| 1. |
- Berg, Danielle A., et al.
(author)
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The COS Legacy Archive Spectroscopy Survey (CLASSY) Treasury Atlas
- 2022
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In: Astrophysical Journal Supplement Series. - : American Astronomical Society. - 0067-0049 .- 1538-4365. ; 261:2
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Journal article (peer-reviewed)abstract
- Far-ultraviolet (FUV; ∼1200–2000 Å) spectra are fundamental to our understanding of star-forming galaxies, providing a unique window on massive stellar populations, chemical evolution, feedback processes, and reionization. The launch of the James Webb Space Telescope will soon usher in a new era, pushing the UV spectroscopic frontier to higher redshifts than ever before; however, its success hinges on a comprehensive understanding of the massive star populations and gas conditions that power the observed UV spectral features. This requires a level of detail that is only possible with a combination of ample wavelength coverage, signal-to-noise, spectral-resolution, and sample diversity that has not yet been achieved by any FUV spectral database. We present the Cosmic Origins Spectrograph Legacy Spectroscopic Survey (CLASSY) treasury and its first high-level science product, the CLASSY atlas. CLASSY builds on the Hubble Space Telescope (HST) archive to construct the first high-quality (S/N1500 Å ≳ 5/resel), high-resolution (R ∼ 15,000) FUV spectral database of 45 nearby (0.002 < z < 0.182) star-forming galaxies. The CLASSY atlas, available to the public via the CLASSY website, is the result of optimally extracting and coadding 170 archival+new spectra from 312 orbits of HST observations. The CLASSY sample covers a broad range of properties including stellar mass (6.2 < log M⋆(M⊙) < 10.1), star formation rate (−2.0 < log SFR (M⊙ yr−1) < +1.6), direct gas-phase metallicity (7.0 < 12+log(O/H) < 8.8), ionization (0.5 < O32 < 38.0), reddening (0.02 < E(B − V) < 0.67), and nebular density (10 < ne (cm−3) < 1120). CLASSY is biased to UV-bright star-forming galaxies, resulting in a sample that is consistent with the z ∼ 0 mass–metallicity relationship, but is offset to higher star formation rates by roughly 2 dex, similar to z ≳ 2 galaxies. This unique set of properties makes the CLASSY atlas the benchmark training set for star-forming galaxies across cosmic time.
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| 2. |
- Erb, Dawn K., et al.
(author)
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Subkiloparsec Imaging of Lyα Emission in a Low-mass, Highly Ionized, Gravitationally Lensed Galaxy at z=1.84
- 2019
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In: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 884:1
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Journal article (peer-reviewed)abstract
- Low-mass, low-metallicity galaxies at low to moderate (z less than or similar to 3) redshifts offer the best opportunity for detailed examination of the interplay between massive stars, ionizing radiation and gas in sources similar to those that likely reionized the universe. We present new narrowband Hubble Space Telescope observations of Ly alpha emission and the adjacent ultraviolet (UV) continuum in the low-mass (M-star = 2 x 10(8) M-circle dot), low-metallicity (Z similar to 1/20 Z(circle dot)), and highly ionized gravitationally lensed galaxy SL2S J02176-0513 at z.=.1.844. The galaxy has strong Lya emission with photometric equivalent width W-Ly alpha(phot)= 218 +/- 12 angstrom , at odds with the Lya escape fraction of 10%. However, the spectroscopic Ly alpha profile suggests the presence of broad absorption underlying the emission, and the total equivalent width is consistent with the escape fraction once this underlying absorption is included. The Lya emission is more spatially extended than the UV continuum, and the 0 14 spatial resolution of HST coupled with the magnification of gravitational lensing enables us to examine the distribution of Ly alpha and the UV continuum on subkiloparsec scales. We find that the peaks of the Ly alpha emission and the UV continuum are offset by 650 pc, and there is no Ly alpha emission arising from the region with the strongest UV light. Our combined spectroscopic and imaging data imply a significant range in neutral hydrogen column density across the object. These observations offer indirect support for a model in which ionizing radiation escapes from galaxies through channels with low column density of neutral gas.
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| 3. |
- Gazagnes, Simon, et al.
