First detection of Lyman continuum escape from a local starburst galaxy

I. Observations of the luminous blue compact galaxy Haro 11 with the Far Ultraviolet Spectroscopic Explorer (FUSE)

¹ Dept. of Astronomy and Space Physics, Box 515, 75120 Uppsala, Sweden e-mail: [nils.bergvall;erik.zackrisson]@astro.uu.se
² Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA e-mail: bg@pha.jhu.edu
³ Instituto de Astrofisica de Andalucia, Granada, Spain e-mail: pepa@iaa.es
⁴ Stockholm Observatory, SCFAB, 106 91 Stockholm, Sweden e-mail: ostlin@astro.su.se

Received: 7 July 2005
Accepted: 2 November 2005

Abstract

Context.The dominating reionization source in the young universe has not yet been identified. Possible candidates include metal poor dwarf galaxies with starburst properties.Aims.We selected an extreme starburst dwarf, the Blue Compact Galaxy Haro 11, with the aim of determining the Lyman continuum escape fraction from UV spectroscopy.Methods.Spectra of Haro 11 were obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE). A weak signal shortwards of the Lyman break is identified as Lyman continuum (LyC) emission escaping from the ongoing starburst. From profile fitting to weak metal lines we derive column densities of the low ionization species. Adopting a metallicity typical of the H II regions of Haro 11, these data correspond to a hydrogen column density of ~10¹⁹cm^-2. This relatively high value indicates that most of the LyC photons escape through transparent holes in the interstellar medium. We then use spectral evolutionary models to constrain the escape fraction of the produced LyC photons.Results.Assuming a normal Salpeter initial mass function we obtain a Lyman continuum escape fraction of 4–10%. We argue that in a hierarchical galaxy formation scenario, the upper limit we derive for the escape rate allows for a substantial contribution to cosmic reionization by starburst dwarf galaxies at high redshifts.