| 1. |
- Della Bruna, Lorenza, 1992-
(författare)
-
Star Formation and feedback at key physical scales for galaxy evolution
- 2021
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Feedback from young, massive stars plays an essential role in the self-regulation of star formation in galaxies, and in shaping the galaxies' global properties. This phenomenon originates at small scales, surrounding the stars, but has been observed to be effective up to galactic-wide scales. The exact mechanism which allows the ionising radiation to escape the star-forming regions (HII regions), initially still embedded in their natal molecular hydrogen gas, is still unknown. Constraining the escape of ionising photons from HII regions is also relevant in order to explain the origin of the diffuse ionised gas (DIG) that is observed to contribute up to 50% to the Ha luminosity of nearby galaxies. I present the results of the study of stellar feedback in two nearby galaxies (NGC 7793 and M83), at spatial scales that critically connect the sources of ionisation with their immediate surroundings. We determine the fraction of DIG and study its properties and origin. We find that in NGC 7793 ionising sources located in the DIG are producing a sufficient amount of hydrogen-ionising (LyC) photons to explain the diffuse gas emission. In M83, on the other hand, the DIG is ionised by a mixed contribution of photoionisation and shocks. We investigate the link between LyC leakage from HII regions and their stellar and gas properties. We find that the age spread of the stellar population in the region does not seem to imply a higher leakage. Also the ionisation structure of the regions (e.g. the presence of "channels" that are transparent to the LyC photons) appears to be uncorrelated with escape in our sample. In M83, we also study the relative importance of different types of stellar feedback. We find that the pressure exerted by the ionised gas is always dominant over the direct radiation pressure. When the total HII region pressure is compared to the environmental pressure, we observe that regions near the galactic centre are in equilibrium with the surroundings, whereas regions in the disk are overpressured and are therefore expanding. We also find that changes in the local environmental conditions are the dominant factor in setting the ionised gas pressure, and that the pressure terms are linked to the physical properties (age and mass) of the young star clusters powering the regions. In the near future, observations from the James Webb Space Telescope will allow us to study the most embedded star-forming regions with a resolution comparable to the present one.
|
|
| 2. |
- Ejdetjärn, Timmy
(författare)
-
Exploring the nature of ISM turbulencein disc galaxies
- 2024
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Galaxy formation is a continuous process that started only a few hundred million yearsafter the Big Bang. The first galaxies were very volatile, with bursts of star formationand disorganised gas motions. However, even as these galaxies evolved to have orderlyrotating gas discs, the gas within the disc, referred to as the interstellar medium (ISM),still remained highly turbulent. In fact, the ISM is supersonically turbulent, meaning thatthe disorganised gas motion exceeds the speed of sound in the medium. This supersonicturbulence has been connected to several crucial properties related to galaxy evolution; forexample, increasing (and decreasing in some regions) the ISM gas density, star formation,and gas mixing.Many observation have shown that all of the gas phases in the ISM experience su-personic levels of turbulence, with line widths (an observational method to quantify theamount of turbulence) as high as σg ≲ 100 km s−1 in high-redshift (younger) disc galaxies,while local quiescent discs have σg ≲ 40 km s−1 . However, the ISM contains a variety ofgas phases that cover a wide range of temperatures and densities, which exhibit differentlevels of turbulence. For example, the warm ionised gas phase represents the upper limitsquoted above, while colder denser gas only reaches σg ≲ 40 km s−1 and σg ≲ 15 km s−1 inhigh-redshift and local galaxies, respectively.The physical processes driving this turbulence are not fully understood, but a combi-nation of stellar feedback (e.g. supernova) and gravitational instability (e.g. during cloudcollapse) have been suggested to provide a majority of the turbulent energy. In particular,stellar feedback is crucial in the formation of warm ionised gas and may therefore have asignificant contribution on the turbulence within ionised gas. Furthermore, heterogeneousdata of widely different galaxies (in terms of e.g. mass and size) at different resolutions(which causes artificial line broadening) complicates understanding the underlying cause.A commonly used tracer of ionised gas is the Hα emission line and has been usedextensively in high-redshift surveys. However, the contribution of the Hα signal comesfrom two primary sources: the radiatively ionised regions around massive newborn starsembedded in molecular gas (called H II regions) and diffuse ionised gas (DIG) filling theentire galactic disc. Observations have found that these two sources contribute, on average,roughly the same amount to the Hα signal (although with a large spread), but the levelsof turbulence is starkly different; with the DIG being roughly 2-3 times more turbulethan the gas in H II regions.Numerical simulations have come a long way and are now able to simulate entire discgalaxies at parsec-scale resolution (in regions of interest). Furthermore, galaxy simulationshave been able to reproduce the level of turbulence observed in local and high-redshiftgalaxies. Direct comparisons between numerical and observational studies are crucial tounderstand the relevant physics driving observed correlations. However, numerical andobservational work have different data available and the reduction/analysis varies betweenauthors, and so diligence is required to perform qualitative comparisons.In this work, I perform numerical simulations to investigate ISM turbulence in differentgas phases. My simulations model a Milky Way-like galaxy at two different redshifts(using gas fraction as a proxy for redshift) and with/without stellar feedback physics, toevaluate its impact. I perform mock observations to explore the relation between the starformation rate and turbulence, and investigate what is driving this relation. Additionally, Ianalyse the Hα emission line and compare the contribution in intensity and line broadening(turbulence) from H II regions and DIG.
