arXiv subjects

A. Grazian

Publications and source records attributed to A. Grazian.

At least 307 records · Page 17Linked to original sources

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.

astro-ph.GA↗

arXiv2018

Galaxy Formation in Sterile Neutrino Dark Matter Models

We investigate galaxy formation in models with dark matter (DM) constituted by sterile neutrinos. Given their large parameter space, defined by the combinations of sterile neutrino mass $m_ν$ and mixing parameter $\sin^2(2θ)$ with active neutrinos, we focus on models with $m_ν=7$ keV, consistent with the tentative 3.5 keV line detected in several X-ray spectra of clusters and galaxies. We consider i) two resonant production models with $\sin^2(2θ)=5\,10^{-11}$ and $\sin^2(2θ)=2\,10^{-10}$, to cover the range of mixing parameter consistent with the 3.5 keV line; ii) two scalar-decay models, representative of the two possible cases characterizing such a scenario: a freeze-in and a freeze-out case. We also consider thermal Warm Dark Matter with particle mass $m_X=3$ keV. Using a semi-analytic model, we compare the predictions for the different DM scenarios with a wide set of observables. We find that comparing the predicted evolution of the stellar mass function, the abundance of satellites of Milky Way-like galaxies, and the global star formation history of galaxies with observations does not allow to disentangle the effects of the baryonic physics from those related to the different DM models. On the other hand, the distribution of the stellar-to-halo mass ratios, the abundance of faint galaxies in the UV luminosity function at $z\gtrsim 6$, and the specific star formation and age distribution of local, low-mass galaxies constitute potential probes for the considered DM scenarios. We discuss how next observations with upcoming facilities will enable to rule out or to strongly support DM models based on sterile neutrinos.

astro-ph.CO↗

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.

astro-ph.GA↗

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]

astro-ph.GA↗

arXiv2017

Kiloparsec-scale gaseous clumps and star formation at $z=5-7$

We investigate the morphology of the [CII] emission in a sample of "normal" star-forming galaxies at $5<z<7.2$ in relation to their UV (rest-frame) counterpart. We use new ALMA observations of galaxies at $z\sim6-7$ as well as a careful re-analysis of archival ALMA data. In total 29 galaxies were analysed, 21 of which are detected in [CII]. For several of the latter the [CII] emission breaks into multiple components. Only a fraction of these [CII] components, if any, is associated with the primary UV systems, while the bulk of the [CII] emission is associated either with fainter UV components, or not associated with any UV counterpart at the current limits. By taking into account the presence of all these components, we find that the L$_{\rm [CII]}$-SFR relation at early epochs is fully consistent with the local relation, but it has a dispersion of 0.48$\pm$0.07 dex, which is about two times larger than observed locally. We also find that the deviation from the local L$_{\rm [CII]}$-SFR relation has a weak anti-correlation with the EW(Ly$α$). The morphological analysis also reveals that [CII] emission is generally much more extended than the UV emission. As a consequence, these primordial galaxies are characterised by a [CII] surface brightness generally much lower than expected from the local ${\rm Σ_{[CII}-Σ_{SFR}}$ relation. These properties are likely a consequence of a combination of different effects, namely: gas metallicity, [CII] emission from obscured star-forming regions, strong variations of the ionisation parameter, and circumgalactic gas in accretion or ejected by these primeval galaxies.

astro-ph.GA↗

arXiv2017

Chasing passive galaxies in the early Universe: a critical analysis in CANDELS GOODS-South

