On The Origin of Lyman-alpha Absorption in Nearby Starbursts and Implications for Other Galaxies

(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 explained. 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.Comment: 11 pages, 10 figures. Accepted for publication in Astronomy and Astrophysic

A CANDELS WFC3 Grism Study of Emission-Line Galaxies at z ~ 2: A Mix of Nuclear Activity and Low-Metallicity Star Formation

We present Hubble Space Telescope Wide Field Camera 3 (WFC3) slitless grism spectroscopy of 28 emission-line galaxies at z ~ 2, in the GOODS-S region of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey. The high sensitivity of these grism observations, with >1σ detections of emission lines to f > 2.5 × 10^(–18) erg s^(–1) cm^(–2), means that the galaxies in the sample are typically ~7 times less massive (median M_* = 10^(9.5) M_☉) than previously studied z ~ 2 emission-line galaxies. Despite their lower mass, the galaxies have [O III]/Hβ ratios which are very similar to previously studied z ~ 2 galaxies and much higher than the typical emission-line ratios of local galaxies. The WFC3 grism allows for unique studies of spatial gradients in emission lines, and we stack the two-dimensional spectra of the galaxies for this purpose. In the stacked data the [O III] emission line is more spatially concentrated than the Hβ emission line with 98.1% confidence. We additionally stack the X-ray data (all sources are individually undetected), and find that the average L_([O III])/L_(0.5-10keV) ratio is intermediate between typical z ~ 0 obscured active galaxies and star-forming galaxies. Together the compactness of the stacked [O III] spatial profile and the stacked X-ray data suggest that at least some of these low-mass, low-metallicity galaxies harbor weak active galactic nuclei

First consistent Lyα profile and UV spectral modeling of z ~ 3 LGBs with a 3D radiative transfer code

We developped a 3D Monte Carlo Lyα radiation transfer code to understand the diversity of Lyα line profiles observed in star forming galaxies and related objects (Verhamme et al. 2006). Our code allows for prescribed arbitrary hydrogen density, ionisation, temperature structures, and dust distributions, and arbitrary velocity fields and UV photon sources.Here we present results from the first modelling of the Lyα line and of the UV spectrum with our code of a sample of z ~ 3 Lyman break galaxies observed by Steidel and collaborators (Pettini et al. 2002) and taken from the FORS Deep Field (Tapken et al. 2006). A simple model of an expanding neutral shell surrounding a starburst region can reproduce the whole variety of spectra ranging from double-peaked profiles to asymetric emission lines, P-Cygni profiles or broad absorption. The main determining parameters are the outflow velocity and the dust content. Other parameters such as the hydrogen column density, the intrinsic Lyα emission and hence SFR, and the intrinsic Lyα line widths can be determined consistently taking all radiation transfer effects into accoun

High-redshift galaxies and black holes in the eyes of JWST: a population synthesis model from infrared to X-rays

The first billion years of the Universe is a pivotal time: stars, black holes (BHs) and galaxies form and assemble, sowing the seeds of galaxies as we know them today. Detecting, identifying and understand- ing the first galaxies and BHs is one of the current observational and theoretical challenges in galaxy formation. In this paper we present a population synthesis model aimed at galaxies, BHs and Active Galactic Nuclei (AGNs) at high redshift. The model builds a population based on empirical relations. Galaxies are characterized by a spectral energy distribution determined by age and metallicity, and AGNs by a spectral energy distribution determined by BH mass and accretion rate. We validate the model against observational constraints, and then predict properties of galaxies and AGN in other wavelength and/or luminosity ranges, estimating the contamination of stellar populations (normal stars and high-mass X-ray binaries) for AGN searches from the infrared to X-rays, and vice-versa for galaxy searches. For high-redshift galaxies, with stellar ages < 1 Gyr, we find that disentangling stellar and AGN emission is challenging at restframe UV/optical wavelengths, while high-mass X-ray binaries become more important sources of confusion in X-rays. We propose a color-color selection in JWST bands to separate AGN vs star-dominated galaxies in photometric observations. We also esti- mate the AGN contribution, with respect to massive, hot, metal-poor stars, at driving high ionization lines, such as C IV and He II. Finally, we test the influence of the minimum BH mass and occupa- tion fraction of BHs in low mass galaxies on the restframe UV/near-IR and X-ray AGN luminosity function.Comment: Accepted for publication in the Astrophysical Journa

Physical properties and evolutionary state of the Lyman alpha emitting starburst galaxy IRAS 08339+6517

