arXiv subjects

A. Fontana

Publications and source records attributed to A. Fontana.

At least 109 records · Page 6Linked to original sources

arXiv2017

Characterization of star-forming dwarf galaxies at 0.1 $\lesssim z \lesssim$ 0.9 in VUDS: Probing the low-mass end of the mass-metallicity relation

We present the discovery and spectrophotometric characterization of a large sample of 164 faint ($i_{AB}$ $\sim$ $23$-$25$ mag) star-forming dwarf galaxies (SFDGs) at redshift $0.13$ $\leq z \leq$ $0.88$ selected by the presence of bright optical emission lines in the VIMOS Ultra Deep Survey (VUDS). We investigate their integrated physical properties and ionization conditions, which are used to discuss the low-mass end of the mass-metallicity relation (MZR) and other key scaling relations. We use optical VUDS spectra in the COSMOS, VVDS-02h, and ECDF-S fields, as well as deep multiwavelength photometry, to derive stellar masses, star formation rates (SFR) and gas-phase metallicities. The VUDS SFDGs are compact (median $r_{e}$ $\sim$ $1.2$ kpc), low-mass ($M_{*}$ $\sim$ $10^7-10^9$ $M_{\odot}$) galaxies with a wide range of star formation rates (SFR($Hα$) $\sim 10^{-3}-10^{1}$ $M_{\odot}/yr$) and morphologies. Overall, they show a broad range of subsolar metallicities (12+log(O/H)=$7.26$-$8.7$; $0.04$ $\lesssim Z/Z_{\odot} \lesssim$ $1$). The MZR of SFDGs shows a flatter slope compared to previous studies of galaxies in the same mass range and redshift. We find the scatter of the MZR partly explained in the low mass range by varying specific SFRs and gas fractions amongst the galaxies in our sample. Compared with simple chemical evolution models we find that most SFDGs do not follow the predictions of a "closed-box" model, but those from a gas regulating model in which gas flows are considered. While strong stellar feedback may produce large-scale outflows favoring the cessation of vigorous star formation and promoting the removal of metals, younger and more metal-poor dwarfs may have recently accreted large amounts of fresh, very metal-poor gas, that is used to fuel current star formation.

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arXiv2017

EGG: hatching a mock Universe from empirical prescriptions

This paper introduces EGG, the Empirical Galaxy Generator, a tool designed within the ASTRODEEP collaboration to generate mock galaxy catalogs for deep fields with realistic fluxes and simple morphologies. The simulation procedure is based exclusively on empirical prescriptions -- rather than first principles -- to provide the most accurate match with observations at 0<z<7. In particular, we consider that galaxies can be either quiescent or star-forming, and use their stellar mass (M*) and redshift (z) as the fundamental properties from which all the other observables can be statistically derived. Drawing z and M* from the observed galaxy stellar mass functions, we associate a star formation rate (SFR) to each galaxy from the tight SFR-M* main sequence, while dust attenuation, optical colors and morphologies (including bulge-to-total ratios, sizes and aspect ratios) are obtained from empirical relations that we establish from the high quality Hubble and Herschel observations available in the CANDELS fields. Random scatter is introduced in each step to reproduce the observed distributions of each parameter. Based on these observables, a panchromatic spectral energy distribution (SED) is selected for each galaxy and synthetic photometry is produced by integrating the redshifted SED in common broad-band filters. Finally, the mock galaxies are placed on the sky at random positions with a fixed angular two-point correlation function to implement basic clustering. The resulting flux catalogs reproduce accurately the observed number counts in all broad bands from the ultraviolet up to the sub-millimeter, and can be directly fed to image simulators such as Skymaker. The images can then be used to test source extraction softwares and image-based techniques such as stacking. EGG is open-source, and is made available to the community together with a set of pre-generated catalogs and images.

