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Ionized and atomic interstellar medium in star-forming galaxies
Stockholm University, Faculty of Science, Department of Astronomy.ORCID iD: 0000-0002-9204-3256
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Star forming galaxies in the local Universe are an important laboratory for learning about conditions in the distant, early Universe. With a high frequency of interactions and mergers, strong star formation activity, and complex kinematics and often disturbed or irregular morphology, these galaxies are believed to be the best local analogs to the galaxies at early times, and can therefore help understand the galaxies we observe at high redshifts in the early Universe. These early galaxies in turn hold the key to cosmological insights about the early Universe, including galaxy formation and early evolution, the onset of the first stars, formation of the cosmic large scale structure, and the Epoch of Reionization.

Many of these galaxies are primarily or only visible in the wavelengths of the Lyman α (Lyα) transition, corresponding to the energy shift in a transition from the first excited energy level to the ground state in atomic Hydrogen. However, Lyα radiation emitted from a galaxy interacts strongly with the neutral hydrogen in and around the galaxy, often transporting it over large distances before it is either absorbed by dust or escapes the galaxy far from the line of sight from its point of origin to Earth. Despite the intrinsic strength of the Lyα line, it is often completely absorbed or spread out over large projected areas of low surface density. The observed strength of Lyα is almost completely decoupled from the intrinsic strength and mainly regulated by the conditions in the gas it travels through. Therefore, to correctly interpret what we observe int Lyα at high redshifts, it is necessary to understand which processes regulate and which conditions facilitate its escape.

Young starburst galaxies are also the main suspect for causing the reionization of the young Universe. To do so, the ionizing photons produced in the central starburst regions of the galaxies need to be able to reach the intergalactic gas. Like Lyα, the ionizing radiation (the Lyman Continuum) also interacts with the neutral medium. While not as strongly as for Lyα, it is still strong enough that at the onset of this project, only two galaxies in the local Universe were confirmed Lyman Continuum leakers. Since then, another few handful local candidates and confirmed leakers have been announced, but still far from the escape fractions needed at high redshift to reionize the early Universe. Identifying which properties of the ISM govern Lyman Continuum escape, and how these evolve with redshift, is a hot topic in extragalactic astronomy these years.

This thesis consists of projects which, in each their way, aim to deepen our understanding of these matters. One project, the Lyman Alpha Reference Sample (LARS), aims to understand which processes govern Lyα radiative transfer through careful, in-depth studies of a sample of 14 local starburst galaxies with a selection of powerful telescopes and instruments. My contribution to this was a spectroscopic analysis of the central star-forming regions to understand their physical properties (Paper I), and of neutral Hydrogen interstellar and circumgalactic systems which interact with Lyα radiation on its way out of the galaxies (Paper II). In Paper III, I performed a deeper, more detailed spectroscopic analysis of the central recombination regions in two local-universe starburst galaxies, of which one is a known Lyman Continuum leaker. Finally, in Papers IV and V, we shift focus focus somewhat to combine information in Lyα and Lyman Continuum (and, in the case of Paper IV, other auxiliary data) from observations of the two first known Lyman Continuum leakers, to understand which configurations of neutral gas would allow for the combination of Lyman Continuum leakage and Lyα spectral and physical morphology that is observed.

Abstract [sv]

Aktivt stjärnbildande galaxer är en viktig och värdefull källa till kunskap om förhållanden i det unga, avlägsna universum. Med en hög frekvens av interaktioner och krockar, stark stjärnbildningsaktivitet, komplicerad kinematik och ofta rubbade eller helt oregelbundna former, är det en vanlig uppfattning bland astronomer att dessa galaxer är det närmaste vi kommer en lokal analog till galaxerna i universums barndom. De är därför mycket värdefulla som nyckel till att förstå vad vi ser när vi observerar unga galaxer vid hög rödförskjutning i det unga universum. Dessa unga galaxer i sin tur är viktiga nycklar till kosmologisk insikt om universums tidigaste epoker, t.ex. de första stjärnornas uppkomst, hur universums storskaliga struktur bildats, och det tidiga universums återjonisering.

