arXiv subjects

Alexandra Le Reste

Publications and source records attributed to Alexandra Le Reste.

16 recordsLinked to original sources

arXiv2026

Specific Star Formation Rate Enhancement across the Galaxy Merger Sequence: Insights from Citizen Science Classifications

We present an analysis of specific star formation rates (sSFR) across the galaxy merger sequence using visual classifications from the Zooniverse citizen science project "Cosmic Disco: Characterizing Galaxy Collisions". Our sample comprises 4884 galaxy systems pre-selected as merger candidates from SDSS DR17 ($0.01 < z < 0.05$, $M_* > 10^{8.5}M_\odot$) using Zoobot, of which 3690 were classified as mergers spanning pre-interaction through post-coalescence stages by citizen scientist volunteers. We find a weak but statistically significant positive correlation between $\log(\mathrm{sSFR})$ and visual merger stage ($r = 0.161$, $p = 7.23 \times 10^{-23}$), with a best-fit relation $\log\left(sSFR\right)=(0.148\pm0.015)\, S_{\rm Merg}-(1.865\pm0.038)$. The large RMS scatter (0.661 dex) reflects visual merger stages capturing wide merger timescales, and our results corroborate previous findings of increasing SFR enhancement with merger progression. This work shows that citizen science is a viable complement to automated and pair-based approaches to evaluate timescales for galaxies across the merger sequence.

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arXiv2026

The NIRISS PASSAGE Spectroscopic Redshift Catalog in COSMOS

We present the Parallel Application of Slitless Spectroscopy to Analyze Galaxy Evolution (PASSAGE) spectroscopic redshift catalog in the COSMOS field. PASSAGE is a JWST Cycle 1 Near Infrared Imager and Slitless Spectrograph (NIRISS) wide-field slitless spectroscopy (WFSS) pure-parallel survey, obtaining near-infrared spectra of thousands of extragalactic sources. 15 out of 63 PASSAGE fields fall within the Hubble Space Telescope (HST) COSMOS footprint, of which 11 overlap with COSMOS-Web, a JWST treasury survey providing additional space-based photometry. We present our custom line-finding algorithm and visual inspection effort used to identify emission lines and derive the spectroscopic redshifts for line-emitting sources in PASSAGE. The line-finding algorithm identifies between ~200 and 950 line-emitting candidates per field, of which typically 47% were identified as true emission lines post visual inspection. We identify 2183 emission line sources at 0.08 5), multi-line emitting sources. This agreement weakens for PASSAGE single-line emitters with ambiguous identities. These single-line emitters are likely mis-identified around 18% of the time based on comparisons to photometric redshifts. We derive stellar masses using PASSAGE photometry and spectroscopic redshifts, in broad agreement with existing COSMOS-Web stellar masses, but with some discrepancy driven by redshift disagreements. We publicly release this spectroscopic redshift catalog, which will enable community-led science in prime extragalactic fields and serve as a crucial dataset for validating Euclid and Roman spectroscopy.

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arXiv2025

Supernovae Driven Winds Impede Lyman Continuum Escape from Dwarf Galaxies in First 10 Myr

Observations suggest that UV-bright, compact star-forming galaxies produce enough ionizing (Lyman continuum; LyC) photons to reionize the Universe. Yet, the efficiency of LyC escape and the roles of radiation, stellar winds, and supernovae remain uncertain. Using medium-resolution spectra of six nearly identical local star-forming galaxies, we directly trace, for the first time, the evolution of a multiphase wind through individual spectral lines alongside measurements of the LyC escape fraction. We find that LyC escape peaks early, during a period dominated by intense radiation and stellar winds but lacking a fast galactic wind. As the starbursts age, supernovae drive and accelerate the wind, progressively suppressing LyC escape. These results highlight the need for cosmological simulations to incorporate early feedback as a key driver of reionization.

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arXiv2025

The Ly$\alpha$ and Continuum Origins Survey. III. Investigating the Link between Galaxy Morphology, Merger Properties and LyC Escape