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arXiv2017

Evolution of cosmic star formation in the SCUBA-2 Cosmology Legacy Survey

We present a new exploration of the cosmic star-formation history and dust obscuration in massive galaxies at redshifts $0.5< z<6$. We utilize the deepest 450 and 850$μ$m imaging from SCUBA-2 CLS, covering 230arcmin$^2$ in the AEGIS, COSMOS and UDS fields, together with 100-250$μ$m imaging from Herschel. We demonstrate the capability of the T-PHOT deconfusion code to reach below the confusion limit, using multi-wavelength prior catalogues from CANDELS/3D-HST. By combining IR and UV data, we measure the relationship between total star-formation rate (SFR) and stellar mass up to $z\sim5$, indicating that UV-derived dust corrections underestimate the SFR in massive galaxies. We investigate the relationship between obscuration and the UV slope (the IRX-$β$ relation) in our sample, which is similar to that of low-redshift starburst galaxies, although it deviates at high stellar masses. Our data provide new measurements of the total SFR density (SFRD) in $M_\ast>10^{10}M_\odot$ galaxies at $0.5 10$. One third of this is accounted for by 450$μ$m-detected sources, while one fifth is attributed to UV-luminous sources (brighter than $L^\ast_{UV}$), although even these are largely obscured. By extrapolating our results to include all stellar masses, we estimate a total SFRD that is in good agreement with previous results from IR and UV data at $z\lesssim3$, and from UV-only data at $z\sim5$. The cosmic star-formation history undergoes a transition at $z\sim3-4$, as predominantly unobscured growth in the early Universe is overtaken by obscured star formation, driven by the build-up of the most massive galaxies during the peak of cosmic assembly.

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arXiv2016

CANDELS Multiwavelength Catalogs: Source Identification and Photometry in the CANDELS COSMOS Survey Field

We present a multi-wavelength photometric catalog in the COSMOS field as part of the observations by the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS). The catalog is based on Hubble Space Telescope Wide Field Camera 3 (HST/WFC3) and Advanced Camera for Surveys (ACS) observations of the COSMOS field (centered at RA: $10^h00^m28^s$, Dec:$+02^{\circ}12^{\prime}21^{\prime\prime}$). The final catalog has 38671 sources with photometric data in forty two bands from UV to the infrared ($\rm \sim 0.3-8\,μm$). This includes broad-band photometry from the HST, CFHT, Subaru, VISTA and Spitzer Space Telescope in the visible, near infrared and infrared bands along with intermediate and narrow-band photometry from Subaru and medium band data from Mayall NEWFIRM. Source detection was conducted in the WFC3 F160W band (at $\rm 1.6\,μm$) and photometry is generated using the Template FITting algorithm. We further present a catalog of the physical properties of sources as identified in the HST F160W band and measured from the multi-band photometry by fitting the observed spectral energy distributions of sources against templates.

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arXiv2016

Characterizing elusive, faint dusty star-forming galaxies: a lensed, optically undetected ALMA galaxy at z~3.3

We present the serendipitous ALMA detection of a faint submillimeter galaxy (SMG) lensed by a foreground z~1 galaxy. By optimizing the source detection to deblend the system, we accurately build the full spectral energy distribution of the distant galaxy from the I814 band to radio wavelengths. It is extremely red, with a I-K colour larger than 2.5. We estimate a photometric redshift of 3.28 and determine the physical parameters. The distant galaxy turns out to be magnified by the foreground lens by a factor of ~1.5, which implies an intrinsic Ks-band magnitude of ~24.5, a submillimeter flux at 870um of ~2.5 mJy and a SFR of ~150-300Msun/yr, depending on the adopted tracer. These values place our source towards the faint end of the distribution of observed SMGs, and in particular among the still few faint SMGs with a fully characterized spectral energy distribution, which allows us not only to accurately estimate its redshift but also to measure its stellar mass and other physical properties. The galaxy studied in this work is a representative of the population of faint SMGs, of which only few objects are known to date, that are undetected in optical and therefore are not typically accounted for when measuring the cosmic star formation history (SFH). This faint galaxy population thus likely represents an important and missing piece in our understanding of the cosmic SFH. Its observation and characterization is of major importance to achieve a solid picture of galaxy evolution.

