The Hubble Frontier Fields (HFF) program combines the capabilities of the Hubble Space Telescope (HST) with the gravitational lensing of massive galaxy clusters to probe the distant Universe to an unprecedented depth. Here, we present the results of
the first combined HST and Spitzer observations of the cluster Abell 2744. We combine the full near-infrared data with ancillary optical images to search for gravitationally lensed high-redshift (z > 6) galaxies. We report the detection of 15 I814-dropout candidates at z ~ 6-7 and one Y105-dropout at z ~ 8 in a total survey area of 1.43 arcmin^2 in the source plane. The predictions of our lens model allow us to also identify five multiply-imaged systems lying at redshifts between z ~ 6 and z ~ 8. Thanks to constraints from the mass distribution in the cluster, we were able to estimate the effective survey volume corrected for completeness and magnification effects. This was in turn used to estimate the rest-frame ultraviolet luminosity function (LF) at z ~ 6-8. Our LF results are generally in agreement with the most recent blank field estimates, confirming the feasibility of surveys through lensing clusters. Although based on a shallower observations than what will be achieved in the final dataset including the full ACS observations, the LF presented here extends down to Muv ~ -18.5 at z ~ 7 with one identified object at Muv ~ -15 thanks to the highly-magnified survey areas. This early study forecasts the power of using massive galaxy clusters as cosmic telescopes and its complementarity to blank fields.
[abridged] Among the different observational techniques used to select high-redshift galaxies, the hydrogen recombination line Lyman-alpha (Lya) is of particular interest as it gives access to the measurement of cosmological quantities such as the st
ar formation rate of distant galaxy populations. However, the interpretation of this line and the calibration of such observables is still subject to serious uncertainties. Therefore, it important to understand under what conditions the Lya line can be used as a reliable star formation diagnostic tool. We use a sample of 24 Lya emitters at z ~ 0.3 with an optical spectroscopic follow-up to calculate the Lya escape fraction and its dependency upon different physical properties. We also examine the reliability of Lya as a star formation rate indicator. We combine these observations with a compilation of Lya emitters selected at z = 0 - 0.3 to assemble a larger sample. The Lya escape fraction depends clearly on the dust extinction following the relation fesc(Lya) = C(Lya) x 10^(-0.4 E(B-V) k(Lya)), but with a shallower slope than previously reported, with k(Lya) ~ 6.67 and C(Lya) = 0.22. However, the correlation does not follow the expected curve for a simple dust attenuation. We explore the various mechanisms than lead to fesc(Lya) values above the continuum extinction curve, i.e. to an enhancement of the Lya output. We also observe that the strength of Lya and the escape fraction appear unrelated to the galaxy metallicity. Regarding the reliability of Lya as a star formation rate (SFR) indicator, we show that the deviation of SFR(Lya) from the true SFR (as traced by the UV continuum) is a function of the observed SFR(UV), which can be seen as the decrease of fesc(Lya) with increasing UV luminosity. Moreover, we observe a redshift-dependence of this relationship revealing the underlying evolution of fesc(Lya) with redshift.
We present near-infrared emission line counts and luminosity functions from the HST WFC3 Infrared Spectroscopic Parallels (WISP) program for 29 fields (0.037 deg^2) observed using both the G102 and G141 grisms. Altogether we identify 1048 emission li
ne galaxies with observed equivalent widths greater than 40 Angstroms, 467 of which have multiple detected emission lines. The WISP survey is sensitive to fainter flux levels (3-5x10^-17 ergs/s/cm^2) than the future space near-infrared grism missions aimed at baryonic acoustic oscillation cosmology (1-4x10^-16 ergs/s/cm^2), allowing us to probe the fainter emission line galaxies that the shallower future surveys may miss. Cumulative number counts of 0.7<z<1.5 galaxies reach 10,000 deg^-2 above an H-alpha flux of 2x10^-16 ergs/s/cm^2. H-alpha-emitting galaxies with comparable [OIII] flux are roughly 5 times less common than galaxies with just H-alpha emission at those flux levels. Galaxies with low H-alpha/[OIII] ratios are very rare at the brighter fluxes that future near-infrared grism surveys will probe; our survey finds no galaxies with H-alpha/[OIII] < 0.95 that have H-alpha flux greater than 3x10^-16 ergs/s/cm^2. Our H-alpha luminosity function contains a comparable number density of faint line emitters to that found by the NICMOS near-infrared grism surveys, but significantly fewer (factors of 3-4 less) high luminosity emitters. We also find that our high redshift (z=0.9-1.5) counts are in agreement with the high redshift (z=1.47) narrow band H-alpha survey of HiZELS (Sobral et al. 2013), while our lower redshift luminosity function (z=0.3-0.9) falls slightly below their z=0.84 result. The evolution in both the H-alpha luminosity function from z=0.3--1.5 and the [OIII] luminosity function from z=0.7-2.3 is almost entirely in the L* parameter, which steadily increases with redshift over those ranges.
