We present reduced and calibrated high resolution Lyman-alpha (Lya) images for a sample of 6 local star forming galaxies. Targets were selected to represent a range in luminosity and metallicity and to include both known Lya emitters and non-emitters
. Far ultraviolet imaging was carried out with the Solar Blind Channel of the ACS on HST in the F122M (Lya on-line) and F140LP (continuum) filters. The resulting Lya images are the product of careful modeling of both the stellar and nebular continua, facilitated by supporting HST imaging in Ha and 5 continuum bands, combined with Starburst99 evolutionary synthesis models, and prescriptions for dust extinction on the continuum. In all, the resulting morphologies in Lya, Ha, and UV-continuum are qualitatively very different and we show that the bulk of Lya emerges in a diffuse component resulting from resonant scattering events. Lya escape fractions, computed from integrated Ha luminosities and recombination theory, are found never to exceed 14%. Internal dust extinction is estimated in each pixel and used to correct Lya fluxes. However, the extinction corrections are far too small (factors from 2.6 to infinity) to reconcile the global Lya luminosities with recombination theory. Surprisingly, when comparing the global equivalent widths of Lya and Ha, the two quantities appear anti-correlated, which may be due to the evolution of mechanical feedback. This calls for caution in the interpretation of Lya observations. The images presented have a physical resolution 3 orders of magnitude better than attainable at high-z from the ground with current instrumentation and our images may therefore serve as useful templates for comparing with observations and modeling of primeval galaxy formation. We therefore provide the reduced Lya, Ha, and continuum images to the community.
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.