Många av dessa tidiga, avlägsna galaxern är primärt eller uteslutande synliga i våglängdsområdet runt den så kallade Lyman α (Lyα) linjen; våglängden motsvarande energin i övergången från första exciterade energinivån till grundnivån i en neutral väteatom. Lyα-strålning utsänds från de centrala delarna av en galax och växelverkar starkt med neutrala vätemoln i och omkring galaxen. Ofta transporteras en Lyα-foton genom talrika växelverkningar lång bort från sin ursprungspunkt innan den antingen absorberas av stoftpartiklar eller slipper ut ur galaxen till den intergalaktiska rymden.  Trots Lyα-linjens ursprungliga styrka absorberas den ofta totalt, eller sprids ut över stora projicerade arealer med relativt låg ytdensitet. Den ljusstyrka som observeras i Lyα är statistiskt nästan fullständigt frånkopplad från den ursprungliga styrkan och regleras i huvudsak av fysiska förhållanden i gasen, som strålningen passerar genom. För att kunna tolka observationer av Lyα vid hög rödförskjutning rätt är det därför viktigt att förstå, vilka processer som reglerar, och vilka förhållanden som tillåter, att strålningen slipper ut ur galaxen.

Unga, så kallade Starburst-galaxer anses även vara den mest sannolika huvudorsaken till återjoniseringen av det unga universum. För att kunna göra detta måste de joniserande UV-fotonerna (i det så kallade Lyman Continuum), som produceras i de centrala starburst-regionerna av dessa galaxer, lyckas slippa igenom galaxens många och ofta tjocka vätemoln och ut ur galaxen. Precis som med Lyα, interagerar även dessa Lyman Continuum-fotoner med neutrala vätemoln, fast inte lika starkt som Lyα. Dock är det tillräckligt starkt för att astronomer, när detta projekt påbörjades, ba- ra skulle känna till två galaxer som utstrålar mätbara mängder Lyman Continuum-strålning. Sedan dess har det hittats ett tiotals kandidater och bekräftade fall där Lyman Continuum-fotoner observerats från lokala galaxer, men fortfarande långt ifrån tillräckligt med joniserande strålning för att återjonisera det unga universum. Astronomer arbetar därför med att identifiera vilka egenskaper hos det interstellara mediet som reglerar Lyman Continuum-utstrålning, och hur dessa utvecklar sig med rördförskjutning över kosmologisk tid.

Denna avhandling består av projekt som, var på sitt sätt, syftar till att förbättra vår förståelse av dessa frågor. Det första projektet, LARS, har som målsättning att förstå vilka processer som reglerar Lyα-strålningstransport genom fördjupade studier av 14 lokala, stjärnbildande galaxer med en rad av kraftfulla teleskop och instrument. Mitt bidrag var spektroskopisk analys av de centrala stjärnbildande regionerna för att förstå dessa regioners fysiska egenskaper (Artikel I); och av neutrala vätemolnen genom vilka Lyα-strålningen måste transporteras för att nå fram till våra teleskop (Artikel II). I Artikel III företar vi ännu djupare, mer detaljerade spektroskopiska studier av de heta centrala, högt joniserade regionerna i två lokala, stjärnbildande galaxer. I Artikel IV och V flyttas fokus till att kombinera den tillgängliga informationen om Lyα och Lyman Continuum-strålning i de två första kända Lyman Continuum-utstrålande galaxerna för att förstå vilka konfigurationer av det interstellara mediet som kan föranleda utstrålning av joniserande fotoner och Lyα i den utformning det observeras.

Place, publisher, year, edition, pages
Stockholm: Department of Astronomy, Stockholm University , 2016. , 74 p.
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
URN: urn:nbn:se:su:diva-134526ISBN: 978-91-7649-545-2ISBN: 978-91-7649-546-9OAI: oai:DiVA.org:su-134526DiVA: diva2:1033949
Public defence
2016-12-01, sal FC61, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Submitted. Paper 4: Submitted. Paper 5: Manuscript.

