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Lyman-alpha imaging of starburst galaxies in the local universe and beyond
Stockholm University, Faculty of Science, Department of Astronomy.
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
Astronomy
Identifiers
URN: urn:nbn:se:su:diva-7023ISBN: 978-91-7155-462-8 (print)OAI: oai:DiVA.org:su-7023DiVA: diva2:197485
Public defence
2007-09-19, sal FB53, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00
Opponent
Supervisors
Available from: 2007-08-23 Created: 2007-08-22Bibliographically approved
List of papers
1. HST/ACS Lyman-alpha imaging of the nearby starburst ESO 338-IG04
Open this publication in new window or tab >>HST/ACS Lyman-alpha imaging of the nearby starburst ESO 338-IG04
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2005 In: Astronomy and Astrophysics, Vol. 438, no 1, 71-85 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:su:diva-24410 (URN)
Note
Part of urn:nbn:se:su:diva-7023Available from: 2007-08-23 Created: 2007-08-22Bibliographically approved
2. On the narrowband detection properties of high-redshift Lyman-alpha emitters
Open this publication in new window or tab >>On the narrowband detection properties of high-redshift Lyman-alpha emitters
2006 In: Astronomy and Astrophysics, Vol. 460, no 3, 681-694 p.Article in journal (Refereed) Published
Identifiers
urn:nbn:se:su:diva-24411 (URN)
Note
Part of urn:nbn:se:su:diva-7023Available from: 2007-08-23 Created: 2007-08-22Bibliographically approved
3. Continuum subtracting Lyman-alpha images: Low redshift studies using the Solar Blind Channel of HST/ACS
Open this publication in new window or tab >>Continuum subtracting Lyman-alpha images: Low redshift studies using the Solar Blind Channel of HST/ACS
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.

Keyword
galaxies: starburst, methods: data analysis, techniques: image processing, techniques: photometric
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-24412 (URN)10.1088/0004-6256/138/3/911 (DOI)000269509700018 ()
Available from: 2007-08-23 Created: 2007-08-22 Last updated: 2017-12-13Bibliographically approved
4. The escape of Lyman photons from a young starburst: The case of Haro 11
Open this publication in new window or tab >>The escape of Lyman photons from a young starburst: The case of Haro 11
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2007 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 382, no 4, 1465-1480 p.Article in journal (Refereed) Published
Abstract [en]

Lyman α (Lyα) is one of the dominant tools used to probe the star-forming galaxy population at high redshift (z). However, astrophysical interpretations of data drawn from Lyα alone hinge on the Lyα escape fraction which, due to the complex radiative transport, may vary greatly. Here, we map the Lyα emission from the local luminous blue compact galaxy Haro 11, a known emitter of Lyα and the only known candidate for low-z Lyman continuum emission. To aid in the interpretation, we perform a detailed ultraviolet and optical multiwavelength analysis and model the stellar population, dust distribution, ionizing photon budget, and star-cluster population. We use archival X-ray observations to further constrain properties of the starburst and estimate the neutral hydrogen column density.

The Lyα morphology is found to be largely symmetric around a single young star-forming knot and is strongly decoupled from other wavelengths. From general surface photometry, only very slight correlation is found between Lyα and Hα, E(BV), and the age of the stellar population. Only around the central Lyα bright cluster do we find the Lyα/Hα ratio at values predicted by the recombination theory. The total Lyα escape fraction is found to be just 3 per cent. We compute that ∼90 per cent of the Lyα photons that escape do so after undergoing multiple resonance scattering events, masking their point of origin. This leads to a largely symmetric distribution and, by increasing the distance that photons must travel to escape, decreases the escape probability significantly. While dust must ultimately be responsible for the destruction of Lyα, it plays a little role in governing the observed morphology, which is regulated more by interstellar medium kinematics and geometry. We find tentative evidence for local Lyα equivalent width in the immediate vicinity of star clusters being a function of cluster age, consistent with hydrodynamic studies. We estimate the intrinsic production of ionizing photons and put further constraints of ∼9 per cent on the escaping fraction of photons at 900 Å.

Place, publisher, year, edition, pages
Wiley-Blackwell, 2007
Keyword
* galaxies: evolution; * galaxies: individual: Haro 11; * galaxies: star clusters; * ultraviolet: galaxies
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-24413 (URN)10.1111/j.1365-2966.2007.12482.x (DOI)
Note
Part of urn:nbn:se:su:diva-7023Available from: 2007-08-23 Created: 2007-08-22 Last updated: 2017-12-13Bibliographically approved
5. The Lyman alpha morphology of local starburst galaxies: release of calibrated images
Open this publication in new window or tab >>The Lyman alpha morphology of local starburst galaxies: release of calibrated images
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2009 (English)In: Astronomical Journal, ISSN 0004-6256, E-ISSN 1538-3881, 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.

Keyword
galaxies: individual (Haro 11, SBS 033596;052, IRAS 08339+6517, Tol 65, NGC6090, Tololo 1924-416); galaxies: ISM; galaxies: starburst; ultraviolet: galaxies
Identifiers
urn:nbn:se:su:diva-24414 (URN)10.1088/0004-6256/138/3/923 (DOI)000269509700019 ()
Note
Part of urn:nbn:se:su:diva-7023Available from: 2007-08-23 Created: 2007-08-22 Last updated: 2017-12-13Bibliographically approved

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