The Lyman-alpha (Lya) recombination line is a fundamental tool for galaxy evolution studies and modern observational cosmology. However, subsequent interpretations are still prone to a number of uncertainties. Besides numerical efforts, empirical dat
a are urgently needed for a better understanding of Lya escape process. We empirically estimate the Lyman-alpha escape fraction fesc(Lya) in a statistically significant sample of z ~ 0 - 0.3 galaxies in order to calibrate high-redshift Lyman-alpha observations. An optical spectroscopic follow-up of a sub-sample of 24 Lyman-alpha emitters (LAEs) detected by GALEX at z ~ 0.2-0.3, combined with a UV-optical sample of local starbursts, both with matched apertures, allow us to quantify the dust extinction through Balmer lines, and to estimate the Lyman-alpha escape fraction from the Halpha flux corrected for extinction in the framework of the recombination theory. The global escape fraction of Lyman-alpha radiation spans nearly the entire range of values, from 0.5 to 100 %, and fesc(Lya) clearly decreases with increasing nebular dust extinction E(B-V). Several objects show fesc(Lya) greater than fesc(continuum) which may be an observational evidence for clumpy ISM geometry or for an aspherical ISM. Selection biases and aperture size effects may still prevail between z ~ 0.2-0.3 LAEs and local starbursts, which may explain the difference observed for fesc(Lya).
(Abridged) Despite the privileged position that Lyman-alpha (Lya) emission line holds in the exploration of the distant universe and modern observational cosmology, the origin of the observed diversity of lya profiles remains to be thoroughly explain
ed. Observations of nearby star forming galaxies bring their batch of apparent contradictions between Lya emission and their physical parameters, and call for a detailed understanding of the physical processes at work. IZw 18, one of the most metal-poor galaxies known is of particular interest in this context. We use a 3D Lya radiation transfer code to model Hubble Space Telescope (HST) observations of IZw 18 and to fit its Lya spectrum. Different geometrical configurations of the source and the neutral gas are explored. The integrated Lya profile of NW region of IZw 18 is reproduced using the observed small amount of dust (E(B-V) ~ 0.05) and a spherical HI shell with N(HI) = 6.5 x 10^(21) cm^(-2). Such a high column density makes it possible to transform a strong Lya emission (EW(Lya) = 60 A) into a damped absorption even with a small extinction. When a slab geometry is applied and a given line of sight is chosen, the Lya profile can be successfully reproduced with no dust at all and N(HI) = 3 x 10^(21) cm^(-2). The spatial variations of the profile shape are naturally explained by radiation transfer effects. In the case of outflowing Inter Stellar Medium (ISM), as commonly observed in Lyman Break Galaxies (LBGs), a high N(H) and dust content are required to observe Lya in absorption. For nearly static neutral gas as observed in IZw 18 and other low luminosity galaxies only a small amount of dust is required provided a sufficiently high N(H) covers the galaxy.
Lyman-alpha is now widely used to investigate the galaxy formation and evolution in the high redshift universe. However, without a rigorous understanding of the processes which regulate the Lya escape fraction, physical interpretations of high-z obse
rvations remain questionable. We examine six nearby star-forming galaxies to disentangle the role of the dust from other parameters such as gas kinematics, geometry and ISM morphology in the obscuration of Ly-alpha. Thereby we aim to understand the Ly-a escape physics and infer the implications for high-redshift studies. We use HST/ACS to produce continuum-subtracted Lya maps, and ground-based observations (ESO/NTT and NOT) to map the Halpha emission and the extinction E(B-V) in the gas phase derived from the Balmer decrement Halpha/Hbeta. When large outflows are present, the Lya emission appears not to correlate with the dust content, confirming the role of the HI kinematics in the escape of Lya photons. In the case of a dense, static HI covering, we observe a damped absorption with a declining relationship between Lya and E(B-V). We found that the Lya escape fraction does not exceed 10% in all our galaxies and is mostly about 3% or below. Finally, because of the radiative transfer complexity of the Lya line, star formation rate based on Lya luminosity is underestimated with respect to that derived from UV luminosity. The failure of simple dust correction to recover the intrinsic Lya/Ha ratio or the total star formation rate should prompt us to be more cautious when interpreting high-z observations and related properties, such as SFRs based on Lya alone. To this end we propose a more realistic calibration for SFR(Lya) which accounts for dust attenuation and resonant scattering effects via the Lya escape fraction.
(Abridged) Lyman-alpha (Lya) is a dominant probe of the galaxy population at high-z. However, interpretation of data drawn from Lya alone hinges on the Lya escape fraction which, due to the complex radiative transport, may vary greatly. Here we map t
he Lya emission from local starburst Haro 11, a Lya emitter and the only known candidate for low-z Lyman continuum emission (LyC). To aid in the interpretation we perform a detailed multi-wavelength analysis and model the stellar population, dust distribution, ionising photon budget, and star-cluster population. We use archival X-ray observations to further constrain properties of the starburst and estimate the HI column density. The Lya morphology is found to be strongly decoupled from stellar and nebular (H-alpha) morphologies. General surface photometry finds only very slight correlation between Lya and H-halpha, E(B-V), and stellar age. Only around the central Lya-bright cluster do we find the Lya/Ha ratio at values predicted by recombination theory. The total Lya escape fraction is found to be just 3%. We compute that ~90% of the Lya photons that escape do so after undergoing multiple resonance scattering events, masking their point of origin. This leads to a largely symmetric distribution and, by increasing the distance that photons must travel to escape, decreases the escape probability significantly. While dust must ultimately be responsible for the destruction of Lya, it plays little role in governing the observed morphology, which is regulated more by ISM kinematics and geometry. We find tentative evidence for local Lya equivalent width in the immediate vicinity of star-clusters being a function of cluster age, consistent with hydrodynamic studies. We estimate the ionising photon production and further constrain the escape fraction at 900 AA to <~9% .
