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Lyman-alpha radiative transfer in Star-forming galaxies
Stockholm University, Faculty of Science, Department of Astronomy.
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis focuses on the intrinsically strongest spectral signature of star-forming galaxies: the Lyman alpha recombination line of the hydrogen atom (hereafter Lya). Located at the wavelength of 1215.67 Å in the restframe far-ultraviolet spectra of star-forming galaxies, the Lya line proves to be a vital tracer and a powerful emission-line window to discover and to study the remote young star-forming galaxies of the early the Universe.

Although intrinsically very strong, the Lya line is also a resonant line. As a consequence, the transport of Lya photons inside the interstellar medium (ISM) of star-forming galaxies is very complex and depends on many ISM quantities (HI mass, dust content, HI gas kinematics and ISM clumpiness). All this process has serious effects on the emergent features of the Lya line (strength, equivalent width and line profile) that need to be understood for ensuring a proper interpretation of all very promising Lya-oriented studies in astrophysics and cosmology. This is precisely the aim of this thesis to go deeper into our understanding of the complex radiative transport experienced by the Lya line in star-forming galaxies.

In this work, we carry out both numerical and observational studies of Lya transport inside the ISM of galaxies.

In Paper I and II, we perform detailed numerical studies that examine the effects of a clumpy ISM on the strength and the shape of the Lya line. Although poorly studied until now, the effects of a clumpy ISM on Lya have been routinely invoked to explain the origin of anomalously strong Lya line observed from high-redshift galaxies. Some analytical studies suggest indeed an higher transmission of Lya photons over UV continuum ones from clumpy ISMs, resulting in an enhanced Lya equivalent width EW(Lya). Our results show that although clumpiness facilitates the escape of Lya, it is highly unlikely that any real ISM should result in any enhancement of EW(Lya). Other possible causes are discussed in our papers, leading to the conclusion that the observed high EW(Lya) are more likely produced by cooling radiation or anisotropic escape of Lya radiation.

Both Paper III and IV are related to the LARS project. This is an ambitious observational program in which 14 nearby star-forming galaxies have been observed with the Hubble Space Telescope (HST) with the aim to investigate how Lya is transported out of galaxies and what effects each ISM quantity produces on the Lya line. While Paper III examines the Lya properties and morphology of individual galaxies, Paper IV presents a detailed study of the surprising Lya emission line of Mrk1486 (the fifth galaxy of the sample).

Place, publisher, year, edition, pages
Stockholm: Department of Astronomy, Stockholm University , 2015. , 84 p.
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
URN: urn:nbn:se:su:diva-111550ISBN: 978-91-7649-087-7 (print)OAI: oai:DiVA.org:su-111550DiVA: diva2:775684
Public defence
2015-02-12, FA31, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Submitted.

Available from: 2015-01-21 Created: 2015-01-04 Last updated: 2015-01-20Bibliographically approved
List of papers
1. Lyman alpha line and continuum radiative transfer in a clumpy interstellar medium
Open this publication in new window or tab >>Lyman alpha line and continuum radiative transfer in a clumpy interstellar medium
2014 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 562, A52- p.Article in journal (Refereed) Published
Abstract [en]

Aims. We study the effects of an inhomogeneous interstellar medium (ISM) on the strength and the shape of the Lyman alpha (Ly alpha) line in starburst galaxies. Methods. Using our 3D Monte Carlo Ly alpha radiation transfer code, we have studied the radiative transfer of Ly alpha, UV, and optical continuum photons in homogeneous and clumpy shells of neutral hydrogen and dust surrounding a central source. Our simulations predict the Ly alpha and continuum escape fraction, the Ly alpha equivalent width EW(Ly alpha), the Ly alpha line profile, and their dependence on the geometry of the gas distribution and the main input physical parameters. Results. The ISM clumpiness is found to have a strong impact on the Ly alpha line radiative transfer, leading to a strong dependence of the emergent features of the Ly alpha line (escape fraction, EW(Ly alpha)) on the ISM morphology. Although a clumpy and dusty ISM appears more transparent to radiation (both line and continuum) compared to an equivalent homogeneous ISM of equal dust optical depth, we find that the Ly alpha photons are, in general, still more attenuated than UV continuum radiation. As a consequence, the observed equivalent width of the Ly alpha line (EWobs(Ly alpha)) is lower than the intrinsic one (EWint(Ly alpha)) for nearly all clumpy ISM configurations being considered. There are, however, special conditions under which Ly alpha photons escape more easily than the continuum, resulting in an enhanced EWobs(Ly alpha). The requirement for this to happen is that the ISM is almost static (galactic outflows <= 200 km s(-1)), extremely clumpy (with density contrasts >10(7) in HI between clumps and the interclump medium), and very dusty (E(B - V) > 0.30). When these conditions are fulfilled, the emergent Ly alpha line profile generally shows no velocity shift and little asymmetry. Otherwise, the Ly alpha line profile is very similar to the one expected for homogeneous media. Conclusions. Given the asymmetry and velocity shifts generally observed in star-forming galaxies with Ly alpha emission, we therefore conclude that clumping is unlikely to significantly enhance their relative Ly alpha/UV transmission.

