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

The escape fraction of hydrogen ionizing photons (f_esc) from galaxies has been suggested to be 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 more nearby universe. Here, we apply a new background subtraction routine on archival data from the Far Ultraviolet Spectroscopic Explorer (FUSE), in order to study local galaxies in search for possible Lyman Continuum (LyC) leakage. In the process, for the first time a stacked spectrum in the LyC is produced for local galaxies. With this small sample, we also make a more tentative approach to look for possible correlations between f_esc and physical parameters such as internal absorption E(B-V)_i, mass, H I mass, specific star formation rate (SSFR), metallicity, and Ly-alpha emission. Eight star forming galaxies with redshifts z > 0.015 from the FUSE archive were re-examined. Also, a sub-sample 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(1036{\AA}) line. 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. We find an absolute escape fraction for Tol 1247-232 of f_esc = 2.4(+0.9/-0.8) %. The stacked sample show an excess in the LyC with f_esc = 1.4(+0.6/-0.5) %, but we note that there might be important selection biases involved. With the small sample, we suggest a possible trend for higher f_esc 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.Comment: Accepted for publication in A&A. 16 pages, 7 picture

GRB 090417B and its Host Galaxy: A Step Towards an Understanding of Optically-Dark Gamma-Ray Bursts

GRB 090417B was an unusually long burst with a T_90 duration of at least 2130 s and a multi-peaked light curve at energies of 15-150 keV. It was optically dark and has been associated with a bright star-forming galaxy at a redshift of 0.345 that is broadly similar to the Milky Way. This is one of the few cases where a host galaxy has been clearly identified for a dark gamma-ray burst and thus an ideal candidate for studying the origin of dark bursts. We find that the dark nature of GRB 090417B cannot be explained by high redshift, incomplete observations, or unusual physics in the production of the afterglow. Assuming the standard relativistic fireball model for the afterglow we find that the optical flux is at least 2.5 mag fainter than predicted by the X-ray flux. The Swift/XRT X -ray data are consistent with the afterglow being obscured by a dense, localized sheet of dust approximately 30-80 pc from the burst along the line of sight. Our results suggest that this dust sheet imparts an extinction of A_V >~ 12 mag, which is sufficient to explain the missing optical flux. GRB 090417B is an example of a gamma-ray burst that is dark due to the localized dust structure in its host galaxy.Comment: Accepted for publication in Ap

On the Escape of Lyman Radiation from Local Galaxies

Cosmic reionization was most likely initiated by star forming dwarf galaxies. Little is known about the physical mechanisms allowing ionizing Lyman continuum (LyC) photons to escape from galaxies, but to learn more we can study local galaxies in detail. Until now, there has however only been one claim of a local LyC leaking galaxy, the disputed case of Haro 11. The lack of local detections could in part be a combined effect of technical problems and search strategies. Re-examining the FUSE (Far Ultraviolet Spectroscopic Explorer) data of Haro 11 led us to develop a new model for the spectral reduction, by which we could confirm an escape fraction of LyC photons (fesc) of 3.3±0.7%. In addition, eight more galaxies from the FUSE archive were examined leading to a new detection, Tol 1247-232, with fesc = 2.4±0.5%. The low value derived from the stacked spectrum of the whole sample, fesc = 1.4±0.4%, could be an indication of an evolving fesc scenario and/or an effect of probing the wrong targets. Local LyC candidates are normally selected among starburst galaxies with high equivalent widths in Hα. This can however give preference to ionization bounded H II regions with low escape fractions. In an attempt to overcome this selection bias, we developed a novel method to select LyC leaking galaxies. The selection is based on a blue continuum and weak emission lines, properties that in combination can be explained only by models with very high fesc. Using these criteria, we selected a sample of leaking candidates at z≈0.03 to be observed in Hα and Johnson B filters. The sample galaxies have properties that strongly favour leakage. Among these are clear signs of mergers and interaction with neighbouring galaxies, off-centre major star forming regions and spectral properties indicating previous starburst activity. The Lyman-alpha (Lyα) line is often used as a tracer for the distant galaxies believed to have reionized the universe. Here, for the first time local face-on spiral galaxies are studied in Lyα imaging. All three galaxies are emitting Lyα photons in the polar direction far out in the spiral arms, in clear contrast to previously studied irregular galaxies where strong emission is seen from the nuclei. If the small sample studied here is representative, it will have implications for detecting Lyα galaxies at high redshifts as it would depend strongly on the viewing angle

Lyman continuum leaking galaxies : Search strategies and local candidates

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 H alpha. 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 LyCleaking candidates at z similar to 0.03 which we have observed with ESO/NTT in broadband B and H alpha. 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 H alpha with those of the stellar continua shows that the scale lengths in H alpha 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

The Lyman continuum escape and ISM properties in Tololo 1247-232-new insights from HST and VLA

Low- and intermediate-mass galaxies are widely discussed as cause of reionization at redshift z similar to 10-6. However, observational proof of galaxies that are leaking ionizing radiation (Lyman continuum; LyC) is a currently ongoing challenge and the list of LyC emitting candidates is still short. Tololo 1247-232 is among those very few galaxies with recently reported leakage. We performed intermediate resolution ultraviolet spectroscopy with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope and confirm ionizing radiation emerging from Tololo 1247-232. Adopting an improved data reduction procedure, we find that LyC escapes from the central stellar clusters, with an escape fraction of 1.5 +/- 0.5 per cent only, i.e. the lowest value reported for the galaxy so far. We further make use of far-ultraviolet absorption lines of Si II and Si IV as a probe of the neutral and ionized interstellar medium ( ISM). We find that most of the ISM gas is ionized, likely facilitating LyC escape from density bounded regions. Neutral gas covering as a function of line-of-sight velocity is derived using the apparent optical depth method. The ISM is found to be sufficiently clumpy, supporting the direct escape of LyC photons. We further report on broad-band UV and optical continuum imaging as well as narrow-band imaging of Lya, Ha and H beta. Using stellar population synthesis, an Lya escape fraction of 8 per cent was derived. We also performed Very Large Array 21cm imaging. The hydrogen hyperfine transition was not detected, but a deep upper limit atomic gas mass of less than or similar to 10(9) M-circle dot could be derived. The upper limit gas fraction defined as M-HI/M-* is only 20 per cent. Evidence is found that the HI gas halo is relatively small compared to the Lyman Alpha Reference Sample (Hayes et al. 2013, 2014; Ostlin et al. 2014)