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Lyman continuum leaking galaxies: Search strategies and local candidates
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
Department of Astronomy, Stockholm University, Oscar Klein Center, AlbaNova, Stockholm.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
2013 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 554, A38- p.Article in journal (Other academic) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
2013. Vol. 554, A38- p.
Keyword [en]
Galaxies, starbursts, Lyman continuum, leakage
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
URN: urn:nbn:se:uu:diva-160862DOI: 10.1051/0004-6361/201118433ISI: 000320444200037OAI: oai:DiVA.org:uu-160862DiVA: diva2:453319
Available from: 2011-11-02 Created: 2011-11-02 Last updated: 2017-12-08Bibliographically approved
In thesis
1. On the Escape of Lyman Radiation from Local Galaxies
Open this publication in new window or tab >>On the Escape of Lyman Radiation from Local Galaxies
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2011. 55 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 880
Keyword
Galaxies, starbursts, spiral galaxies, Lyman continuum, Lyman alpha, leakage, cosmic reionization, escape fraction
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:uu:diva-160864 (URN)978-91-554-8220-6 (ISBN)
Public defence
2011-12-15, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2011-11-23 Created: 2011-11-02 Last updated: 2012-10-16

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