|
|
| 3. |
- Duval, Florent, 1986-
(författare)
-
Lyman-alpha radiative transfer in Star-forming galaxies
- 2015
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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).
|
|
| 4. |
- Menacho Menacho, Veronica, 1983-
(författare)
-
Impact of feedback on the ISM of extreme starburst galaxies : The case of Haro 11
- 2020
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Blue compact galaxies (BCGs) are compact, metal-poor, starbursting galaxies with characteristics similar to what is expected for the young high-redshifted galaxies. BCGs are among the most active in producing a large number of massive star clusters, each containing thousands of massive stars. During their short life, massive stars are continuously injecting energy, heat and momentum into the ISM via their intense radiation, stellar winds, and later on supernova explosions. These feedback mechanisms impact directly the star's surroundings, but when this feedback originates from a concentration of massive star clusters, it can strongly affect the condition of the gas of the entire galaxy.This thesis presents a detailed analysis of the ionized gas condition and the effect of strong feedback in Haro 11, an extreme starbursting BCG and the closest Lyman continuum (LyC) leaking galaxy. We exploit the spectro-photometric capabilities of the MUSE instrument, by slicing the galaxy spectra in a sequence of maps in velocity bins, in order to obtain a 3D information of the galaxy. Haro 11 has a rich population of massive and predominantly young star clusters, concentrated in three compact knots within its 4 x 4 kpc$^2$ centre. We find that the localised stellar feedback is strongly impacting the global kinematics and the condition of the gas up to further distances in the halo. Many kpc-scale structures such as filaments, shells and bubbles were traced in our data. Moreover, the strong feedback seems to have developed kpc-scale bubbles, outflows and galactic ionized cones with drastic consequences for the likely escape of Ly$\alpha$ and LyC photons, gas and metals out of the galaxy. The extended halo around Haro 11 is governed by photoionization processes and/or shocks from recurrent supernovae originated in the central starburst region. Due to the galaxy's extreme ISM condition, commonly used emission lines diagnostics produce, in part, large discrepancies in the ionized gas properties.The results presented in this work highlight: a) the strong impact of stellar feedback affecting the ISM at all scales in starburst systems; b) the fact that traditional relations drawn up from averaged measurements of emission lines or from simplified models, fail in probing the condition of the gas in extreme environments. This is an appeal to revisit the standard relations by including more realistic models where several physical processes are simultaneously at work; c) the method applied here can be used to explore in detail the high sensitive, high spatial-resolution data from future facilities such as JWST/ELT.
|
|
| 5. |
- Messa, Matteo, 1988-
(författare)
-
Young Star Clusters and Clumps in the Local Universe : The effect of galactic environment on star formation
- 2019
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Stars do not form in isolation, but rather out of a hierarchical structure set by the turbulence of the interstellar medium. At the densest peak of the gas distribution, the star formation process can produce young star clusters (YSCs), which are gravitationally bound systems of stars with mass between ~100 and 106 MSun and typical size of few parsecs. At larger scales, clusters are themselves arranged into cluster complexes, on scales of hundreds of parsecs and up to kiloparsec scales, which are usually referred to as ‘star-forming clumps’.Observations of local star-forming galaxies show that YSCs form over a wide range of galactic environment. However, it is not yet clear if and how the galactic environment relates to the properties of star clusters. I present the results obtained by studying the YSC population of the nearby spiral galaxy M51. We find that the cluster mass function, dN/dM, can be described by a power-law with a -2 slope and an exponential truncation at 105 MSun, consistent with what is observed in similar galaxies in the literature. The shape of the mass function is similar when looking at increasing galactocentric distances. We observe significant differences, however, when comparing clusters located in the spiral arm with those the inter-arm environments. On average, more massive clusters are formed in the spiral arms, as also previously found for the YSC progenitors, the giant molecular clouds (GMCs). Finally, we see that clusters are more quickly disrupted in denser environments, as expected if their disruption is mainly caused by tidal interaction with dense gas structures like the GMCs.I have also undertaken the analysis of the interplay between galactic scale properties and larger star forming units, the stellar clumps. The analysis has been conducted in a sample of 14 low-redshift starburst galaxies, the Lyman-Alpha Reference Sample (LARS). The elevated star formation rate densities of such galaxies allow to form clumps with densities comparable to clumps at high-redshift, typically more massive and denser than what is normally observed in the local universe. The clumps in the LARS galaxies contribute to a large fraction to the UV flux of the galaxy itself (in many galaxies > 50%), resulting in galaxies which appear ‘clumpy’. In agreement with formation theories we observe that clumpiness is higher in galaxies with higher SFR surface density and dominated by turbulent gas motion.