We search for passive galaxies at $z>3$ in the GOODS-South field, using different techniques based on photometric data, and paying attention to develop methods that are sensitive to objects that have become passive shortly before the epoch of observation. We use CANDELS HST catalogues, ultra-deep $Ks$ data and new IRAC photometry, performing spectral energy distribution fitting using models with abruptly quenched star formation histories. We then single out galaxies which are best fitted by a passively evolving model, and having only low probability $( < 5 \%)$ star-forming solutions. We verify the effects of including nebular lines emission, and we consider possible solutions at different redshifts. The number of selected sources dramatically depends on the models used in the SED fitting. Without including emission lines and with photometric redshifts fixed at the CANDELS estimate, we single out 30 candidates; the inclusion of nebular lines emission reduces the sample to 10 objects; allowing for solutions at different redshifts, only 2 galaxies survive as robust candidates. Most of the candidates are not far-infrared emitters, corroborating their association with passive galaxies. Our results translate into an upper limit in the number density of $\sim 0.173$ arcmin$^2$ above the detection limit. However, we conclude that the selection of passive galaxies at $z>3$ is still subject to significant uncertainties, being sensitive to assumptions in the SED modeling adopted and to the relatively low S/N of the objects. By means of dedicated simulations, we show that JWST will greatly enhance the accuracy, allowing for a much more robust classification.

astro-ph.GA↗

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]

astro-ph.GA↗

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.

astro-ph.GA↗

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.

astro-ph.GA↗

arXiv2017

Observing the very low-surface brightness dwarfs in a deep field in the VIRGO cluster: constraints on Dark Matter scenarios

We report the discovery of 11 very faint (r< 23), low surface brightness (μ_r< 27 mag/arcsec^2) dwarf galaxies in one deep field in the Virgo cluster, obtained by the prime focus cameras (LBC) at the Large Binocular Telescope (LBT). These extend our previous sample to reach a total number of 27 galaxies in a field of just of 0.17 deg^2 located at a median distance of 390 kpc from the cluster center. Their association with the Virgo cluster is supported by their separate position in the central surface brightness - total magnitude plane with respect to the background galaxies of similar total magnitude. For a significant fraction (26\%) of the sample the association to the cluster is confirmed by spectroscopic follow-up. We show that the mere abundance of satellite galaxies corresponding to our observed number in the target field provides extremely tight constraints on Dark Matter models with suppressed power spectrum compared to the Cold Dark Matter case, independently of the galaxy luminosity distribution. In particular, requiring the observed number of satellite galaxies not to exceed the predicted abundance of Dark Matter sub-halos yields a limit m_X >3 keV at 1-σ and m_X > 2.3 keV at 2-σ confidence level for the mass of thermal Warm Dark Matter particles. Such a limit is competitive with other limits set by the abundance of ultra-faint satellite galaxies in the Milky Way, is completely independent of baryon physics involved in galaxy formation, and has the potentiality for appreciable improvements with next observations. We extend our analysis to Dark Matter models based on sterile neutrinos, showing that our observations set tight constraints on the combination of sterile neutrino mass m_ν and mixing parameter sin^2(2θ). We discuss the robustness of our results with respect to systematics.

astro-ph.CO↗

arXiv2017

The ASTRODEEP Frontier Fields Catalogues III. Multiwavelength photometry and rest frame properties of MACS-J0717 and MACS-J1149

We present the multiwavelength photometry of two Frontier Fields massive galaxy clusters MACS-J0717 and MACS-J1149 and their parallel fields, ranging from HST to ground based K and Spitzer IRAC bands, and the public release of photometric redshifts and rest frame properties of galaxies found in cluster and parallel pointings. This work was done within ASTRODEEP project and aims to provide a reference for future investigations of the extragalactic populations. To fully exploit the depth of the images and detect faint sources we used an accurate procedure which carefully removes the foreground light of bright cluster sources and the intra-cluster light thus enabling detection and measurement of accurate fluxes in crowded cluster regions. This same procedure has been successfully used to derive the photometric catalogue of MACS-J0416 and Abell-2744. The obtained multi-band photometry was used to derive photometric redshifts, magnification and physical properties of sources. In line with the first two FF catalogues released by ASTRODEEP, the photometric redshifts reach $\sim$4$\%$ accuracy. Moreover we extend the presently available samples to galaxies intrinsically as faint as H160$\sim$32-34 mag thanks the magnification factors induced to strong gravitational lensing. Our analysis allows us to probe galaxy masses larger then 10$^{7}$ M$\odot$ and/or SFR=0.1-1M$\odot$/yr out to redshift z$>6$.

astro-ph.GA↗

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.

astro-ph.GA↗

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.

astro-ph.GA↗

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.

astro-ph.GA↗

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.

