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The Lyman alpha morphology of local starburst galaxies: release of calibrated images
Stockholm University, Faculty of Science, Department of Astronomy.
Stockholm University, Faculty of Science, Department of Astronomy.
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2009 (English)In: Astronomical Journal, ISSN 0004-6256, Vol. 138, 923-940 p.Article in journal (Refereed) Published
Abstract [en]

We present reduced and calibrated high resolution Lyman-alpha (Lyα) images for a sample of six local star-forming galaxies. Targets were selected to represent a range in luminosity and metallicity and to include both known Lyα emitters and nonemitters. Far ultraviolet imaging was carried out with the Solar Blind Channel of the Advanced Camera for Surveys on the Hubble Space Telescope (HST) in the F122M (Lyα online) and F140LP (continuum) filters. The resulting Lyα images are the product of careful modeling of both the stellar and nebular continua, facilitated by supporting HST imaging at λ 2200, 3300, 4400, 5500, Hα, and 8000 Å, combined with Starburst 99 evolutionary synthesis models, and prescriptions for dust extinction on the continuum. In all, the resulting morphologies in Lyα, Hα, and UV continuum are qualitatively very different and we show that the bulk of Lyα emerges in a diffuse component resulting from resonant scattering events. Lyα escape fractions, computed from integrated Hα luminosities and recombination theory, are found never to exceed 14%. Internal dust extinction is estimated in each pixel and used to correct Lyα fluxes. However, the extinction corrections are far too small (by factors from 2.6 to infinity) to reconcile the emerging global Lyα luminosities with standard recombination predictions. Surprisingly, when comparing the global equivalent widths of Lyα and Hα, the two quantities appear to be anticorrelated, which may be due to the evolution of mechanical feedback from the starburst. This calls for caution in the interpretation of Lyα observations in terms of star formation rates. The images presented have a physical resolution 3 orders of magnitude better than attainable at high redshifts from the ground with current instrumentation and our images may therefore serve as useful templates for comparing with observations and modeling of primeval galaxy formation. We therefore provide the reduced Lyα, Hα, and continuum images to the community.

Place, publisher, year, edition, pages
2009. Vol. 138, 923-940 p.
Keyword [en]
galaxies: individual (Haro 11, SBS 033596;052, IRAS 08339+6517, Tol 65, NGC6090, Tololo 1924-416); galaxies: ISM; galaxies: starburst; ultraviolet: galaxies
URN: urn:nbn:se:su:diva-24414DOI: 10.1088/0004-6256/138/3/923ISI: 000269509700019OAI: diva2:197484
Part of urn:nbn:se:su:diva-7023Available from: 2007-08-23 Created: 2007-08-22 Last updated: 2010-01-17Bibliographically approved
In thesis
1. Lyman-alpha imaging of starburst galaxies in the local universe and beyond
Open this publication in new window or tab >>Lyman-alpha imaging of starburst galaxies in the local universe and beyond
2007 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The last decade has seen huge advances in studies of astrophysical cosmology, primarily as a result of developments in telescopic facilities. One of the primary observational signatures of actively star forming galaxies in the distant universe is the Lyman-alpha emission line (Lyα). The line is used either to search for objects or as a spectral feature for definite redshift confirmation. In recent years, high-z Lyα surveys have been used to constrain cosmic star formation history, investigate large scale structure, and examine the neutral hydrogen fraction of the universe. This doctoral thesis is directly concerned with studies of the Lyα emission from star-forming galaxies and the validity of Lyα as a cosmological tool.

The approach is to study a sample of local actively star forming galaxies using data obtained with the Hubble Space Telescope (HST). Imaging observations have been performed in the Lyα line, Hα, and various continuum bandpasses in the ultraviolet and optical wavelength domains. Sophisticated tools have been developed for the analysis of the images, resulting also in theoretical exploration of Lyα-related observables from galaxies at high-z. Model simulations are presented, along with a methodology by which to interpret high-z survey data. HST imaging results call into question the interpretations of many high-z Lyα surveys. More specifically, the first direct observational evidence is presented for the emission of Lyα photons after resonant scattering in neutral hydrogen and low surface-brightness Lyα halos are found as a result. Imaging reveals Lyα morphologies that systematically differ from morphologies probed by stellar light or non-resonant tracers of the nebular gas. Based upon Hα observations and recombination theory, the fraction of Lyα photons that escape is found never to exceed 20% in any of the observed targets, despite the violent star-formation known to be taking place. Even after internal dust corrections, a deficit from the predicted Lyα/Hα line ratio is always found. The interpretation is that scattering events systematically enhance the probability of absorption of Lyα by dust grains. If these galaxies are representative of those that fall into the data-sets of high-z Lyα surveys, some cosmological estimates may be in error by an order of magnitude.

Place, publisher, year, edition, pages
Stockholm: Institutionen för astronomi, 2007. 63 p.
National Category
Astronomy, Astrophysics and Cosmology
Research subject
urn:nbn:se:su:diva-7023 (URN)978-91-7155-462-8 (ISBN)
Public defence
2007-09-19, sal FB53, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00
Available from: 2007-08-23 Created: 2007-08-22Bibliographically approved

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