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arXiv2016

Host galaxies of luminous z$\sim$0.6 quasars: Major mergers are not prevalent at the highest AGN luminosities

Galaxy interactions are thought to be one of the main triggers of Active Galactic Nuclei (AGN), especially at high luminosities, where the accreted gas mass during the AGN lifetime is substantial. Evidence for a connection between mergers and AGN, however, remains mixed. Possible triggering mechanisms remain particularly poorly understood for luminous AGN, which are thought to require triggering by major mergers, rather than secular processes. We analyse the host galaxies of a sample of 20 optically and X-ray selected luminous AGN (log($L_{bol}$ [erg/s]) $>$ 45) at z $\sim$ 0.6 using HST WFC3 data in the F160W/H band. 15/20 sources have resolved host galaxies. We create a control sample of mock AGN by matching the AGN host galaxies to a control sample of non-AGN galaxies. Visual signs of disturbances are found in about 25% of sources in both the AGN hosts and control galaxies. Using both visual classification and quantitative morphology measures, we show that the levels of disturbance are not enhanced when compared to a matched control sample. We find no signs that major mergers play a dominant role in triggering AGN at high luminosities, suggesting that minor mergers and secular processes dominate AGN triggering up to the highest AGN luminosities. The upper limit on the enhanced fraction of major mergers is $\leqslant$20%. While major mergers might increase the incidence of (luminous AGN), they are not the prevalent triggering mechanism in the population of unobscured AGN.

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arXiv2016

T-PHOT version 2.0: improved algorithms for background subtraction, local convolution, kernel registration, and new options

We present the new release v2.0 of T-PHOT, a publicly available software package developed to perform PSF-matched, prior-based, multiwavelength deconfusion photometry of extragalactic fields. New features included in the code are presented and discussed: background estimation, fitting using position dependent kernels, flux prioring, diagnostical statistics on the residual image, exclusion of selected sources from the model and residual images, individual registration of fitted objects. These new options improve on the performance of the code, allowing for more accurate results and providing useful aids for diagnostics.

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arXiv2016

Tracing the reionization epoch with ALMA: [CII] emission in z~7 galaxies

We present new results on [CII]158$μ$ m emission from four galaxies in the reionization epoch. These galaxies were previously confirmed to be at redshifts between 6.6 and 7.15 from the presence of the Ly$α$ emission line in their spectra. The Ly$α$ emission line is redshifted by 100-200 km/s compared to the systemic redshift given by the [CII] line. These velocity offsets are smaller than what is observed in z~3 Lyman break galaxies with similar UV luminosities and emission line properties. Smaller velocity shifts reduce the visibility of Ly$α$ and hence somewhat alleviate the need for a very neutral IGM at z~7 to explain the drop in the fraction of Ly$α$ emitters observed at this epoch. The galaxies show [CII] emission with L[CII]=0.6-1.6 x10$^8 L_\odot$: these luminosities place them consistently below the SFR-L[CII] relation observed for low redshift star forming and metal poor galaxies and also below z =5.5 Lyman break galaxies with similar star formation rates. We argue that previous undetections of [CII] in z~7 galaxies with similar or smaller star formation rates are due to selection effects: previous targets were mostly strong Ly$α$ emitters and therefore probably metal poor systems, while our galaxies are more representative of the general high redshift star forming population .

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arXiv2016

The impact of the Star Formation Histories on the SFR-M$_{*}$ relation at z$\ge$2

In this paper we investigate the impact of different star formation histories (SFHs) on the relation between stellar mass M$_{*}$ and star formation rate (SFR) using a sample of galaxies with reliable spectroscopic redshift zspec>2 drawn from the VIMOS Ultra-Deep Survey (VUDS). We produce an extensive database of dusty model galaxies, calculated starting from the new library of single stellar population (SSPs) models presented in Cassara' et al. 2013 and weighted by a set of 28 different SFHs based on the Schmidt function, and characterized by different ratios of the gas infall time scale $τ_{infall}$ to the star formation efficiency $ν$. The treatment of dust extinction and re-emission has been carried out by means of the radiative transfer calculation. The spectral energy distribution (SED) fitting technique is performed by using GOSSIP+, a tool able to combine both photometric and spectroscopic information to extract the best value of the physical quantities of interest, and to consider the Intergalactic Medium (IGM) attenuation as a free parameter. We find that the main contribution to the scatter observed in the $SFR-M_{*}$ plane is the possibility of choosing between different families of SFHs in the SED fitting procedure, while the redshift range plays a minor role. The majority of the galaxies, at all cosmic times, are best-fit by models with SFHs characterized by a high $τ_{\rm infall}/ν$ ratio. We discuss the reliability of the presence of a small percentage of dusty and highly star forming galaxies, in the light of their detection in the FIR.

