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The origins of Lyman continuum (LyC) photons responsible for the reionization of the universe are as of yet unknown and highly contested. Detecting LyC photons from the Epoch of Reionization is not possible due to absorption by the intergalactic medium, which has prompted the development of several indirect diagnostics to infer the rate at which galaxies contribute LyC photons to reionize the universe by studying lower-redshift analogs. We present the Low-redshift Lyman Continuum Survey (LzLCS) comprising measurements made with the Hubble Space Telescope Cosmic Origins Spectrograph for a z = 0.2–0.4 sample of 66 galaxies. After careful processing of the far-UV spectra, we obtain a total of 35 Lyman continuum emitters (LCEs) detected with 97.725% confidence, nearly tripling the number of known local LCEs. We estimate escape fractions from the detected LyC flux and upper limits on the undetected LyC flux, finding a range of LyC escape fractions up to 50%. Of the 35 LzLCS LCEs, 12 have LyC escape fractions greater than 5%, more than doubling the number of known local LCEs with cosmologically relevant LyC escape.

We compare mock ultraviolet C ii and Si ii absorption and emission line features generated using a ∼109 M ⊙ virtual galaxy with observations of 131 z ∼ 3 galaxies from the vandels survey. We find that the mock spectra reproduce reasonably well a large majority (83%) of the vandels spectra (χ 2 < 2), but do not resemble the most massive objects (⪆1010 M ⊙), which exhibit broad absorption features. Interestingly, the best-matching mock spectra originate from periods of intense star formation in the virtual galaxy, where its luminosity is 4 times higher than in periods of relative quiescence. Furthermore, for each galaxy, we predict the Lyman continuum (LyC) escape fractions ( fesc(pred)LyC ) using the environment of the virtual galaxy. We derive an average fesc(pred)LyC of 0.01 ± 0.02, consistent with other estimates from the literature. The fesc(pred)LyC are tightly correlated with the Lyα escape fractions and highly consistent with observed empirical trends. Additionally, galaxies with larger fesc(pred)LyC exhibit bluer β slopes, more Lyα flux, and weaker low-ionization absorption lines. Building upon the good agreement between fesc(pred)LyC and observationally established LyC diagnostics, we examine the LyC leakage mechanisms in the simulation. We find that LyC photon leakage is enhanced in directions where the observed flux dominantly emerges from compact regions depleted of neutral gas and dust, mirroring the scenario inferred from observational data. In general, this study further highlights the potential of high-resolution radiation hydrodynamics simulations in analyzing UV absorption and emission line features and providing valuable insights into the LyC leakage of star-forming galaxies.

Ultraviolet absorption line spectroscopy is a sensitive diagnostic for the properties of interstellar and circumgalactic gas. Down-the-barrel observations, where the absorption is measured against the galaxy itself, are commonly used to study feedback from galactic outflows and to make predictions about the leakage of H i ionizing photons into the intergalactic medium. Nonetheless, the interpretation of these observations is challenging, and observational compromises are often made in terms of signal-to-noise ratio, spectral resolution, or the use of stacking analyses. In this paper, we present a novel quantitative assessment of UV absorption line measurement techniques by using mock observations of a hydrodynamical simulation. We use a simulated galaxy to create 22,500 spectra in the commonly used Si ii lines while also modeling the signal-to-noise ratio and spectral resolution of recent rest-frame UV galaxy surveys at both high and low redshifts. We show that the residual flux of absorption features is easily overestimated for single line measurements and for stacked spectra. Additionally, we explore the robustness of the partial covering model for estimating column densities from spectra and find underpredictions on an average of 1.25 dex. We show that the underprediction is likely caused by high-column-density sight lines that are optically thick to dust making them invisible in UV spectra.

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 (106–109 M ⊙) 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.

The Epoch of Reionization (EoR) provides critical insights into the role of early galaxies in shaping the ionization state of the Universe. However, because of the opacity of the intergalactic medium, it is often not possible to make direct measurements of the ionizing photon escape fraction ( fescLyC ) of high-redshift (z ≳ 4) galaxies. To explore the agreement and systematics of common indirect approaches, we applied six empirically calibrated diagnostics to predict fescLyC for the 45 nearby star-forming galaxies from the COS Legacy Spectroscopic SurveY. These methods—based on ultraviolet (UV) absorption lines, the UV continuum slope, Lyα kinematics, a multivariate model, radiation-hydrodynamic simulations, and nebular emission-line ratios—enable us to explore systematic differences between predictions and assess how galactic properties influence inferred LyC escape. Despite significant variations in method predictions, there is broad consistency in the resulting weak and strong LyC leaker classifications, with approximately half exhibiting predicted escape fractions >1%. We find evidence for two different pathways of LyC escape in nearby star-forming galaxies: (1) an early escape model driven by very young stellar populations, and (2) a delayed escape model that is consistent with supernova-driven outflows and time-dependent interstellar medium clearing. The early escape model is favored among galaxies with a single, intense burst of recent star formation. In contrast, the delayed escape model is common among galaxies with more extended starburst histories. To interpret ionizing photon escape during the EoR, it will be necessary to recognize and understand this diversity in LyC escape mechanisms.