Though Lyman alpha emission (Lya) is one of the most used tracers of massive star formation at high redshift, a correct understanding of radiation transfer effects by neutral gas is required to properly quantify the star formation rate along the history of the Universe. We are embarked in a program to study the properties of the Lya emission (spectral profile, spatial distribution, relation to Balmer lines intensity,...) in several local starburst galaxies. We present here the results obtained for IRAS 08339+6517. Using evolutionary population synthesis models, we have characterized the properties of the starburst (UV continuum, Halpha, total infrared and X-ray emissions, etc.), which transformed 1.4e+8 Mo of gas into stars around 5-6 Myr ago. In addition to the central compact emission blob, we have identified a diffuse Lya emission component smoothly distributed over the whole central area of IRAS 08339+6517. This diffuse emission is spatially decoupled from the UV continuum, the Halpha emission or the Halpha/Hbeta ratio. Both locally and globally, the Lya/Halpha ratio is lower than the Case B predictions, even after reddening correction, with an overall Lya escape fraction of only 4%. We conclude that in IRAS 08339+6517 the resonant scattering of Lya photons by an outflowing shell of neutral gas causes their highly-efficient destruction by dust, which explains the low Lya escape fraction measured. These results stress again the importance of a proper correction of scattering and transfer effects when using Lya to derive the star formation rate in high-redshift galaxies.Comment: Accepted for publication in A&A, 17 pages, 13 figures, 8 tables. If problems with quality of images, see https://cloud.cab.inta-csic.es/public.php?service=files&file=%2Fotih%2Ffiles%2Foti_mas%2Firas%2Firas-v53.ack_referee.pd

Multiwavelength analysis of the Lyman alpha emitting galaxy Haro 2: relation between the diffuse Lyman alpha and soft X-ray emissions

In order to use Lyman alpha (Lya) emission as star formation tracer in cosmological studies, we must understand how the resonant scattering affects the escape fraction of the Lya photons. Thus, high spatial resolution multiwavelength studies of nearby Lya emitters, like Haro 2, are highly needed. For that purpose, we have used Chandra X-ray and HST (UV, optical and NIR) images of Haro 2, and STIS and ground-based spectral images along its major and minor axes, to characterize the Lya emission and the properties of the stellar population. The UV, Ha (Halpha) and FIR luminosities of the Haro 2 nuclear starburst are reproduced using evolutionary synthesis models assuming a young stellar population with ages ~3.5-5.0 Myr, affected by differential interstellar extinctions. The observed X-ray emission is attributed to gas heated by the mechanical energy released by the starburst (soft component) and a Ultra-Luminous X-ray source candidate (hard). Both compact and diffuse Lya components are observed. Whereas Lya is spatially decoupled from Balmer lines emission, Balmer decrement and UV continuum, the diffuse Lya component is spatially correlated with the diffuse soft X-ray emission. Moreover, unlike the compact one, diffuse Lya shows luminosities larger than predicted from Ha, assuming case B recombination and dust extinction as derived from Ha/Hbeta. We propose that, whereas the compact Lya emission is associated to the massive stellar clusters and is affected by outflows and dust extinction, the diffuse Lya originates in gas ionized by the hot plasma responsible for the soft X-ray radiation, as suggested by their spatial correlation and by the measured L(Ha)/LsoftX ratios. Calibration of Lya as star formation rate tracer should therefore include both effects (destruction vs. enhancement) to avoid biases in the study of galaxies at cosmological distances.Comment: Accepted for publication in A&A, 18 pages, 8 figures, 9 tables. If problems with quality of images, see http://www.cab.inta-csic.es/users/otih/haro2-v63.clean.pd

Empirical Estimate of Lyman-alpha Escape Fraction in a Statistical Sample of Lyman-alpha Emitters

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 data 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).Comment: 4 pages, 2 figures, accepted for publication in Astronomy and Astrophysic

Extremely Small Sizes for Faint z~2-8 Galaxies in the Hubble Frontier Fields: A Key Input For Establishing their Volume Density and UV Emissivity

