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On the Escape of Lyman Radiation from Local Galaxies
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Observational Astronomy.
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 [en]
Galaxies, starbursts, spiral galaxies, Lyman continuum, Lyman alpha, leakage, cosmic reionization, escape fraction
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
URN: urn:nbn:se:uu:diva-160864ISBN: 978-91-554-8220-6 (print)OAI: oai:DiVA.org:uu-160864DiVA: diva2:453321
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
List of papers
1. Analyzing low signal-to-noise FUSE spectra Confirmation of Lyman continuum escape from Haro 11
Open this publication in new window or tab >>Analyzing low signal-to-noise FUSE spectra Confirmation of Lyman continuum escape from Haro 11
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.

Keyword
intergalactic medium, galaxies: starburst, galaxies: fundamental parameters, galaxies: evolution, diffuse radiation, ultraviolet: galaxies
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-159256 (URN)10.1051/0004-6361/201015654 (DOI)000293942700006 ()
Available from: 2011-09-26 Created: 2011-09-26 Last updated: 2017-12-08Bibliographically approved
2. Escape of Lyman continuum radiation from local galaxies: Detection of leakage from the young starburst Tol 1247-232
Open this publication in new window or tab >>Escape of Lyman continuum radiation from local galaxies: Detection of leakage from the young starburst Tol 1247-232
Show others...
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.

Keyword
Galaxies, starburst, Lyman continuum, leakage
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:uu:diva-160861 (URN)10.1051/0004-6361/201118370 (DOI)000319858700106 ()
Available from: 2011-11-02 Created: 2011-11-02 Last updated: 2017-12-08Bibliographically approved
3. Lyman continuum leaking galaxies: Search strategies and local candidates
Open this publication in new window or tab >>Lyman continuum leaking galaxies: Search strategies and local candidates
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.

Keyword
Galaxies, starbursts, Lyman continuum, leakage
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:uu:diva-160862 (URN)10.1051/0004-6361/201118433 (DOI)000320444200037 ()
Available from: 2011-11-02 Created: 2011-11-02 Last updated: 2017-12-08Bibliographically approved
4. Lyman-alpha imaging of local face-on spiral galaxies
Open this publication in new window or tab >>Lyman-alpha imaging of local face-on spiral galaxies
Show others...
(English)Manuscript (preprint) (Other academic)
Keyword
Galaxies, Spiral galaxies, Lyman-alpha, escape, imaging
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:uu:diva-160863 (URN)
Available from: 2011-11-02 Created: 2011-11-02 Last updated: 2012-10-18

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