Keyword
galaxies: starburst, galaxies: ISM, galaxies: high-redshift, ultraviolet: galaxies, radiative transfer, line: profiles
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-102983 (URN)10.1051/0004-6361/201220455 (DOI)000332161800007 ()
Note

AuthorCount:4;

Available from: 2014-04-25 Created: 2014-04-25 Last updated: 2017-12-05Bibliographically approved
2. ON THE (NON-) ENHANCEMENT OF THE Ly alpha EQUIVALENT WIDTH BY A MULTIPHASE INTERSTELLAR MEDIUM
Open this publication in new window or tab >>ON THE (NON-) ENHANCEMENT OF THE Ly alpha EQUIVALENT WIDTH BY A MULTIPHASE INTERSTELLAR MEDIUM
2013 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 766, no 2, 124- p.Article in journal (Refereed) Published
Abstract [en]

It has been suggested that radiative transfer effects may explain the unusually high equivalent widths (EWs) of the Ly alpha line, observed occasionally from starburst galaxies, especially at high redshifts. If the dust is locked up inside high-density clouds dispersed in an empty intercloud medium, the Ly alpha photons could scatter off of the surfaces of the clouds, effectively having their journey confined to the dustless medium. The continuum radiation, on the other hand, does not scatter, and would thus be subject to absorption inside the clouds. This scenario is routinely invoked when Ly alpha EWs higher than what is expected theoretically are observed, although the ideal conditions under which the results are derived usually are not considered. Here we systematically examine the relevant physical parameters in this idealized framework, testing whether any astrophysically realistic scenarios may lead to such an effect. It is found that although clumpiness indeed facilitates the escape of Ly alpha, it is highly unlikely that any real interstellar media should result in a preferential escape of Ly alpha over continuum radiation. Other possible causes are discussed, and it is concluded that the observed high EWs are more likely to be caused by cooling radiation from cold accretion and/or anisotropic escape of the Ly alpha radiation.

Keyword
galaxies: ISM, radiative transfer, scattering
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-89706 (URN)10.1088/0004-637X/766/2/124 (DOI)000316320900058 ()
Funder
Knut and Alice Wallenberg FoundationSwedish Research Council
Note

AuthorCount:3;

Available from: 2013-05-08 Created: 2013-05-06 Last updated: 2017-12-06Bibliographically approved
3. THE LYMAN ALPHA REFERENCE SAMPLE. II. HUBBLE SPACE TELESCOPE IMAGING RESULTS, INTEGRATED PROPERTIES, AND TRENDS
Open this publication in new window or tab >>THE LYMAN ALPHA REFERENCE SAMPLE. II. HUBBLE SPACE TELESCOPE IMAGING RESULTS, INTEGRATED PROPERTIES, AND TRENDS
Show others...
2014 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 782, no 1, 6Article in journal (Refereed) Published
Abstract [en]

We report new results regarding the Ly alpha output of galaxies, derived from the Lyman Alpha Reference Sample, and focused on Hubble Space Telescope imaging. For 14 galaxies we present intensity images in Ly alpha, H alpha, and UV, and maps of H alpha/H beta, Ly alpha equivalent width (EW), and Ly alpha/H alpha. We present Ly alpha and UV radial light profiles and show they are well-fitted by Sersic profiles, but Ly alpha profiles show indices systematically lower than those of the UV (n approximate to 1-2 instead of greater than or similar to 4). This reveals a general lack of the central concentration in Ly alpha that is ubiquitous in the UV. Photometric growth curves increase more slowly for Ly alpha than the far ultraviolet, showing that small apertures may underestimate the EW. For most galaxies, however, flux and EW curves flatten by radii approximate to 10 kpc, suggesting that if placed at high-z only a few of our galaxies would suffer from large flux losses. We compute global properties of the sample in large apertures, and show total Ly alpha luminosities to be independent of all other quantities. Normalized Ly alpha throughput, however, shows significant correlations: escape is found to be higher in galaxies of lower star formation rate, dust content, mass, and nebular quantities that suggest harder ionizing continuum and lower metallicity. Six galaxies would be selected as high-z Ly alpha emitters, based upon their luminosity and EW. We discuss the results in the context of high-z Ly alpha and UV samples. A few galaxies have EWs above 50 angstrom, and one shows f(esc)(Ly alpha) of 80%; such objects have not previously been reported at low-z.

Keyword
cosmology: observations, galaxies: starburst
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-102756 (URN)10.1088/0004-637X/782/1/6 (DOI)000331848200006 ()
Note

AuthorCount:19;

Available from: 2014-04-22 Created: 2014-04-22 Last updated: 2017-12-05Bibliographically approved
4. LARS VIII: Lyman alpha escape from the edge-on disk galaxy Mrk1486
Open this publication in new window or tab >>LARS VIII: Lyman alpha escape from the edge-on disk galaxy Mrk1486
Show others...
(English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746Article in journal (Refereed) Submitted
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-112787 (URN)
Available from: 2015-01-15 Created: 2015-01-15 Last updated: 2017-12-05

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