HST / ACS Lyman α imaging of the nearby starburst ESO 338 - IG 04 ⋆

ESO 338-IG04 (Tololo 1924-416) is a well known, luminous (M V = −19.3) Blue Compact Galaxy in the local universe. It has a complex morphology indicating a recent merger and/or close interaction and contains a central young starburst with compact star clusters of ages 40Myr. The galaxy was imaged using the Advanced Camera for Surveys (ACS) onboard the Hubble Space Telescope (HST) in the Lyman α 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 Lyα line. The method allows us to disentangle the degenerate effects of age and reddening by careful sampling of the UV continuum slope and 4000Å discontinuity. Our results are in qualitative agreement with the models of Lyα escape being regulated by kinematical properties of the interstellar medium. The line-only image shows Lyα in both emission and absorption. Most notably, Lyα emission is seen from central bright young clusters and is in spatial agreement with the features present in a longslit spectrum taken with the Space Telescope Imaging Spectrograph. The Lyα is also seen in diffuse emission regions surrounding the central starburst where photons escape after one or more resonant scatterings in H. Quantitative photometry reveals a total flux in the Lyα line of f Lyα,TOT = 194 × 10 −14 erg s −1 cm −2. The Lyα flux in a 10 × 20 ′′ elliptical aperture centred on the brightest central star cluster measures f Lyα,IUE = 134 × 10 −14 erg s −1 cm −2 with an equivalent width of 22.6Å. 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 Lyα maps of nearby galaxies. Analysis of parameter dependencies show our technique to be largely parameter independent, producing Lyα maps indistinguishable from one another by eye and with Lyα fluxes consistent with one another to better than 50%. We see large amounts of diffuse Lyα emission that dominates the total Lyα output which are interpreted as centrally produced Lyα photons scattered by neutral hydrogen. By comparison of Lyα fluxes with Hα fluxes of a previous study, we estimate that each observed Lyα photon has undergone 2 additional scatterings. We …