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Continuum subtracting Lyman-alpha images: Low redshift studies using the Solar Blind Channel of HST/ACS
Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
2009 (English)In: Astronomical Journal, ISSN 0004-6256, E-ISSN 1538-3881, Vol. 138, no 3, 911-922 p.Article in journal (Refereed) Published
Abstract [en]

We are undertaking an imaging study of local star-forming galaxies in the Lyman-alpha (Lyα) emission line using the Solar Blind Channel (SBC) of the Advanced Camera for Surveys onboard the Hubble Space Telescope. Observations have been obtained in Lyα and H-alpha (Hα) and six line-free continuum filters between ~1500 Å and the I band. In a previous article, we demonstrated that the production of Lyα line-only images (i.e., continuum subtraction) in the SBC-only data set is nontrivial and that supporting data is a requirement. We here develop various methods of continuum subtraction and assess their relative performance using a variety of spectral energy distributions (SEDs) as input. We conclude that simple assumptions about the behavior of the ultraviolet continuum consistently lead to results that are wildly erroneous, and determine that a spectral fitting approach is essential. Moreover, fitting of a single component stellar or stellar+nebular spectrum is not always sufficient for realistic template SEDs and, in order to successfully recover the input observables, care must be taken to control the contribution of nebular gas and any underlying stellar population. Independent measurements of the metallicity must first be obtained, while details of the initial mass function play only a small role. We identify the need to bin together pixels in our data to obtain signal-to-noise ratios (S/Ns) of around 10 in each band before processing. At S/N = 10, we are able to recover Lyα fluxes accurate to within around 30% for Lyα lines with intrinsic equivalent width (W Lyα) of 10 Å. This accuracy improves to 10% for W Lyα = 100 Å. We describe the method of image processing applied to the observations presented in Östlin et al. and the associated data release. We also present simulations for an observing strategy for an alternative low-redshift Lyα imaging campaign using ACS/SBC using adjacent combinations of long-pass filters to target slightly higher redshift.

Place, publisher, year, edition, pages
2009. Vol. 138, no 3, 911-922 p.
Keyword [en]
galaxies: starburst, methods: data analysis, techniques: image processing, techniques: photometric
National Category
Astronomy, Astrophysics and Cosmology
URN: urn:nbn:se:su:diva-24412DOI: 10.1088/0004-6256/138/3/911ISI: 000269509700018OAI: diva2:197482
Available from: 2007-08-23 Created: 2007-08-22 Last updated: 2011-05-30Bibliographically 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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