The Lyman continuum (LyC) cannot be observed at the epoch of reionization (z ≳ 6) owing to intergalactic H i absorption. To identify LyC emitters (LCEs) and infer the fraction of escaping LyC, astronomers have developed various indirect diagnostics of LyC escape. Using measurements of the LyC from the Low-redshift Lyman Continuum Survey (LzLCS), we present the first statistical test of these diagnostics. While optical depth indicators based on Lyα, such as peak velocity separation and equivalent width, perform well, we also find that other diagnostics, such as the [O iii]/[O ii] flux ratio and star formation rate surface density, predict whether a galaxy is an LCE. The relationship between these galaxy properties and the fraction of escaping LyC flux suggests that LyC escape depends strongly on H i column density, ionization parameter, and stellar feedback. We find that LCEs occupy a range of stellar masses, metallicities, star formation histories, and ionization parameters, which may indicate episodic and/or different physical causes of LyC escape.

We calculate the redshift evolution of the global 21cm signal in the first billion years using a semi-analytic galaxy formation model, DELPHI, that jointly tracks the assembly of dark matter halos and their constituent baryons including the impact of supernova feedback and dust enrichment. Employing only two redshift- and mass-independent free parameters, our model predicts galaxy populations in accord with data from both the James Webb Space Telescope (JWST) and the Atacama Large Millimetre Array (ALMA) at \\(z 5-12\\). In addition to this ``fiducial\" model, which fully incorporates the impact of dust attenuation, we also explore an unphysical ``maximal\" model wherein galaxies can convert a 100\\% of their gas into stars instantaneously (and supernova feedback is ignored) required to explain JWST data at \\(z >=13\\). We also explore a wide range of values for our ıt 21cm parameters that include the impact of X-ray heating (\\(f_ X,h =0.02-2.0\\)) and the escape fraction of Lyman Alpha photons (\\(f_ = 0.01-1.0\\)). Our key findings are: (i) the fiducial model predicts a global 21cm signal which reaches a minimum brightness temperature of \\( T_ b, min -215\\) mK at a redshift \\(z_ min 14\\); (ii) since the impact of dust on galaxy properties (such as the star formation rate density) only becomes relevant at \\(z <= 8\\), dust does not have a sensible impact on the global 21cm signal; (iii) the ``maximal\" model predicts \\(T_ b, min= -210\\) mK as early as \\(z_ min 18\\); (iv) galaxy formation and 21cm parameters have a degenerate impact on the global 21cm signal. A combination of the minimum temperature and its redshift will therefore be crucial in constraining galaxy formation parameters and their coupling to the 21cm signal at these early epochs.

Recent observations with the James Webb Space Telescope are yielding tantalizing hints of an early population of massive, bright galaxies at \\(z > 10\\), with Atacama Large Millimeter Array (ALMA) observations indicating significant dust masses as early as \\(z 7\\). To understand the implications of these observations, we use the DELPHI semi-analytic model that jointly tracks the assembly of dark matter halos and their baryons, including the key processes of dust enrichment. Our model employs only two redshift- and mass-independent free parameters (the maximum star-formation efficiency and the fraction of supernova energy that couples to gas) that are tuned against all available galaxy data at \\(z 5-9\\) before it is used to make predictions up to \\(z 20\\). Our key results are: (i) the model under-predicts the observed ultraviolet luminosity function (UV LF) at \\(z > 12\\); observations at \\(z>16\\) lie close to, or even above, a \"maximal\" model where all available gas is turned into stars; (ii) UV selection would miss 34\\% of the star formation rate density at \\(z 5\\), decreasing to 17\\% by \\(z 10\\) for bright galaxies with \\(M_UV < -19\\); (iii) the dust mass (\\(M_d\\)) evolves with the stellar mass (\\(M_*\\)) and redshift as \\((M_d) = 1.194(M_*) + 0.0975z - 5.433\\); (iv) the dust temperature increases with stellar mass, ranging between \\(30-33\\) K for \\(M_* 10^9-11M_\\) galaxies at \\(z 7\\). Finally, we predict the far infrared LF at \\(z 5-20\\), testable with ALMA observations, and caution that spectroscopic redshifts and dust masses must be pinned down before invoking unphysical extrema in galaxy formation models.

The field of high redshift galaxy formation has been revolutionised by JWST, which is yielding unprecedented insights on galaxy assembly at early times. Our key aim is to study the physical mechanisms that can explain the unexpected abundance of bright galaxies at \\(z 11\\), as well as their metal enrichment and spectral properties. We also use recent data to determine the key sources of reionisation. To do so, we implement cold gas fractions and star formation efficiencies derived from the SPHINX20 high-resolution radiation-hydrodynamics simulation into DELPHI, a semi-analytic model that tracks the assembly of dark matter halos and their baryonic components from \\(z 4.5-40\\). In addition, we explore two different methodologies to boost galaxy luminosities at \\(z 11\\): a stellar initial mass function (IMF) that becomes increasingly top-heavy with decreasing metallicity and increasing redshift (eIMF model), and star formation efficiencies that increase with increasing redshift (eSFE model). Our key findings are: (i) both the eIMF and eSFE models can explain the abundance of bright galaxies at \\(z 11\\); (ii) dust attenuation plays an important role for the bright-end of the UV LF at \\(z 11\\); (iii) the mass-metallicity relation is in place as early as \\(z 17\\) in all models although its slope is model-dependent; (iv) within the spread of both models and observations, all of our models are in good agreement with current estimates of \\(\\) slopes at \\(z 5-17\\) and Balmer break strengths at \\(z 6-10\\); (v) in the eIMF model, galaxies at \\(z12\\) or with \\(M_UV-18\\) show values of \\(_ion 10^25.55~ [Hz~erg^-1]\\), twice larger than in other models; (vi) star formation in galaxies below \\(10^9M_\\) is the key driver of reionisation, providing the bulk (\\( 85\\%\\)) of ionising photons down to its midpoint at \\(z 7\\).