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Observing Lyman alpha emitters - How does Lyman alpha escape from galaxies?
Stockholm University, Faculty of Science, Department of Astronomy.
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: urn:nbn:se:su:diva-125604ISBN: 978-91-7649-336-6 (print)OAI: oai:DiVA.org:su-125604DiVA: diva2:894354
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
List of papers
1. Limits on Lyman Continuum Escape from z = 2.2 Hα-emitting Galaxies
Open this publication in new window or tab >>Limits on Lyman Continuum Escape from z = 2.2 Hα-emitting Galaxies
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2015 (English)In: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 814, no 1, L10Article in journal (Refereed) Published
Abstract [en]

The leakage of Lyman continuum (LyC) photons from star-forming galaxies is an elusive parameter. When observed, it provides a wealth of information on star formation in galaxies and on the geometry of the interstellar medium, and puts constraints on the role of star-forming galaxies in the reionization of the universe. Hα-selected galaxies at  trace the highest star formation population at the peak of cosmic star formation history, providing a base for directly measuring LyC escape. Here we present this method and highlight its benefits as well as caveats. We also use the method on 10 Hα emitters in the Chandra Deep Field South at  also imaged with the Hubble Space Telescope in the ultraviolet. We find no individual LyC detections, and our stack puts a 5σ upper limit on the average absolute escape fraction of <24%, consistent with similar studies. With future planned observations, the sample sizes should rapidly increase and the method presented here should provide very robust constraints on the escape fraction.

Keyword
dark ages, reionization, first stars, galaxies: evolution, ultraviolet: galaxies
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-123705 (URN)10.1088/2041-8205/814/1/L10 (DOI)000365035400010 ()
Available from: 2015-12-02 Created: 2015-12-02 Last updated: 2017-12-01Bibliographically approved
2. 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
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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.

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
3. Trident: A three-pronged galaxy survey I. Lyman alpha emitting galaxies at z similar to 2 in GOODS North
Open this publication in new window or tab >>Trident: A three-pronged galaxy survey I. Lyman alpha emitting galaxies at z similar to 2 in GOODS North
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2015 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 580, A91Article in journal (Refereed) Published
Abstract [en]

Context: Lyman alpha (Ly alpha) emitting galaxies (LAEs) are used to probe the distant universe and are therefore important for galaxy evolution studies and for providing clues to the nature of the epoch of reionization. However, the exact circumstances under which Lya escapes a galaxy are still not fully understood. Aims. The Trident project is designed to simultaneously examine Ly alpha, H alpha, and Lyman continuum emission from galaxies at redshift z similar to 2, thus linking these three aspects of ionizing radiation in galaxies. In this paper, we outline the strategy of this project and examine the properties of LAEs in the GOODS North field.

Methods: We performed a narrowband LAE survey in GOODS North using existing filters and two custom made filters at the Nordic Optical Telescope with MOSCA. We use complementary broadband archival data in the field to make a careful candidate selection and perform optical to near-IR SED fitting. We also estimate far-IR luminosities by matching our candidates to detections in Spitzer/MIPS 24 mu m and Herschel/PACS catalogues. Results. We find a total of 25 LAE candidates, probing mainly the bright end of the LAE luminosity function with L-Ly alpha similar to 1-15 x 10(42) erg s(-1). They display a range of masses of similar to 0.5-50x10(9) M-circle dot, and average ages from a few tens of Myr to 1 Gyr when assuming a constant star formation history. The majority of our candidates also show signs of recent elevated star formation. Three candidates have counterparts in the GOODS-Herschel far-IR catalogue, with luminosities consistent with ultra-luminous infrared galaxies (ULIRGs).

Conclusions: The wide range of parameters derived from our SED fitting, as well as part of our sample being detected as ULIRGs, seems to indicate that at these Ly alpha luminosities, LAEs do not necessarily have to be young dwarfs, and that a lack of dust is not required for Ly alpha to escape.

Keyword
galaxies: photometry, galaxies: structure
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-121175 (URN)10.1051/0004-6361/201525728 (DOI)000360020200091 ()
Available from: 2015-10-01 Created: 2015-09-28 Last updated: 2017-12-01Bibliographically approved
4. Neutral gas in Lyman-alpha emitting galaxies Haro 11 and ESO 338-IG04 measured through sodium absorption
Open this publication in new window or tab >>Neutral gas in Lyman-alpha emitting galaxies Haro 11 and ESO 338-IG04 measured through sodium absorption
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2013 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 552, A95Article in journal (Refereed) Published
Abstract [en]

