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THE LYMAN ALPHA REFERENCE SAMPLE. II. HUBBLE SPACE TELESCOPE IMAGING RESULTS, INTEGRATED PROPERTIES, AND TRENDS
Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Universite de Toulouse, France.
Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
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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.

Place, publisher, year, edition, pages
2014. Vol. 782, no 1, 6
Keyword [en]
cosmology: observations, galaxies: starburst
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
URN: urn:nbn:se:su:diva-102756DOI: 10.1088/0004-637X/782/1/6ISI: 000331848200006OAI: oai:DiVA.org:su-102756DiVA: diva2:713233
Note

AuthorCount:19;

Available from: 2014-04-22 Created: 2014-04-22 Last updated: 2017-12-05Bibliographically approved
In thesis
1. Lyman-alpha radiative transfer in Star-forming galaxies
Open this publication in new window or tab >>Lyman-alpha radiative transfer in Star-forming galaxies
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:nbn:se:su:diva-111550 (URN)978-91-7649-087-7 (ISBN)
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
2. Observing Lyman alpha emitters - How does Lyman alpha escape from galaxies?
Open this publication in new window or tab >>Observing Lyman alpha emitters - How does Lyman alpha escape from galaxies?
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The Lyman alpha (Lyα) emission line has grown to become one of the most successful tools for finding galaxies at high redshift. At redshifts corresponding to the early cosmic times of reionization and primeval galaxy formation, the wavelength of Lyα is still accessible with ground-based facilities. Lyα is a resonance line which undergoes a complicated radiative transfer process through the neutral gas inside galaxies. This process is still not fully understood. The precise distribution and kinematics of stars, gas and dust all seem to affect the amount of Lyα that eventually escapes the galaxy. Observational studies of Lyα emitting galaxies are necessary for understanding this process in detail.

From previous observations and simulations, it is evident that outflows of neutral gas can facilitate the escape of Lyα photons, as the Doppler effect shifts the frequency out of resonance. In Paper I we explore the connection between Lyα escape and outflows of neutral gas as measured with the Na D absorption feature in two nearby Lyα emitting galaxies. We find suprisingly little evidence for such a connection, and speculate how the Na D absorption is perhaps not measuring the velocity of the gas which is the most important for Lyα escape.

Papers II and III address LARS - the Lyman Alpha Reference Sample - a project in which 14 nearby galaxies and their Lyα emission are studied in detail using the Hubble Space Telescope. The two papers describe how we directly image the Lyα emission and absorption in these galaxies, and relate it to their physical properties. We find that Lyα escape is more probable in galaxies with younger age, and lower mass, dust content and instantaneous star formation rate, whereas the total Lyα luminosity appears to be independent of these factors.

Papers IV and V then turn to higher redshifts, exploring Lyα and Lyman Continuum escape at z ∼ 2. In Paper IV we find 25 Lyα-emitting galaxies (LAEs) using the photometric narrow-band technique, and we explore their multi-wavelength properties. Our results are well in line with similar studies at this redshift. We also find several luminous infrared galaxies (LIRGs) in the sample, which may seem surprising given their high dust content, but we also review similar previous findings in the literature.

Paper V describes the method of using Hα-emitting galaxies (HAEs) in order to accurately ascertain the Lyman Continuum escape fraction from a galaxy population, a number which is crucial for the understanding of the role of galaxies during the epoch of reionization. An Hα-selected sample is less biased towards dust-free systems than UV-selected samples, which are typically used for this type of study. We also use the method on 10 strongly clustered HAEs and constrain the Lyman Continuum escape fraction to <24%, but stress that this number is strongly affected by cosmic variance and that further studies of HAEs could provide very robust constraints on the escape fraction.

Place, publisher, year, edition, pages
Stockholm: Department of Astronomy, Stockholm University, 2016. 52 p.
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-125604 (URN)978-91-7649-336-6 (ISBN)
Public defence
2016-03-10, sal FB42, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)
Opponent
Supervisors
Available from: 2016-02-16 Created: 2016-01-14 Last updated: 2017-02-24Bibliographically approved

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