(author)
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Interpreting the Si ii and C ii Line Spectra from the COS Legacy Archive Spectroscopic SurveY Using a Virtual Galaxy from a High-resolution Radiation-hydrodynamic Simulation
- 2023
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In: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 952:2
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Journal article (peer-reviewed)abstract
- Observations of low-ionization state metal lines provide crucial insights into the interstellar medium (ISM) of galaxies, yet, disentangling the physical processes responsible for the emerging line profiles is difficult. This work investigates how mock spectra generated using a single galaxy in a radiation-hydrodynamical simulation can help us interpret observations of a real galaxy. We create 22,500 C ii and Si ii spectra from the virtual galaxy at different times and through multiple lines of sight and compare them with the 45 observations of low-redshift star-forming galaxies from the COS Legacy Spectroscopic SurveY (classy). We find that the mock profiles provide accurate replicates of the observations of 38 galaxies with a broad range of stellar masses (106–109M⊙) and metallicities (0.02–0.55 Z⊙). Additionally, we highlight that aperture losses explain the weakness of the fluorescent emission in several classy spectra and must be accounted for when comparing simulations to observations. Overall, we show that the evolution of a single simulated galaxy can produce a large diversity of spectra whose properties are representative of galaxies of comparable or smaller masses. Building upon these results, we explore the origin of the continuum, residual flux, and fluorescent emission in the simulation. We find that these different spectral features all emerge from distinct regions in the galaxy's ISM, and their characteristics can vary as a function of the viewing angle. While these outcomes challenge simplified interpretations of down-the-barrel spectra, our results indicate that high-resolution simulations provide an optimal framework to interpret these observations.
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| 4. |
- James, Bethan L., et al.
(author)
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CLASSY. II. A Technical Overview of the COS Legacy Archive Spectroscopic Survey
- 2022
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In: Astrophysical Journal Supplement Series. - : American Astronomical Society. - 0067-0049 .- 1538-4365. ; 262:2
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Journal article (peer-reviewed)abstract
- The COS Legacy Archive Spectroscopic SurveY (CLASSY) is designed to provide the community with a spectral atlas of 45 nearby star-forming galaxies that were chosen to cover similar properties to those seen at high z (z > 6). The prime high-level science product of CLASSY is accurately coadded UV spectra, ranging from ∼1000 to 2000 Å, derived from a combination of archival and new data obtained with HST's Cosmic Origins Spectrograph (COS). This paper details the multistage technical processes of creating this prime data product and the methodologies involved in extracting, reducing, aligning, and coadding far-ultraviolet and near-ultraviolet (NUV) spectra. We provide guidelines on how to successfully utilize COS observations of extended sources, despite COS being optimized for point sources, and best-practice recommendations for the coaddition of UV spectra in general. Moreover, we discuss the effects of our reduction and coaddition techniques in the scientific application of the CLASSY data. In particular, we find that accurately accounting for flux calibration offsets can affect the derived properties of the stellar populations, while customized extractions of NUV spectra for extended sources are essential for correctly diagnosing the metallicity of galaxies via C iii] nebular emission. Despite changes in spectral resolution of up to ∼25% between individual data sets (due to changes in the COS line-spread function), no adverse affects were observed on the difference in velocity width and outflow velocities of isolated absorption lines when measured in the final combined data products, owing in part to our signal-to-noise regime of S/N < 20.
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| 5. |
- Matthee, Jorryt, et al.
(author)
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(Re)Solving reionization with Lyα : how bright Lyα Emitters account for the z ≈ 2–8 cosmic ionizing background
- 2022
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 512:4, s. 5960-5977
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Journal article (peer-reviewed)abstract
- The cosmic ionizing emissivity from star-forming galaxies has long been anchored to UV luminosity functions. Here, we introduce an emissivity framework based on Lyα emitters (LAEs), which naturally hones in on the subset of galaxies responsible for the ionizing background due to the intimate connection between production and escape of Lyα and LyC photons. Using constraints on the escape fractions of bright LAEs (LLyα > 0.2L*) at z ≈ 2 obtained from resolved Lyα profiles, and arguing for their redshift-invariance, we show that: (i) quasars and LAEs together reproduce the relatively flat emissivity at z ≈ 2–6, which is non-trivial given the strong evolution in both the star formation density and quasar number density at these epochs and (ii) LAEs produce late and rapid reionization between z ≈ 6−9 under plausible assumptions. Within this framework, the >10 × rise in the UV population-averaged fesc between z ≈ 3–7 naturally arises due to the same phenomena that drive the growing LAE fraction with redshift. Generally, a LAE dominated emissivity yields a peak in the distribution of the ionizing budget with UV luminosity as reported in latest simulations. Using our adopted parameters (fesc=50 per cent, ξion = 1025.9 Hz erg−1 for half the bright LAEs), a highly ionizing minority of galaxies with MUV < −17 accounts for the entire ionizing budget from star-forming galaxies. Rapid flashes of LyC from such rare galaxies produce a ‘disco’ ionizing background. We conclude proposing tests to further develop our suggested Lyα-anchored formalism.
