HST/ACS Lyman α imaging of the nearby starburst ESO 338-IG04

ESO338-IG04 (Tololo 1924-416) is a well-known, luminous (M-V = -19.3) Blue Compact Galaxy in the local universe. Its complex morphology indicates a recent merger and/or close interaction, and it contains a central young starburst with compact star clusters of ages less than or similar to 40 Myr. The galaxy was imaged using the Advanced Camera for Surveys onboard the Hubble Space Telescope (HST) in the Lyman alpha line and continuum. Using the Starburst99 synthetic spectra and other imaging data from the Wide Field and Planetary Camera 2, we developed a technique that allows us to make the first photometrically valid subtraction of continuum from the Ly alpha line. The method allows us to disentangle the degenerate effects of age and reddening by careful sampling of the UV continuum slope and 4000 angstrom discontinuity. Our results are in qualitative agreement with the models of Lya escape being regulated by kinematical properties of the interstellar medium. The line-only image shows Lya in both emission and absorption. Most notably, Lya emission is seen from central bright young clusters and is in spatial agreement with features present in a longslit spectrum taken with the Space Telescope Imaging Spectrograph. Lya is also seen in diffuse emission regions surrounding the central starburst where photons escape after one or more resonant scatterings in HI. Quantitative photometry reveals a total flux in the Ly alpha line of f(Ly alpha,TOT) = 194 x 10(-14) erg s(-1) cm(-2). The Ly alpha flux in a 10 x 20 elliptical aperture centred on the brightest central star cluster measures f(Ly alpha,IUE) = 134 x 10(-14) erg s(-1) cm(-2) with an equivalent width of 22.6 angstrom. This is in close agreement with previous studies made using spectra from the IUE satellite to which our aperture was created to match. Thus we demonstrate that we have software in place to create line-only Lya maps of nearby galaxies. Analysis of parameter dependencies show our technique to be largely parameter independent, producing Lya maps indistinguishable from one another by eye and with Ly alpha fluxes consistent with one another to better than 50%. We see large amounts of diffuse Ly alpha emission that dominates the total Ly alpha output which are interpreted as centrally produced Ly alpha photons scattered by neutral hydrogen. By comparison of Lya fluxes with Ha fluxes of a previous study, we estimate that each observed Ly alpha photon has undergone greater than or similar to 2 additional scatterings. We see that Ly alpha line kinematics closely correlate with other kinematic tracers but, within these data, find no evidence for Lya emission or absorption from star clusters being a function of age.