Escape of Lyman continuum radiation from local galaxies - Detection of leakage from the young starburst Tol 1247-232

摘要

Context. It has been suggested that the escape fraction of hydrogen ionizing photons (fesc) from galaxies is evolving with time, but the picture is far from clear. While evidence for significant escape fractions has been found at high redshifts in several studies, the picture looks different in the closer universe. The lack of local detections could in principle be a combined effect of an evolving escape fraction, the low number galaxies observed, the selection criteria of these targets, and technical problems associated with the instrument best adapted for low redshift targets, the Far Ultraviolet Spectroscopic Explorer (FUSE). Aims. In an attempt to tackle the last of these effects, we apply a new background routine developed for FUSE data to study galaxies from the archive in search of possible Lyman continuum (LyC) leakage. In the process, for the first time a stacked spectrum in the LyC has been produced for local galaxies. With this small sample, we also make a more tentative approach to looking for possible correlations between fesc and physical parameters, such as internal absorption E(B???V)i, mass, H?i mass, specific star formation rate (SSFR), metallicity, and Lyα emission. Methods. Eight star forming galaxies with redshifts z?>?0.015 from the FUSE archive were re-examined. Also, a subsample of an additional four galaxies with lower redshifts were included, for which the escape fraction was estimated from residual flux in the low ionization interstellar C?ii λ1036 ? line. Results. Out of the eight galaxies, only one was found to have significant LyC leakage, Tol 1247-232 (S/N?=?5.2). This is the second detection of a leaking galaxy in the local universe. From the first case, Haro 11, we derive an intrinsic Lyman break amplitude for starbursts at this young age of (f1500??/f900??)int = 1.5-0.5+0.6, which gives an absolute escape fraction for Tol 1247?232 of fesc = 2.4-0.8+0.9. Tol 1247-232 exhibits an extremely blue far-UV slope reminiscent of high redshift LyC leaking galaxies, and although it does not classify as an AGN in the BPT diagram or by other available diagnostics, a minor AGN cannot be completely excluded. The stacked sample shows an excess in the LyC with fesc = 1.4-0.5+0.6 %, but we note that there might be important selection biases involved, since the galaxies were originally handpicked for their star forming qualities. With the small sample, we suggest a possible trend toward higher fesc with lower mass and with enhanced SSFR. None of the galaxies with high values of E(B???V)i were found to show any sign of leakage.