Available from: 2016-11-08 Created: 2016-10-10 Last updated: 2016-11-09Bibliographically approved
List of papers
1. 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: 2016-10-10Bibliographically approved
2. THE LYMAN ALPHA REFERENCE SAMPLE. V. THE IMPACT OF NEUTRAL ISM KINEMATICS ANDGEOMETRY ON Lyα ESCAPE
Open this publication in new window or tab >>THE LYMAN ALPHA REFERENCE SAMPLE. V. THE IMPACT OF NEUTRAL ISM KINEMATICS ANDGEOMETRY ON Lyα ESCAPE
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2015 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 805, no 14, 14Article in journal (Refereed) Published
Abstract [en]

We present high-resolution far-UV spectroscopy of the 14 galaxies of the Lyα Reference Sample; a sample of strongly star-forming galaxies at low redshifts (0.028 < z < 0.18). We compare the derived properties to global properties derived from multi-band imaging and 21 cm H i interferometry and single-dish observations, as well as archival optical SDSS spectra. Besides the Lyα line, the spectra contain a number of metal absorption features allowing us to probe the kinematics of the neutral ISM and evaluate the optical depth and and covering fraction of the neutral medium as a function of line of sight velocity. Furthermore, we show how this, in combination with the precise determination of systemic velocity and good Lyα spectra, can be used to distinguish a model in which separate clumps together fully cover the background source, from the "picket fence" model named by Heckman et al. We find that no one single effect dominates in governing Lyα radiative transfer and escape. Lyα escape in our sample coincides with a maximum velocity-binned covering fraction of 0.9 and bulk outflow velocities of 50 km s−1, although a number of galaxies show these characteristics and yet little or no Lyα escape. We find that Lyα peak velocities, where available, are not consistent with a strong backscattered component, but rather with a simpler model of an intrinsic emission line overlaid by a blueshifted absorption profile from the outflowing wind. Finally, we find a strong anticorrelation between Hα equivalent width and maximum velocity-binned covering factor, and propose a heuristic explanatory model.

Keyword
cosmology: observations, galaxies: ISM, galaxies: kinematics and dynamics, galaxies: starburst
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-134520 (URN)10.1088/0004-637X/805/1/14 (DOI)000354991300014 ()
Projects
The Lyman Alpha Reference Sample
Available from: 2016-10-10 Created: 2016-10-10 Last updated: 2016-11-22Bibliographically approved
3. OF NEEDLES AND HAYSTACKS: SPATIALLY RESOLVED GAS KINEMATICS IN STARBURST GALAXIESHARO 11 AND ESO 338 FROM NUV/OPTICAL LONG-SLIT SPECTROSCOPY
Open this publication in new window or tab >>OF NEEDLES AND HAYSTACKS: SPATIALLY RESOLVED GAS KINEMATICS IN STARBURST GALAXIESHARO 11 AND ESO 338 FROM NUV/OPTICAL LONG-SLIT SPECTROSCOPY
(English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357Article in journal (Refereed) Submitted
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-134522 (URN)
Available from: 2016-10-10 Created: 2016-10-10 Last updated: 2016-10-18Bibliographically approved
4. The Lyman Continuum Escape and ISM properties inTololo 1247-232 – New Insights from HST and VLA
Open this publication in new window or tab >>The Lyman Continuum Escape and ISM properties inTololo 1247-232 – New Insights from HST and VLA
(English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966Article in journal (Refereed) Submitted
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-134524 (URN)
Available from: 2016-10-10 Created: 2016-10-10 Last updated: 2016-10-18Bibliographically approved
5. NEUTRAL ISM, LYMAN-ALPHA AND LYMAN-CONTINUUM IN NEARBY STARBURST HARO 11
Open this publication in new window or tab >>NEUTRAL ISM, LYMAN-ALPHA AND LYMAN-CONTINUUM IN NEARBY STARBURST HARO 11
(English)Manuscript (preprint) (Other academic)
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-134521 (URN)
Available from: 2016-10-10 Created: 2016-10-10 Last updated: 2016-10-18Bibliographically approved

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