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Register a new userLy${alpha}$-Lyman Continuum connection in 3.5 < z < 4.3 star-forming galaxies from the VUDS survey
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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<z<4.3 to explore the validity of some of the proposed indirect indicators of LyC radiation. We created subsamples of galaxies with EWLya>70{AA}, 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>70{AA}), 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.
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Questions as to what drove the bulk reionization of the Universe, how that reionization proceeded, and how the hard ionizing radiation reached the intergalactic medium remain open and debated. Observations probing that epoch are severely hampered by the increasing amounts of neutral gas with increasing redshift, so a small, but growing number of experiments are targeting star forming galaxies ($zsim3$) as proxies. However, these studies, while providing fantastic detail, are time intensive, contain relatively few targets, and can suffer from selection biases. As a complementary alternative, we investigate whether stacking the already vast (and growing) numbers of low-resolution ($Delta lambda / lambda = 800$) Lyman-$alpha$ Emitting (LAE) galaxy spectra from the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) can be used to measure ionizing photons (restframe 880-910AA) escaping their galaxy hosts. As a blind survey, HETDEX avoids the biases from continuum selected galaxies and its planned 540 square degree coverage promotes the statistical power of large numbers. In this paper, we confirm the feasibility of Lyman continuum detection by carefully selecting a sample of lyccount high redshift ($zsim$3) LAEs from a subset of HETDEX observations, stacking their spectra and measuring a $gtrsim$3$sigma$ detection of $0.10 mu$Jy restframe Lyman continuum emission, uncorrected for attenuation in the intergalactic medium, over the full sample stack ($3.0 < z < 3.5$ and $-22.0 lesssim M_{text{UV}} lesssim -19.0$).
We present the observations of Lyman continuum (LyC) emission in the afterglow spectra of GRB 191004B at $z=3.5055$, together with those of the other two previously known LyC-emitting long gamma-ray bursts (LGRB) (GRB 050908 at $z=3.3467$, and GRB 060607A at $z=3.0749$), to determine their LyC escape fraction and compare their properties. From the afterglow spectrum of GRB 191004B we determine a neutral hydrogen column density at the LGRB redshift of $log(N_{rm HI}/cm^{-2})= 17.2 pm 0.15$, and negligible extinction ($A_{rm V}=0.03 pm 0.02$ mag). The only metal absorption lines detected are CIV and SiIV. In contrast to GRB 050908 and GRB 060607A, the host galaxy of GRB 191004B displays significant Ly$alpha$ emission. From its Ly$alpha$ emission and the non-detection of Balmer emission lines we constrain its star-formation rate (SFR) to $1 leq$ SFR $leq 4.7$ M$_{odot} yr^{-1}$. We fit the Ly$alpha$ emission with a shell model and find parameters values consistent with the observed ones. The absolute LyC escape fractions we find for GRB 191004B, GRB 050908 and GRB 060607A are of $0.35^{+0.10}_{-0.11}$, $0.08^{+0.05}_{-0.04}$ and $0.20^{+0.05}_{-0.05}$, respectively. We compare the LyC escape fraction of LGRBs to the values of other LyC emitters found from the literature, showing that LGRB afterglows can be powerful tools to study LyC escape for faint high-redshift star-forming galaxies. Indeed we could push LyC leakage studies to much higher absolute magnitudes. The host galaxies of the three LGRB presented here have all $M_{rm 1600} > -19.5$ mag, with the GRB 060607A host at $M_{rm 1600} > -16$ mag. LGRB hosts may therefore be particularly suitable for exploring the ionizing escape fraction in galaxies that are too faint or distant for conventional techniques. Furthermore the time investment is very small compared to galaxy studies. [Abridged]
A large number of high-redshift galaxies have been discovered via their narrow-band Lya line or broad-band continuum colors in recent years. The nature of the escaping process of photons from these early galaxies is crucial to understanding galaxy evolution and the cosmic reionization. Here, we investigate the escape of Lya, non-ionizing UV-continuum (l = 1300 - 1600 angstrom in rest frame), and ionizing photons (l < 912 angstrom) from galaxies by combining a cosmological hydrodynamic simulation with three-dimensional multi-wavelength radiative transfer calculations. The galaxies are simulated in a box of 5^3 h^-3 Mpc^3 with high resolutions using the Aquila initial condition which reproduces a Milky Way-like galaxy at redshift z=0. We find that the escape fraction (fesc) of these different photons shows a complex dependence on redshift and galaxy properties: fesc(Lya) and fesc(UV) appear to evolve with redshift, and they show similar, weak correlations with galaxy properties such as mass, star formation, metallicity, and dust content, while fesc(Ion) remains roughly constant at ~ 0.2 from z ~ 0 - 10, and it does not show clear dependence on galaxy properties. fesc(Lya) correlates more strongly with fesc(UV) than with fesc(Ion). In addition, we find a relation between the emergent Lya luminosity and the ionizing photon emissivity of Lyman Alpha Emitters (LAEs). By combining this relation with the observed luminosity functions of LAEs at different redshift, we estimate the contribution from LAEs to the reionization of intergalactic medium (IGM). Our result suggests that ionizing photons from LAEs alone are not sufficient to ionize IGM at z > 6, but they can maintain the ionization of IGM at z ~ 0 - 5.
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 $zsim 4$ 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 $880-910AA$. 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 $895AA$) and non-ionizing emission (at $sim 1500 AA$) for all individual sources. We also produce a normalized stacked spectrum of all sources. Assuming an intrinsic average $L_{ u}(1470)/L_{ u}(895)$ 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$alpha$ equivalent width (EW). From the stacked spectrum and assuming a mean transmissivity for the sample, we measure a relative escape fraction $f_{esc}^{rel}=0.09pm0.04$. 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$alpha$ emission line. Our results imply that the LyC flux emitted by $V=25-26$ star-forming galaxies at z$sim$4 is at most very modest, in agreement with previous upper limits from studies based on broad and narrow band imaging.
Spatially resolved studies of high redshift galaxies, an essential insight into galaxy formation processes, have been mostly limited to stacking or unusually bright objects. We present here the study of a typical (L$^{*}$, M$_star$ = 6 $times 10^9$ $M_odot$) young lensed galaxy at $z=3.5$, observed with MUSE, for which we obtain 2D resolved spatial information of Ly$alpha$ and, for the first time, of CIII] emission. The exceptional signal-to-noise of the data reveals UV emission and absorption lines rarely seen at these redshifts, allowing us to derive important physical properties (T$_esim$15600 K, n$_esim$300 cm$^{-3}$, covering fraction f$_csim0.4$) using multiple diagnostics. Inferred stellar and gas-phase metallicities point towards a low metallicity object (Z$_{mathrm{stellar}}$ = $sim$ 0.07 Z$_odot$ and Z$_{mathrm{ISM}}$ $<$ 0.16 Z$_odot$). The Ly$alpha$ emission extends over $sim$10 kpc across the galaxy and presents a very uniform spectral profile, showing only a small velocity shift which is unrelated to the intrinsic kinematics of the nebular emission. The Ly$alpha$ extension is $sim$4 times larger than the continuum emission, and makes this object comparable to low-mass LAEs at low redshift, and more compact than the Lyman-break galaxies and Ly$alpha$ emitters usually studied at high redshift. We model the Ly$alpha$ line and surface brightness profile using a radiative transfer code in an expanding gas shell, finding that this model provides a good description of both observables.
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