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

Abstract

Abstract. ESO 338-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 40 Myr. The galaxy was imaged using the Advanced Camera for Surveys 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 4000 Å 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 features present in a longslit spectrum taken with the Space Telescope Imaging Spectrograph. 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 22.6 Å. 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 see that Lyα line kinematics closely correlate with other kinematic tracers but, within these data, find no evidence for Lyα emission or absorption from star clusters being a function of age.

The Lyman alpha reference sample

“…Creating the continuum-subtracted Lyα map of nearby starforming galaxies in the SBC-only data set is nontrivial and requires a good knowledge of many parameters: the stellar UV continuum emission near Lyα, the stellar absorption below Lyα, and the presence of UV interstellar absorption lines (LIS) in the spectral energy distribution (SED) of the galaxy (Hayes et al 2005Hayes & Östlin 2006). We derived the Lyα map of Mrk 1486 throughout from a continuum-subtraction method using the Lyman-alpha eXtraction Software (LaXs; Hayes et al 2009) and our full set of HST data (see Table 1).…”

Neutral gas in Lyman-alpha emitting galaxies Haro 11 and ESO 338-IG04 measured through sodium absorption

“…HST UV imaging of the two galaxies Hayes et al 2005Hayes et al , 2007Östlin et al 2009) gives us information on the Ly α and UV continuum morphologies. Haro 11 is a blue compact galaxy at z = 0.02 with three main condensations or "knots".…”

“…The ongoing starburst is about 40 Myr old and was probably triggered by a merger with a small galaxy or from an interaction with remaining debris from a previous encounter with the companion galaxy ESO 338-IG04b (Östlin et al 2001;Cannon et al 2004). When we discuss ESO 338-IG04 below, we mainly focus on the bright central region, which we refer to as knot A (Hayes et al 2005, cf. Fig.…”

“…1; also known as cluster # 23 in the "inner" sample of Östlin et al 1998). Hayes et al (2005) present a detailed description of a HST/STIS long-slit spectrum across ESO 338-IG04. The spectrum shows diffuse Ly α emission in several regions, but over knot A the Ly α emission is weak and shows only a hint of blueshifted absorption.…”

Lyα EMISSION FROM HIGH-REDSHIFT SOURCES IN COSMOS

“…Analysis of nearby LAE galaxies MasHesse et al 2003;Hayes et al 2005;Ostlin et al 2009;Atek et al 2009;Scarlata et al 2009) indicates that Lyα emission is affected by ISM geometry, gas kinematics, and dust. However, the order of importance of each of these factors is not clearly established and could possibly vary from object to object (Schaerer 2007).…”