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Escape of Lyman continuum radiation from local galaxies: Detection of leakage from the young starburst Tol 1247-232
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.
Université de Toulouse, UPS-OMP, IRAP, Toulouse, France.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
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2013 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 553, A106- p.Article in journal (Refereed) Published
Abstract [en]

Context. It has been suggested that the escape fraction of hydrogen ionizing photons (f(esc)) 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 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. 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 lambda 1036 angstrom 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 (f(1500) (angstrom)/f(900) (angstrom))(int) = 1.5(-0.5)(+0.6), which gives an absolute escape fraction for Tol 1247-232 of f(esc) = 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 f(esc) = 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 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.

Place, publisher, year, edition, pages
2013. Vol. 553, A106- p.
Keyword [en]
Galaxies, starburst, Lyman continuum, leakage
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
URN: urn:nbn:se:uu:diva-160861DOI: 10.1051/0004-6361/201118370ISI: 000319858700106OAI: oai:DiVA.org:uu-160861DiVA: diva2:453318
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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Leitet, ElisabetBergvall, Nils

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