case agrees with observations only at $z<3$, when this form of feedback is particularly effective. We also present scatter plots in the $b-N$ and in the $N_{\rm CIV}-N_{\rm HI}$ planes, showing that there is rough agreement between observations and simulations only when feedback is taken into account.

astro-ph.CO↗