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arXiv2016

Chandra counterparts of CANDELS GOODS-S sources

Improving the capabilities of detecting faint X-ray sources is fundamental to increase the statistics on faint high-z AGN and star-forming galaxies. We performed a simultaneous Maximum Likelihood PSF fit in the [0.5-2] keV and [2-7] keV energy bands of the 4 Ms{\em Chandra} Deep Field South (CDFS) data at the position of the 34930 CANDELS H-band selected galaxies. For each detected source we provide X-ray photometry and optical counterpart validation. We validated this technique by means of a raytracing simulation. We detected a total of 698 X-ray point-sources with a likelihood $\mathcal{L}$$>$4.98 (i.e. $>$2.7$σ$). We show that the prior knowledge of a deep sample of Optical-NIR galaxies leads to a significant increase of the detection of faint (i.e. $\sim$10$^{-17}$ cgs in the [0.5-2] keV band) sources with respect to "blind" X-ray detections. By including previous X-ray catalogs, this work increases the total number of X-ray sources detected in the 4 Ms CDFS, CANDELS area to 793, which represents the largest sample of extremely faint X-ray sources assembled to date. Our results suggest that a large fraction of the optical counterparts of our X-ray sources determined by likelihood ratio actually coincides with the priors used for the source detection. Most of the new detected sources are likely star-forming galaxies or faint absorbed AGN. We identified a few sources sources with putative photometric redshift z$>$4. Despite the low number statistics and the uncertainties on the photo-z, this sample significantly increases the number of X--ray selected candidate high-z AGN.

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arXiv2016

The evolution of the equivalent width of the Ha emission line and specific star-formation rate in star-forming galaxies at 1<z<5

We present the results of a study which uses spectral energy distribution (SED) fitting to investigate the evolution of the equivalent width (EW) of the Halpha emission line in star-forming galaxies over the redshift interval 1 =1 in the CANDELS UDS and GOODS-S fields. Confining our analysis to a constant stellar mass range (9.5<log(M/Msun)<10.5), we find that the median EW(Ha) evolves only modestly with redshift, reaching a rest-frame value of EW(Ha)=301+/-30 Angs by redshift z~4.5. Furthermore, using estimates of star-formation rate (SFR) based on both UV luminosity and Ha line flux, we use our galaxy samples to compare the evolution of EW(Ha) and specific star-formation rate (sSFR). Our results indicate that over the redshift range 1<z<5, the evolution displayed by EW(Ha) and sSFR is consistent, and can be adequately parameterized as: propto (1+z)^(1.0+/-0.2). As a consequence, over this redshift range we find that the sSFR and rest-frame EW(Ha) of star-forming galaxies with stellar masses M~10^(10) Msun are related by: EW(Ha)/Ang=(63+/-7)sSFR/Gyr^(-1). Given the current uncertainties in measuring the SFRs of high-redshift galaxies, we conclude that EW(Ha) provides a useful independent tracer of sSFR for star-forming galaxies out to redshifts of z=5.

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arXiv2016

Constraints on photoionization feedback from number counts of ultra-faint high-redshift galaxies in the Frontier Fields

We exploit a sample of ultra-faint high-redshift galaxies (demagnified HST $H_{160}$ magnitude $>30$) in the Frontier Fields clusters A2744 and M0416 to constrain a theoretical model for the UV luminosity function (LF) in the presence of photoionization feedback. The objects have been selected on the basis of accurate photometric redshifts computed from multi-band photometry including 7 HST bands and deep $K_s$ and IRAC observations. Magnification is computed on an object-by-object basis from all available lensing models of the two clusters. We take into account source detection completeness as a function of luminosity and size, magnification effects and systematics in the lens modeling of the clusters under investigation. We find that our sample of high-$z$ galaxies constrain the cut-off halo circular velocity below which star-formation is suppressed by photo-ionization feedback to $v_c^{\rm cut} < 50$ km s$^{-1}$. This circular velocity corresponds to a halo mass of $\approx5.6\times10^9~M_\odot$ and $\approx2.3\times10^9~M_\odot$ at $z=5$ and 10 respectively: higher mass halos can thus sustain continuous star formation activity without being quenched by external ionizing flux. More stringent constraints are prevented by the uncertainty in the modeling of the cluster lens, as embodied by systematic differences among the lens models available.

