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Analyzing low signal-to-noise FUSE spectra Confirmation of Lyman continuum escape from Haro 11
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.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
2011 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 532, A107- p.Article in journal (Refereed) Published
Abstract [en]

Context. Galaxies are believed to be the main providers of Lyman continuum (LyC) photons during the early phases of the cosmic reionization. Little is known however, when it comes to escape fractions and the mechanisms behind the leakage. To learn more, one may look at local objects, but so far only one low-z galaxy has shown any signs of emitting LyC radiation. With data from the Far Ultraviolet Spectroscopic Explorer (FUSE), we previously found an absolute escape fraction of ionizing photons (f(esc)) of 4-10% for the blue compact galaxy Haro 11. However, using a revised version of the reduction pipeline on the same data set, Grimes and collaborators were unable to confirm this and derived an upper limit of f(esc) less than or similar to 2%. Aims. We attempt to determine whether Haro 11 is emitting ionizing radiation to a significant level or not. We also investigate the performance of the reduction pipeline for faint targets such as Haro 11, and introduce a new approach to the background subtraction. Methods. The final version of the reduction pipeline, CalFUSE v3.2, was applied to the same Haro 11 data set as the two previous authors used. At these faint flux levels, both FUSE and CalFUSE are pushed to their limits, and a detailed analysis was undertaken to monitor the performance of the pipeline. We show that non-simultaneous background estimates are insuffient when working with data of low signal-to-noise ratio (S/N), and a new background model was developed based on a direct fit to the detector response. Results. We find that one has to be very careful when using CalFUSE v3.2 on low S/N data, and especially when dealing with sources where signal might originate from off-center regions. Applying the new background fit, a significant signal is detected in the LyC in both detector segments covering these wavelengths. Thus, the leakage is confirmed with a flux density of f(900) = 4.0 x 10(-15) erg s(-1) cm(-2) angstrom(-1) (S/N = 4.6), measured on the airglow free regions in the LyC for the night-only data. This corresponds to an absolute escape fraction of ionizing photons from Haro 11 of f(esc) = 3.3 +/- 0.7%. We confirm these results by investigating the two-dimensional data, the count rates, and the residual flux in CII lambda 1036 angstrom.

Place, publisher, year, edition, pages
2011. Vol. 532, A107- p.
Keyword [en]
intergalactic medium, galaxies: starburst, galaxies: fundamental parameters, galaxies: evolution, diffuse radiation, ultraviolet: galaxies
National Category
Physical Sciences
URN: urn:nbn:se:uu:diva-159256DOI: 10.1051/0004-6361/201015654ISI: 000293942700006OAI: oai:DiVA.org:uu-159256DiVA: diva2:443536
Available from: 2011-09-26 Created: 2011-09-26 Last updated: 2012-10-16Bibliographically 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.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 880
Galaxies, starbursts, spiral galaxies, Lyman continuum, Lyman alpha, leakage, cosmic reionization, escape fraction
National Category
Astronomy, Astrophysics and Cosmology
Research subject
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)
Available from: 2011-11-23 Created: 2011-11-02 Last updated: 2012-10-16

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