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| 6. |
- Naidu, Rohan P., et al.
(author)
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The synchrony of production and escape : half the bright Lyα emitters at z ≈ 2 have Lyman continuum escape fractions ≈50 per cent
- 2022
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 510:3, s. 4582-4607
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Journal article (peer-reviewed)abstract
- The ionizing photon escape fraction [Lyman continuum (LyC) fesc] of star-forming galaxies is the single greatest unknown in the reionization budget. Stochastic sightline effects prohibit the direct separation of LyC leakers from non-leakers at significant redshifts. Here we circumvent this uncertainty by inferring fesc using resolved (R > 4000) Lyman α (Lyα) profiles from the X-SHOOTER Lyα survey at z = 2 (XLS-z2). With empirically motivated criteria, we use Lyα profiles to select leakers (fesc>20 per centfesc>20 per cent) and non-leakers (fesc<5 per centfesc<5 per cent) from a representative sample of >0.2L* Lyman α emitters (LAEs). We use median stacked spectra of these subsets over λrest ≈ 1000–8000 Å to investigate the conditions for LyC fesc. Our stacks show similar mass, metallicity, MUV, and βUV. We find the following differences between leakers versus non-leakers: (i) strong nebular C IV and He II emission versus non-detections; (ii) [O III]/[O II] ≈ 8.5 versus ≈3; (iii) Hα/Hβ indicating no dust versus E(B − V) ≈ 0.3; (iv) Mg II emission close to the systemic velocity versus redshifted, optically thick Mg II; and (v) Lyα fesc of ≈50 per cent≈50 per cent versus ≈10 per cent≈10 per cent. The extreme equivalent widths (EWs) in leakers ([O III]+Hβ≈1100Hβ≈1100 Å rest frame) constrain the characteristic time-scale of LyC escape to ≈3–10 Myr bursts when short-lived stars with the hardest ionizing spectra shine. The defining traits of leakers – extremely ionizing stellar populations, low column densities, a dust-free, high-ionization state interstellar medium (ISM) – occur simultaneously in the fesc>20 per centfesc>20 per cent stack, suggesting they are causally connected, and motivating why indicators like [O III]/[O II] may suffice to constrain fesc at z > 6 with the James Webb Space Telescope (JWST). The leakers comprise half of our sample, have a median LyCfesc≈50 per centfesc≈50 per cent (conservative range: 20−55 per cent20−55 per cent), and an ionizing production efficiency log(ξion/Hz erg−1)≈25.9log(ξion/Hz erg−1)≈25.9 (conservative range: 25.7–25.9). These results show LAEs – the type of galaxies rare at z ≈ 2, but that become the norm at higher redshift – are highly efficient ionizers, with extreme ξion and prolific fesc occurring in sync.
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| 7. |
- Rivera-Thorsen, Thøger Emil, 1979-
(author)
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Ionized and atomic interstellar medium in star-forming galaxies
- 2016
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Doctoral thesis (other academic/artistic)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.
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| 8. |
- Xu, Xinfeng, et al.
(author)
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Tracing Lyα and LyC Escape in Galaxies with Mg ıı Emission
- 2022
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In: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 933:2
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Journal article (peer-reviewed)abstract
- Star-forming galaxies are considered the likeliest source of the H i ionizing Lyman continuum (LyC) photons that reionized the intergalactic medium at high redshifts. However, above z ≳ 6, the neutral intergalactic medium prevents direct observations of LyC. Therefore, recent years have seen the development of indirect indicators for LyC that can be calibrated at lower redshifts and applied in the epoch of reionization. Emission from the Mg ıı λλ2796, 2803 doublet has been proposed as a promising LyC proxy. In this paper, we present new Hubble Space Telescope/Cosmic Origins Spectrograph observations for eight LyC emitter candidates, selected to have strong Mg ii emission lines. We securely detect LyC emission in 50% (4/8) of the galaxies with 2σ significance. This high detection rate suggests that strong Mg ii emitters might be more likely to leak LyC than similar galaxies without strong Mg ıı. Using photoionization models, we constrain the escape fraction of Mg ii as ∼15%–60%. We confirm that the escape fraction of Mg ıı correlates tightly with that of Lyα, which we interpret as an indication that the escape fraction of both species is controlled by resonant scattering in the same low column density gas. Furthermore, we show that the combination of the Mg ıı emission and dust attenuation can be used to estimate the escape fraction of LyC statistically. These findings confirm that Mg ıı emission can be adopted to estimate the escape fraction of Lyα and LyC in local star-forming galaxies and may serve as a useful indirect indicator at the epoch of reionization.
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