Reducing low signal-to-noise FUSE spectra: confirmation of Lyman continuum escape from Haro 11

Abstract

Galaxies are believed to be the main providers of Lyman continuum (LyC) photons during the early phases of the cosmic reionization. Little is known however, when it comes to escape fractions and the mechanisms behind the leakage. To learn more one may look at local objects, but so far only one low-z galaxy has shown any sign of emitting LyC radiation. With data from the Far Ultraviolet Spectroscopic Explorer (FUSE), Bergvall et al. (2006) found an absolute escape fraction of ionizing photons (f_esc) of 4-10% for the blue compact galaxy Haro 11. However, using a newer version of the reduction pipeline on the same data set, Grimes et al. (2007) could not confirm this and derived an upper limit of f_esc \leq 2%. Here, using the last version of the pipeline CalFUSE v3.2, we aim at settling the question if Haro 11 is emitting ionizing radiation to a significant level or not. We also investigate the performance of the reduction pipeline for faint targets such as Haro 11. At these faint flux levels both FUSE and CalFUSE are pushed to their limits, and a detailed analysis was undertaken. We show that non-simultaneous background estimates are insuffient when working with low signal-to-noise data, and a new background model was developed based on a direct fit to the detector response. Applying the new background fit, a significant signal in the LyC emerged in both detector segments covering these wavelengths. Thus, the leakage is confirmed with a flux density of f_900=4.0 *10^{-15} erg/s/cm^2/{\AA} (S/N=4.6), measured on the airglow free regions in the LyC for the night only data. This corresponds to an absolute escape fraction of ionizing photons for Haro 11 of f_esc=3.3 \pm 0.7%. The results were confirmed by investigating the 2D data, the count rates and the residual flux in CII(1036{\AA}).