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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

The ASTRODEEP Frontier Fields Catalogues: I - Multiwavelength photometry of Abell-2744 and MACS-J0416

We present multiwavelength photometric catalogues (HST, Spitzer and Hawk-I K band) for the first two of the Frontier Fields, Abell2744 and MACSJ0416 (plus their parallel fields). To detect faint sources even in the central regions of the clusters, we develop a robust and repeatable procedure that uses the public codes Galapagos and Galfit to model and remove most of the light contribution from both the brightest cluster members as well as the ICL. We perform the detection on the HST H160 processed image to obtain a pure H-selected sample. We also add a sample of sources which are undetected in the H160 image but appear on a stacked infrared image. Photometry in the other HST bands is obtained using SExtractor, performed again on residual images after the Galfit procedure for foreground light removal. Photometry on the Hawk-I and IRAC bands has been obtained using our PSF-matching deconfusion code T-PHOT. A similar procedure, but without the need for the foreground light removal, is adopted for the Parallel fields. The procedure allows for the detection and the photometric measurements of ~2500 sources per field. We deliver and release complete photometric H-detected catalogues, with the addition of a complementary sample of infrared-detected sources. All objects have multiwavelength coverage including B to H HST bands, plus K band from Hawk-I, and 3.6 - 4.5 μm from Spitzer. Full and detailed treatment of photometric errors is included. We perform basic sanity checks on the reliability of our results. The multiwavelength catalogues are publicly available and are ready to be used for scientific purposes. Our procedures allows for the detection of outshined objects near the bright galaxies, which, coupled with the magnification effect of the clusters, can reveal extremely faint high redshift sources. Full analysis on photometric redshifts is presented in a companion Paper II. [abridged]

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arXiv2016

The ASTRODEEP Frontier Fields Catalogues: II - Photometric redshifts and rest-frame properties in Abell-2744 and MACS-J0416

We present the first public release of photometric redshifts, galaxy rest-frame properties and associated magnification values in the cluster and parallel pointings of the first two Frontier Fields, Abell-2744 and MACS-J0416. We exploit a multi-wavelength catalogue ranging from HST to ground-based K and Spitzer IRAC which is specifically designed to enable detection and measurement of accurate fluxes in crowded cluster regions. The multi-band information is used to derive photometric redshifts and physical properties of sources detected either in the H-band image alone or from a stack of four WFC3 bands. To minimize systematics median photometric redshifts are assembled from six different approaches to photo-z estimates. Their reliability is assessed through a comparison with available spectroscopic samples. State of the art lensing models are used to derive magnification values on an object-by-object basis by taking into account sources positions and redshifts. We show that photometric redshifts reach a remarkable ~3-5% accuracy. After accounting for magnification the H band number counts are found in agreement at bright magnitudes with number counts from the CANDELS fields, while extending the presently available samples to galaxies intrinsically as faint as H160~32-33 thanks to strong gravitational lensing. The Frontier Fields allow to probe the galaxy stellar mass distribution at 0.5-1.5 dex lower masses, depending on magnification, with respect to extragalactic wide fields, including sources at Mstar~ 10^7-10^8 Msun at z>5. Similarly, they allow the detection of objects with intrinsic SFRs>1dex lower than in the CANDELS fields reaching 0.1-1 Msun/yr at z~6-10. [abridged]

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arXiv2016

The VIMOS Ultra Deep Survey First Data Release: spectra and spectroscopic redshifts of 698 objects up to z~6 in CANDELS