Context. The Lyman alpha emission line of neutral hydrogen is an important tool for finding galaxies at high redshift, thus for probing the structure of the early universe. However, the resonance nature of the line and its sensitivity to dust and neutral gas is still not fully understood. Aims. We present measurements of the velocity, covering fraction and optical depth of neutral gas in front of two well-known, local blue compact galaxies that show Lyman alpha in emission: ESO 338-IG 04 and Haro 11. We thus observationally test the hypothesis that Lyman alpha can escape through neutral gas by being Doppler shifted out of resonance. Methods. We present integral field spectroscopy, obtained with the GIRAFFE/Argus spectrograph at VLT/FLAMES in Paranal, Chile. The excellent wavelength resolution allowed us to accurately measure the velocity of the ionized and neutral gas through the Ha emission and Na D absorption, which trace the ionized medium and cold interstellar gas, respectively. We also present independent measurements from the VLT/X-shooter spectrograph that confirm our results. Results. For ESO 338-IG 04 we measure no significant shift of neutral gas: the best fit velocity offset is -15 +/- 16 km s(-1). For Haro 11, we see an outflow from knot B at 44 +/- 13 km s(-1), and infalling gas towards knot C with 32 +/- 12 km s(-1). Based on the relative strength of the Na D absorption lines, we estimate low covering fractions of neutral gas (down to 10%) in all three cases. Conclusions. The Na D absorption most likely occurs in dense clumps with higher column densities than the medium in which the bulk of the Ly alpha scattering takes place. Still, we find no strong correlation between outflowing neutral gas and strong Ly alpha emission. The Ly alpha photons from these two galaxies are therefore likely to be escaping due to a low column density and/or covering fraction.

Keyword
galaxies: kinematics and dynamics, galaxies: ISM, galaxies: starburst, galaxies: individual: Haro 11, galaxies: individual: ESO 338-IG04
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-90375 (URN)10.1051/0004-6361/201220702 (DOI)000317912000095 ()
Note

AuthorCount:7;

Available from: 2013-06-12 Created: 2013-06-03 Last updated: 2017-12-06Bibliographically approved
5. THE Ly alpha REFERENCE SAMPLE. I. SURVEY OUTLINE AND FIRST RESULTS FOR MARKARIAN 259
Open this publication in new window or tab >>THE Ly alpha REFERENCE SAMPLE. I. SURVEY OUTLINE AND FIRST RESULTS FOR MARKARIAN 259
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2014 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 797, no 1, 11Article in journal (Refereed) Published
Abstract [en]

The Ly alpha Reference Sample (LARS) is a substantial program with the Hubble Space Telescope (HST) that provides a sample of local universe laboratory galaxies in which to study the detailed astrophysics of the visibility and strength of the Ly alpha line of neutral hydrogen. Ly alpha is the dominant spectral line in use for characterizing high-redshift (z) galaxies. This paper presents an overview of the survey, its selection function, and HST imaging observations. The sample was selected from the combined GALEX+Sloan Digital Sky Survey catalog at z = 0.028-0.19, in order to allow Ly alpha to be captured with combinations of long-pass filters in the Solar Blind Channel (SBC) of the Advanced Camera for Surveys (ACS) onboard HST. In addition, LARS utilizes H alpha and H beta narrowband and u, b, i broadband imaging with ACS and the Wide Field Camera 3 (WFC3). In order to study galaxies in which large numbers of Ly alpha photons are produced (whether or not they escape), we demanded an H alpha equivalent width W(H alpha) >= 100 angstrom. The final sample of 14 galaxies covers far-UV (FUV, lambda similar to 1500 angstrom) luminosities that overlap with those of high-z Ly alpha emitters (LAEs) and Lyman break galaxies (LBGs), making LARS a valid comparison sample. We present the reduction steps used to obtain the Ly alpha images, including our LARS eXtraction software (LaXs), which utilizes pixel-by-pixel spectral synthesis fitting of the energy distribution to determine and subtract the continuum at Ly alpha. We demonstrate that the use of SBC long-pass-filter combinations increase the signal-to-noise ratio by an order of magnitude compared to the nominal Ly alpha filter available in SBC. To exemplify the science potential of LARS, we also present some first results for a single galaxy, Mrk 259 (LARS #1). This irregular galaxy shows bright and extended (indicative of resonance scattering) but strongly asymmetric Ly alpha emission. Spectroscopy from the Cosmic Origins Spectrograph on board HST centered on the brightest UV knot shows a moderate outflow in the neutral interstellar medium (probed by low ionization stage absorption features) and Ly alpha emission with an asymmetric profile. Radiative transfer modeling is able to reproduce the essential features of the Ly alpha line profile and confirms the presence of an outflow. From the integrated photometry we measure an Ly alpha luminosity of L-Ly alpha= 1.3x10(42) erg s(-1) an equivalent width W(Ly alpha) = 45 angstrom and an FUV absolute magnitude M-FUV = -19.2 (AB). Mrk 259 would hence be detectable in high-z Ly alpha and LBG surveys. The total Ly alpha escape fraction is 12%. This number is higher than the low-z average, but similar to that at z > 4, demonstrating that LARS provides a valid comparison sample for high-z galaxy studies.

Keyword
cosmology: observations, galaxies: evolution, galaxies: individual (Mrk 259), galaxies: starburst, radiative transfer, ultraviolet: galaxies
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-111396 (URN)10.1088/0004-637X/797/1/11 (DOI)000345915000011 ()
Note

AuthorCount:23;

Available from: 2015-01-05 Created: 2015-01-02 Last updated: 2017-12-05Bibliographically approved

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