Lyman continuum leaking galaxies. Search strategies and local candidates

Abstract

Context. Star-forming dwarf galaxies may have played an important role in the reionization of the Universe, provided that some fraction of their ionizing radiation were able to escape into the intergalactic medium. Local galaxies exhibiting such Lyman-continuum (LyC) leakage could potentially shed light on the escape mechanisms involved, but only two low-redshift cases of LyC leakage have been identified so far. Here, we argue that this meager harvest may be caused by unsuitable selection criteria. Candidates for LyC leakage are normally selected by indicators of starburst activity, one of which is a high equivalent width in Hα. Such a criterion will guarantee a high production of LyC photons but will also bias the selection in favour of a high column density in the neutral gas, effectively ruling out LyC escape. Aims. In this work we want to investigate whether the lack of local LyC emitters can be caused in part by biased selection criteria, 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. Methods. We introduce a new selection strategy here where the potential LyC leakers are selected by their blue colours and weak emission lines. The selection is based on data from the Sloan Digital Sky Survey (SDSS). We also take a closer look at the properties of 8 LyC leaking candidates at z ∼ 0.03 which we have observed with ESO/NTT in broadband B and Hα. Results. We find that 7 of the 8 target galaxies are involved in interaction with neighbours or show signs of mergers. In 7 cases the young stellar population is clearly displaced relative to the main body of these galaxies, often directly bordering the halo region. In about half of our targets the absorption spectra show young post-starburst signatures. Comparing the scale lengths in Hα with those of the stellar continua shows that the scale lengths in Hα typically are 30% smaller, which is characteristic of galaxies influenced by ram pressure stripping. We tentatively identify a few mechanisms that could improve the conditions 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, whose previous activity has cleared channels for leakage into the intergalactic medium, and 4) a low dust content. Although our target galaxies are rare species in the local universe, we argue that related types could have played a major role in producing ionizing radiation at high redshifts.

PHOTOIONIZATION MODELS FOR THE SEMI-FORBIDDEN C iii] 1909 EMISSION IN STAR-FORMING GALAXIES

“…Since C iii] EWs decrease in density-bounded nebulae ( § 3.3), C iii] could be a possible diagnostic of optical depth. Bergvall et al (2013) predict similar behavior for optical emission lines, suggesting that weak nebular emission could select optically thin galaxies. Emission line ratios may also trace a galaxy's optical depth.…”

The Spectral Evolution of the First Galaxies. Ii. Spectral Signatures of Lyman Continuum Leakage From Galaxies in the Reionization Epoch

“…Galaxies in this mass range may be insufficient to explain the ionization state of the universe at z 9 (e.g., Finkelstein et al 2012a;Ferrara & Loeb 2013;Alvarez et al 2012;Paardekooper et al 2013) and methods based on photometry rather than spectroscopy (in the vein of Ono et al 2010;Bergvall et al 2013;Pirzkal et al 2012Pirzkal et al , 2013 will be necessary to estimate f esc for objects at lower masses. While photometric methods are unlikely to provide strong constraints for individual targets, they can, on the other hand, be applied to larger samples of galaxies.…”

“…The latter quantity, the Lyman continuum (LyC) escape fraction f esc , can be directly measured at z 4 (e.g., Steidel et al 2001;Shapley et al 2006;Bergvall et al 2006;Iwata et al 2009;Bogosavljević 2010;Vanzella et al 2010;Siana et al 2010;Boutsia et al 2011;Nestor et al 2011;Vanzella et al 2012;Nestor et al 2013;Leitet et al 2013). Observations of this type indicate an increase in the typical f esc with redshift in the z ≈ 0-3 interval (Inoue et al 2006;Bergvall et al 2013) and data-constrained models for galaxy-dominated reionization also require a redshift evolution in f esc (e.g., Kuhlen & FaucherGiguère 2012;Fontanot et al 2012;Yue et al 2013;Ferrara & Loeb 2013;Mitra et al 2013). Simulations and theoretical arguments moreover suggest that f esc may vary as a function of galaxy mass and star-formation activity (e.g., Razoumov & Sommer-Larsen 2010;Yajima et al 2009;Fernandez & Shull 2011;Conroy & Kratter 2012).…”

“…Ono et al (2010), Bergvall et al (2013), and Pirzkal et al (2012Pirzkal et al ( , 2013 have previously attempted to assess f esc from photometric data, but spectroscopic signatures are likely to produce more reliable results for individual targets. Recently, Jones et al (2013) presented a spectroscopic method to place upper limits on the LyC escape fraction of high-redshift galaxies using metal absorption lines, but unlike their method, the one proposed here should be able to place both upper and lower limits on the escape fraction.…”

Limits on the LyC signal fromz~ 3 sources with secure redshift and HST coverage in the E-CDFS field

“…It is composed of three main young (< 40 Myr old) star-formation clumps, probably experiencing a merging event. Leitet et al (2011) estimated that ∼ 17% of the intrinsic LyC flux escapes Haro11 (see also Bergvall et al, 2013, for the method). A comparable value was estimated by Leitet et al (2013) for a second blue galaxy, Tol 1247-232, which is also characterized by low metallicity and low dust content.…”