| 1. |
- Melinder, Jens, 1977-, et al.
(författare)
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The Lyα Reference Sample. XIV. Lyα Imaging of 45 Low-redshift Star-forming Galaxies and Inferences on Global Emission
- 2023
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Ingår i: Astrophysical Journal Supplement Series. - 0067-0049 .- 1538-4365. ; 266:1
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Tidskriftsartikel (refereegranskat)abstract
- We present Lyα imaging of 45 low-redshift star-forming galaxies observed with the Hubble Space Telescope. The galaxies have been selected to have moderate to high star formation rates (SFRs) using far-ultraviolet (FUV) luminosity and Hα equivalent width criteria, but no constraints on Lyα luminosity. We employ a pixel stellar continuum fitting code to obtain accurate continuum-subtracted Lyα, Hα, and Hβ maps. We find that Lyα is less concentrated than FUV and optical line emission in almost all galaxies with significant Lyα emission. We present global measurements of Lyα and other quantities measured in apertures designed to capture all of the Lyα emission. We then show how the escape fraction of Lyα relates to a number of other measured quantities (mass, metallicity, star formation, ionization parameter, and extinction). We find that the escape fraction is strongly anticorrelated with nebular and stellar extinction, weakly anticorrelated with stellar mass, but no conclusive evidence for correlations with other quantities. We show that Lyα escape fractions are inconsistent with common dust extinction laws, and discuss how a combination of radiative transfer effects and clumpy dust models can help resolve the discrepancies. We present an SFR calibration based on Lyα luminosity, where the equivalent width of Lyα is used to correct for nonunity escape fraction, and show that this relation provides a reasonably accurate SFR estimate. We also show stacked growth curves of Lyα for the galaxies that can be used to find aperture loss fractions at a given physical radius.
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| 2. |
- Rasekh, Armin, 1988-
(författare)
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The spatial distribution of Lyman alpha from star-forming galaxies in the low redshift Universe
- 2022
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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.
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| 3. |
- Runnholm, Axel, 1992-, et al.
(författare)
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On the evolution of the size of Lyman alpha haloes across cosmic time : no change in the circumgalactic gas distribution when probed by line emission
- 2023
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Ingår i: Monthly notices of the Royal Astronomical Society. - 0035-8711 .- 1365-2966. ; 522:3, s. 4275-4293
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Tidskriftsartikel (refereegranskat)abstract
- Lyman alpha (Ly alpha) is now routinely used as a tool for studying high-redshift galaxies, and its resonant nature means it can trace neutral hydrogen around star-forming galaxies. Integral field spectrograph measurements of high-redshift Ly alpha emitters indicate that significant extended Ly alpha halo emission is ubiquitous around such objects. We present a sample of redshift 0.23 to 0.31 galaxies observed with the Hubble Space Telescope selected to match the star formation properties of high-z samples while optimizing the observations for detection of low surface brightness Ly alpha emission. The Ly alpha escape fractions range between 0.7 and 37 per cent, and we detect extended Ly alpha emission around six out of seven targets. We find Ly alpha halo to UV scale length ratios around 6:1, which is marginally lower than high-redshift observations, and halo flux fractions between 60 and 85 per cent - consistent with high-redshift observations - when using comparable methods. However, our targets show additional extended stellar UV emission: we parametrize this with a new double exponential model. We find that this parametrization does not strongly affect the observed Ly alpha halo fractions. We find that deeper H alpha data would be required to firmly determine the origin of Ly alpha halo emission; however, there are indications that H alpha is more extended than the central FUV profile, potentially indicating conditions favourable for the escape of ionizing radiation. We discuss our results in the context of high-redshift galaxies, cosmological simulations, evolutionary studies of the circumgalactic medium in emission, and the emission of ionizing radiation.
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