astro-ph.GA↗

arXiv2017

Paving the way for the JWST: witnessing globular cluster formation at z>3

We report on five compact, extremely young (<10Myr) and blue (β_UV<-2.5, F_λ=λ^β) objects observed with VLT/MUSE at redshift 3.1169, 3.235, in addition to three objects at z=6.145. These sources are magnified by the Hubble Frontier Field galaxy clusters MACS~J0416 and AS1063. Their de-lensed half light radii (Re) are between 16 to 140pc, the stellar masses are ~1-20 X 10^6 Msun, the magnitudes are m_uv=28.8 - 31.4 (-17 3, elusive even in the deepest blank fields. We discuss how proto-GCs might contribute to the ionization budget of the universe and augment Lya visibility during reionization. This work underlines the crucial role of JWST in characterizing the rest-frame optical and near-infrared properties of such low-luminosity high-z objects.

astro-ph.GA↗

arXiv2017

New constraints on the average escape fraction of Lyman continuum radiation in z~4 galaxies from the VIMOS Ultra Deep Survey (VUDS)

Determining the average fraction of Lyman continuum (LyC) photons escaping high redshift galaxies is essential for understanding how reionization proceeded in the z>6 Universe. We want to measure the LyC signal from a sample of sources in the Chandra Deep Field South (CDFS) and COSMOS fields for which ultra-deep VIMOS spectroscopy as well as multi-wavelength Hubble Space Telescope (HST) imaging are available. We select a sample of 46 galaxies at $z\sim 4$ from the VIMOS Ultra Deep Survey (VUDS) database, such that the VUDS spectra contain the LyC part of the spectra, that is, the rest-frame range $880-910Å$. Taking advantage of the HST imaging, we apply a careful cleaning procedure and reject all the sources showing nearby clumps with different colours, that could potentially be lower-redshift interlopers. After this procedure, the sample is reduced to 33 galaxies. We measure the ratio between ionizing flux (LyC at $895Å$) and non-ionizing emission (at $\sim 1500 Å$) for all individual sources. We also produce a normalized stacked spectrum of all sources. Assuming an intrinsic average $L_ν(1470)/L_ν(895)$ of 3, we estimate the individual and average relative escape fraction. We do not detect ionizing radiation from any individual source, although we identify a possible LyC emitter with very high Ly$α$ equivalent width (EW). From the stacked spectrum and assuming a mean transmissivity for the sample, we measure a relative escape fraction $f_{esc}^{rel}=0.09\pm0.04$. We also look for correlations between the limits in the LyC flux and source properties and find a tentative correlation between LyC flux and the EW of the Ly$α$ emission line. Our results imply that the LyC flux emitted by $V=25-26$ star-forming galaxies at z$\sim$4 is at most very modest, in agreement with previous upper limits from studies based on broad and narrow band imaging.

astro-ph.GA↗

arXiv2017

Analogs of primeval galaxies two billion years after the Big Bang

Deep observations are revealing a growing number of young galaxies in the first billion year of cosmic time. Compared to typical galaxies at later times, they show more extreme emission-line properties, higher star formation rates, lower masses, and smaller sizes. However, their faintness precludes studies of their chemical abundances and ionization conditions, strongly limiting our understanding of the physics driving early galaxy build-up and metal enrichment. Here we study a rare population of UV-selected, sub$-L^{*}$(z=3) galaxies at redshift 2.4$<z<$3.5 that exhibit all the rest-frame properties expected from primeval galaxies. These low-mass, highly-compact systems are rapidly-forming galaxies able to double their stellar mass in only few tens million years. They are characterized by very blue UV spectra with weak absorption features and bright nebular emission lines, which imply hard radiation fields from young hot massive stars. Their highly-ionized gas phase has strongly sub-solar carbon and oxygen abundances, with metallicities more than a factor of two lower than that found in typical galaxies of similar mass and star formation rate at $z\lesssim$2.5. These young galaxies reveal an early and short stage in the assembly of their galactic structures and their chemical evolution, a vigorous phase which is likely to be dominated by the effects of gas-rich mergers, accretion of metal-poor gas and strong outflows.

astro-ph.GA↗