We provide the first observational constraints on the sizes of the faintest galaxies lensed by the Hubble Frontier Fields (HFF) clusters. Ionizing radiation from faint galaxies likely drives cosmic reionization, and the HFF initiative provides a key opportunity to find such galaxies. Yet, we cannot really assess their ionizing emissivity without a robust measurement of their sizes, since this is key to quantifying both their prevalence and the faint-end slope to the UV luminosity function. Here we provide the first such size constraints with 2 new techniques. The first utilizes the fact that the detectability of highly-magnified galaxies as a function of shear is very dependent on a galaxy's size. Only the most compact galaxies will remain detectable in regions of high shear (vs. a larger detectable size range for low shear), a phenomenon we carefully quantify using simulations. Remarkably, however, no correlation is found between the surface density of faint galaxies and the predicted shear, using 87 faint high-magnification mu>10 z~2-8 galaxies seen behind the first 4 HFF clusters. This can only be the case if such faint (~-15 mag) galaxies have significantly smaller sizes than luminous galaxies. We constrain their half-light radii to be <~30 mas (<160-240 pc). As a 2nd size probe, we rotate and stack 26 faint high-magnification sources along the major shear axis. Less elongation is found than even for objects with an intrinsic half-light radius of 10 mas. Together these results indicate that extremely faint z~2-8 galaxies have near point-source profiles in the HFF dataset (half-light radii conservatively <30 mas and likely 5-10 mas). These results suggest smaller completeness corrections and hence much lower volume densities for faint z~2-8 galaxies and shallower faint-end slopes than have been derived in many recent studies (by factors of ~2-3 and by dalpha>~0.1-0.3).Comment: 19 pages, 15 figures, 3 tables, accepted for publication in Ap

Grid of Lya radiation transfer models for the interpretation of distant galaxies

Lya is a key diagnostic for numerous observations of distant star-forming galaxies. It's interpretation requires, however, detailed radiation transfer models. We provide an extensive grid of 3D radiation transfer models simulating the Lya and UV continuum radiation transfer in the interstellar medium of star-forming galaxies. We have improved our Monte Carlo MCLya code, and have used it to compute a grid of 6240 radiation transfer models for homogeneous spherical shells containing HI and dust surrounding a central source. The simulations cover a wide range of parameter space. We present the detailed predictions from our models including in particular the Lya escape fraction fesc, the continuum attenuation, and detailed Lya line profiles. The Lya escape fraction is shown to depend strongly on dust content, but also on other parameters (HI column density and radial velocity). The predicted line profiles show a great diversity of morphologies ranging from broad absorption lines to emission lines with complex features. The results from our simulations are distributed in electronic format. Our models should be of use for the interpretation of observations from distant galaxies, for other simulations, and should also serve as an important base for comparison for future, more refined, radiation transfer models.Comment: Accepted for publication in Astronomy & Astrophysics. Results from simulations available at http://obswww.unige.ch/sf

The first Frontier Fields cluster: 4.5{\mu}m excess in a z~8 galaxy candidate in Abell 2744

We present in this letter the first analysis of a z~8 galaxy candidate found in the Hubble and Spitzer imaging data of Abell 2744, as part of the Hubble Frontier Fields legacy program. We applied the most commonly-used methods to select exceptionally high-z galaxies by combining non-detection and color-criteria using seven HST bands. We used GALFIT on IRAC images for fitting and subtracting contamination of bright nearby sources. The physical properties have been inferred from SED-fitting using templates with and without nebular emission. This letter is focussed on the brightest candidate we found (m$_{F160W}$=26.2) over the 4.9 arcmin$^2$ field of view covered by the WFC3. It shows a non-detection in the ACS bands and at 3.6{\mu}m whereas it is clearly detected at 4.5{\mu}m with rather similar depths. This break in the IRAC data could be explained by strong [OIII]+H{\beta} lines at z~8 which contribute to the 4.5{\mu}m photometry. The best photo-z is found at z~8.0$^{+0.2}_{-0.5}$, although solutions at low-redshift (z~1.9) cannot be completely excluded, but they are strongly disfavoured by the SED-fitting work. The amplification factor is relatively small at {\mu}=1.49$\pm$0.02. The Star Formation Rate in this object is ranging from 8 to 60 Mo/yr, the stellar mass is in the order of M$_{\star}$=(2.5-10) x 10$^{9}$Mo and the size is r~0.35$\pm$0.15 kpc. This object is one of the first z~8 LBG candidates showing a clear break between 3.6{\mu}m and 4.5{\mu}m which is consistent with the IRAC properties of the first spectroscopically confirmed galaxy at a similar redshift. Due to its brightness, the redshift of this object could potentially be confirmed by near infrared spectroscopy with current 8-10m telescopes. The nature of this candidate will be revealed in the coming months with the arrival of new ACS and Spitzer data, increasing the depth at optical and near-IR wavelengths.Comment: 4 pages, 2 figures, Accepted for publication in Astronomy and Astrophysics Letter