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Lyman continuum leaking galaxies - Search strategies and local candidates

Star-forming dwarf galaxies leaking Lyman-continuum (LyC) radiation may have played an important role in the reionization of the Universe. Local galaxies exhibiting LyC leakage could shed light on the escape mechanisms, but so far only two such cases have been identified. Here we want to investigate whether the lack of local LyC emitters can be caused in part by biased selection criteria (e.g. strong H-alpha emission), and we present a novel method of selecting targets with high escape fractions. By applying these criteria, we assemble a sample of observation targets to study their basic properties. We introduce a new strategy where potential LyC leakers are selected by their blue colours and weak (instead of strong) emission lines. We take a closer look at 8 LyC leaking candidates at z ~ 0.03, selected from the Sloan Digital Sky Survey (SDSS), which we observe with ESO/NTT in broadband B and H-alpha. We find that 7 of the 8 galaxies are involved in interaction with neighbours or show signs of mergers. In 7 cases the young stellar population is clearly displaced from the main body of the galaxies. Half of our targets show absorption spectra with post-starburst signatures. The scale lengths in H-alpha are typically 30% smaller than those of the stellar continua, indicating ram pressure stripping. We tentatively identify a few conditions favourable for leakage: 1) the combined effects of ram pressure stripping with supernova winds from young stars formed in the front, 2) merger events that increase the star formation rate and displace stars from gas, 3) starbursts in the centres of post-starburst galaxies, and 4) a low dust content.