|
|
| 6. |
- Micheva, Genoveva, 1977-
(författare)
-
Unveiling the nature of blue compact galaxies
- 2012
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Blue compact galaxies (BCGs) are gas-rich star-forming low redshift galaxies with low metallicities. In some cases the relative strength of the starburst can be so high that it completely dominates the light output of the galaxy, an obstacle which has been countered by deeper optical imaging data and observations in the near infra-red (NIR) regime. This has revealed an older population referred to as the "host". In an effort to study the hosts of BCGs we have analyzed new and extremely deep UBVRIHKs imaging data for 46 high and low luminosity BCGs. For several BCGs the data reveal previously undetected extended low surface brightness components beyond the μB~26 mag arcsec-2 isophote. These are predominantly the luminous BCGs in the sample, and they show tails, plumes, optical bridges between companion galaxies, and other signs of merging or strong tidal interactions. The low luminosity BCGs, on the other hand, are well represented by an exponential disk profile down to the reliability limit of the data at a surface brightness level of μB~28 mag arcsec-2.The burst and host populations are examined separately. The integrated colors of both are compared to predictions from spectral evolutionary models, giving an indication of their respective ages and metallicities. Our analysis suggests that for the luminous BCGs a strong contribution by nebular emission is present almost down to the Holmberg radius, invalidating the host structural parameters obtained from brighter isophotes. Possible evolutionary links to quiescent galaxies like dEs, dIs, and LSBGs are explored by examining the structural parameters derived from two radial ranges typically assumed to be dominated by the underlying host galaxy. In this parameter space the luminous BCGs in our sample deviate from their low luminosity counterparts and from BCG data in the literature. They are instead consistent with the structural properties of giant low surface brightness galaxies with central surface brightnesses μB≥23 mag arcsec-2. We further examine the asymmetry and concentration parameters for the sample and study the correlation between the minimum asymmetry distribution in the optical and NIR vs morphological class, concentration and integrated colors to identify mergers/tidally interacting galaxies. A shift in the asymmetry distribution occurs for low luminosity BCGs from the optical to the NIR. In contrast, we find that the flocculent asymmetry component (due to star formation) completely dominates the composite asymmetry of high luminosity BCGs. We introduce an alternative asymmetry measure which successfully traces the dynamical asymmetry component (due to merging/tidal interaction) of the host.
|
|
| 7. |
- Puschnig, Johannes, 1980-
(författare)
-
Molecular gas and ionizing radiation in star-forming galaxies
- 2021
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In cosmic history, some of the major changes such as reionization were driven by baryons (i.e. the stars and gas in galaxies), despite the fact that they contribute only few percent to the total mass-energy budget in the Universe. This thesis is about the interplay between gas and stars in highly star-forming galaxies and aims to explore the physics that dictates transformation processes that took place at various stages in cosmic history.Using panchromatic observations ranging from the 21cm H I line in the radio regime to the extreme ultraviolet (UV), we studied ionizing radiation from massive stars (direct and through hydrogen recombination lines) as well as the atomic and molecular gas content in 15 highly star-forming local galaxies. The results are brought into cosmological context, taking a step forward towards finding answers to the following open questions in galaxy evolution: Which physical conditions enable galaxies to leak ionizing radiation (and power reionization)? What drives the high Lyman-alpha escape fractions observed in the early Universe? How did the massive stellar clumps found in high redshift galaxies have possibly formed?One of the galaxies we studied is Tololo 1247-232. Our results show that ionizing photons (i.e. Lyman continuum) escape from the region around two central massive stellar clusters. From UV absorption lines we further conclude that bulk of the gas in the galaxy must be ionized and clumpy. Moreover, the 21cm H I data reveal a low upper limit neutral gas fraction. We thus argue that the Lyman continuum escape in Tololo 1247-232 is facilitated by the large amount of ionizing radiation that is produced in the central region and then escapes from clumpy, density bounded regions. This scenario may also explain how early galaxies at z>6 have powered cosmic reionization.Additionally, we performed infrared and molecular gas (traced by CO) observations of galaxies drawn from the "Lyman Alpha Reference Sample'' (LARS). The galaxies were selected as analogues of high-redshift galaxies. Our main discovery is a roughly linear trend between the Lyman-alpha escape fraction and the total gas depletion time. This finding is counter-intuitive, because given the resonant scattering nature of Lyman-alpha photons, an increase in atomic gas should result in longer path lengths out of the galaxy, making photons more prone to absorption. Some other process seems to facilitate Lyman-alpha escape. We speculate that gas accretion enhances the turbulence of the cold gas and shifts the Lyman-alpha photons out of resonance. This scenario would naturally explain elevated Lyman-alpha escape fractions during the phases in cosmic history when galaxies were still accretion-dominated (at high-z) rather than defined by gas depletion.Finally, we present high-resolution interferometric observations of a single galaxy, LARS 8. The galaxy is a proto-typical analogue of normal star-forming galaxies at z~1-2, i.e. it is massive, has a large gas fraction, is rotationally supported and its morphology is dominated by massive clumps. We show that these clumps are the result of an extremely gravitationally unstable gas disc. Large scale instabilities are found across the whole extent of the rotating disc, with only the innermost 500pc being stabilized by its bulgelike structure. Our findings prove that gravitational instabilities may play a significant role in galaxy evolution, in particular at z≃1-3, when galaxies are characterized by massive clumps.
|
|
| 8. |
- Rasekh, Armin, 1988-
(författare)
-
The spatial distribution of Lyman alpha from star-forming galaxies in the low redshift Universe
- 2022
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Lyman alpha (Lyα) emission line from the recombination of hydrogen atoms is predicted to be the strongest spectral feature of galaxies. This emission line is regularly used to detect and characterise high redshift galaxies. However, studying this spectral line is challenging. Lyα is a resonant line, meaning it interacts strongly with neutral hydrogen. Consequently, the interpretation of Lyα observations of galaxies is very complicated. Nonetheless, this complexity provides a wealth of information. One way to learn how to extract this information is to focus on low-z star-forming galaxies. The characteristics of these galaxies, such as their gas kinematics and dust, affect Lyα escape from galaxies. This is the primary motivation for the projects included in this thesis, where we focus on Lyα observations in the low-z Universe.We studied the morphology of Lyα emission in galaxies from the largest galaxy sample with available Lyα imaging, the Lyman Alpha Reference Sample (LARS). We investigated the Lyα light distribution and how it affects the Lyα global observables, such as Lyα luminosity, Lyα escape fraction and Lyα equivalent width. We found that the Lyα luminosity anti-correlate with the Lyα halo luminosity fraction. In other words, in galaxies that are faint in Lyα, most of the Lyα luminosity originates from their Lyα halos. Because of surface brightness dimming, this could introduce a detection bias against low luminosity Lyα emitters at high redshift, which explains why observing faint Lyα emitters at high redshift is hard. Hence, the conclusions drawn from studying data that only sample bright LAEs may have been biased.We also investigated the origin of the extended Lyα halo emission using line-of-sight kinematic information in the LARS sample. We found that the gas kinematics in the centres of galaxies plays a vital role. We discovered that Lyα flux in the central regions of galaxies varies with the intensity of the outflow of gas in the centre. If the outflow is weak, galaxies show Lyα absorption or lower Lyα flux than expected (based on the observed Hα flux), which ultimately affects the measured Lyα halo luminosity fraction.Finally, we studied a sample of six intensely star-forming galaxies in the low-z Universe. Due to the strong emission of Hβ + [O III] nebular emission lines and the compact nature of these galaxies, they are called Green Pea galaxies. We looked for any relationship between the Lyα properties and well-established diagnostics for determining whether a galaxy is a Lyman continuum emitter. We found that one of the primary Lyman continuum emitter diagnostics -- Lyα spectral peak separation -- correlates with the Lyα halo luminosity fraction. This result suggests that the Lyα halo luminosity fraction could be used as a diagnostic of Lyman continuum escape.
|
|
| 9. |
- Rivera-Thorsen, Thøger Emil, 1979-
(författare)
-
Ionized and atomic interstellar medium in star-forming galaxies
- 2016
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
|
|
| 10. |
- Sandberg, Andreas, 1985-
(författare)
-
Observing Lyman alpha emitters - How does Lyman alpha escape from galaxies?
- 2016
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
|
|