This paper describes the first data release (DR1) of the VIMOS Ultra Deep Survey (VUDS). The DR1 includes all low-resolution spectroscopic data obtained in 276.9 arcmin2 of the CANDELS-COSMOS and CANDELS-ECFDS survey areas, including accurate spectroscopic redshifts z_spec and individual spectra obtained with VIMOS on the ESO-VLT. A total of 698 objects have a measured redshift, with 677 galaxies, two type-I AGN and a small number of 19 contaminating stars. The targets of the spectroscopic survey are selected primarily on the basis of their photometric redshifts to ensure a broad population coverage. About 500 galaxies have z_spec>2, 48 with z_spec>4, and the highest reliable redshifts reach beyond z_spec=6. This dataset approximately doubles the number of galaxies with spectroscopic redshifts at z>3 in these fields. We discuss the general properties of the sample in terms of the spectroscopic redshift distribution, the distribution of Lyman-alpha equivalent widths, and physical properties including stellar masses M_star and star formation rates (SFR) derived from spectral energy distribution fitting with the knowledge of z_spec. We highlight the properties of the most massive star-forming galaxies, noting the large range in spectral properties, with Lyman-alpha in emission or in absorption, and in imaging properties with compact, multi-component or pair morphologies. We present the catalogue database and data products. All data are publicly available and can be retrieved from a dedicated query-based database available at http://cesam.lam.fr/vuds.

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arXiv2016

Size evolution of star-forming galaxies with $2<z<4.5$ in the VIMOS Ultra-Deep Survey

We measure galaxy sizes on a sample of $\sim1200$ galaxies with confirmed spectroscopic redshifts $2 \leq z_{spec} \leq 4.5$ in the VIMOS Ultra Deep Survey (VUDS), representative of star-forming galaxies with $i_\mathrm{AB} \leq 25$. We first derive galaxy sizes applying a classical parametric profile fitting method using GALFIT. We then measure the total pixel area covered by a galaxy above a given surface brightness threshold, which overcomes the difficulty of measuring sizes of galaxies with irregular shapes. We then compare the results obtained for the equivalent circularized radius enclosing 100\% of the measured galaxy light $r_T^{100}$ to those obtained with the effective radius $r_{e,\mathrm{circ}}$ measured with GALFIT. We find that the sizes of galaxies computed with our non-parametric approach span a large range but remain roughly constant on average with a median value $r_T^{100}\sim2.2$ kpc for galaxies with $2<z<4.5$. This is in stark contrast with the strong downward evolution of $r_e$ with increasing redshift, down to sizes of $<1$ kpc at $z\sim4.5$. We analyze the difference and find that parametric fitting of complex, asymmetric, multi-component galaxies is severely underestimating their sizes. By comparing $r_T^{100}$ with physical parameters obtained through SED fitting we find that the star-forming galaxies that are the largest at any redshift are, on average, more massive and more star-forming. We discover that galaxies present more concentrated light profiles as we move towards higher redshifts. We interpret these results as the signature of several, possibly different, evolutionary paths of galaxies in their early stages of assembly, including major and minor merging or star-formation in multiple bright regions. (abridged)

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arXiv2016

Hubble imaging of the ionizing radiation from a star-forming galaxy at z=3.2 with fesc>50%

Star-forming galaxies are considered to be the leading candidate sources that dominate the cosmic reionization at z>7, and the search for analogs at moderate redshift showing Lyman continuum (LyC) leakage is currently a active line of research. We have observed a star-forming galaxy at z=3.2 with Hubble/WFC3 in the F336W filter, corresponding to the 730-890A rest-frame, and detect LyC emission. This galaxy is very compact and also has large Oxygen ratio [OIII]5007/[OII]3727 (>=10). No nuclear activity is revealed from optical/near-infrared spectroscopy and deep multi-band photometry (including the 6Ms X-ray, Chandra). The measured escape fraction of ionizing radiation spans the range 50-100\%, depending on the IGM attenuation. The LyC emission is detected at S/N=10 with m(F336W)=27.57+/-0.11 and it is spatially unresolved, with effective radius R_e<200pc. Predictions from photoionization and radiative transfer models are in line with the properties reported here, indicating that stellar winds and supernova explosions in a nucleated star-forming region can blow cavities generating density-bounded conditions compatible with optically thin media. Irrespective to the nature of the ionizing radiation, spectral signatures of these sources over the entire electromagnetic spectrum are of central importance for their identification during the epoch of reionization, when the LyC is unobservable. Intriguingly, the Spitzer/IRAC photometric signature of intense rest-frame optical emissions ([OIII]+Hbeta) observed recently at z~7.5-8.5 is similar to what is observed in this galaxy. Only the James Webb Space Telescope will measure optical line ratios at z>7 allowing a direct comparison with lower redshift LyC emitters, as reported here.

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