Characterizing the mechanisms and galaxy properties conducive to the emission and escape of ionizing (LyC) emission is necessary to accurately model the Epoch of Reionization, and identify the sources that powered it. The Ly$\alpha$ and Continuum Origins Survey (LaCOS) is the first program to obtain uniform, multi-wavelength subkiloparsec imaging for a large sample (42) of galaxies observed in LyC and enable statistically robust studies between LyC and resolved galaxy properties. Here, we characterize the morphology and galaxy merger properties of LaCOS galaxies and investigate their connection with the escape fraction of LyC emission $f_{esc}^{LyC}$. We find strong anticorrelations between $f_{esc}^{LyC}$ and size ($r_{20}$, $r_{50}$, and $r_{80}$) measured in filters containing emission from star-forming regions, and with the asymmetry and clumpiness in F150LP, the bluest filter in our dataset, tracing UV continuum and Ly$\alpha$. We find that $\geq48\%$ of LaCOS galaxies, and $\geq41\%$ of LaCOS LyC-emitters are visually classified as galaxy mergers. Galaxies robustly identified as mergers in LaCOS are at advanced stages of interaction, close to coalescence. The $f_{esc}^{LyC}$ properties of robust mergers and low-probability mergers cannot be differentiated statistically, and we only find significant difference between the two populations in terms of their of their sizes and LyC luminosity: robust mergers having larger values. We conclude that (i) $f_{esc}^{LyC}$ tends to be larger in galaxies with a small number of compact, centrally-located, UV-emitting star-forming regions, (ii) at advanced stages of interaction represent a sizable fraction of LyC-emitting samples at $z\sim0.3$, $z\sim0$, and (iii) mergers can facilitate the escape of LyC photons from galaxies.

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arXiv2025

Identifying the contributors to cosmic reionization with the Habitable World Observatory: LyC escape calibration in faint galaxies

Investigating how small-scale physical processes shape large-scale astrophysical phenomena is one of the key science themes of the Astro2020 decadal Survey. An example of this interplay is Cosmic Reionization, where ionizing photons from galaxies escaped into the intergalactic medium (IGM), driving its transition from neutral to ionized at $z>6$. Star-forming galaxies are thought to be the dominant sources of reionization. However, an open question remains as to whether bright or faint star-forming galaxies are the primary reionization contributors. This depends on the escape fraction $f_{~esc}^{LyC}$ -- the fraction of ionizing photons that successfully escape the galaxy's interstellar and circumgalactic medium to reach the IGM. Performing direct measurements of $f_{~esc}^{LyC}$ during the reionization epoch is not feasible, due to the near-zero transmission of the IGM at $z\gtrsim4$. However, by calibrating $f_{~esc}^{LyC}$ against indirect indicators (i.e. galaxy properties that correlate with $f_{~esc}^{LyC}$) at lower $z$, we can estimate its value in galaxies during reionization. Currently, $f_{~esc}^{LyC}$ calibrations are mostly limited to $\sim 90$ sources brighter $M_{UV} \sim -18$ at $z\sim0.3$. Yet, recent models suggest that fainter galaxies ($-19<M_{UV}<-13$) may be the major contributors to reionization. Our science goal is to extend $f_{~esc}^{LyC}$ calibrations to such faint magnitudes. With the Habitable Worlds Observatory, we aim to achieve a $\sim$ 20-fold increase in statistical power. Additionally, we plan to reach magnitudes as faint as 1/100 $L^*$ at $z\sim0.1$. Suitable targets will be selected from the upcoming Ultraviolet Explorer (UVEX) survey. Through this science case, we will be able to place robust constraints on the role of faint galaxies in the reionization of the Universe and identify its primary contributors.

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arXiv2025

Spatially Resolving the Fundamental Elements of Reionization in Galaxies

Cosmic reionization marks a critical epoch when the first galaxies ionized the intergalactic medium through the escape of Lyman continuum (LyC) radiation. Young, massive star clusters are believed to be the primary LyC sources, yet the physical mechanisms enabling LyC escape remain poorly understood. Most existing studies rely on spatially integrated observations, which lack the resolution to resolve internal galaxy structure and pinpoint where and how LyC photons escape. To address this, we propose a science case for the Habitable Worlds Observatory (HWO) that enables spatially resolved spectroscopy of LyC-emitting star clusters and their environments in low-redshift galaxies. This requires a UV integral field unit (IFU) with coverage down to ~ 900 Angstrom and a spatial resolution of 10-100 pc-capabilities essential for directly detecting LyC escape and mapping the surrounding interstellar medium. With such instrumentation, we will map cluster-scale LyC escape fractions, characterize the physical conditions of the surrounding interstellar medium, and directly observe feedback-driven outflows that facilitate LyC leakage. These observations will enable novel calibrations of indirect LyC indicators at unprecedented spatial resolution and establish direct connections between local LyC processes and those in high-redshift, clumpy star-forming galaxies. In the long run, this program will build the physical framework needed to understand how galaxies reionized the early universe and shaped its subsequent evolution.

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arXiv2025

Evolution of the Ionizing Photon Luminosity Function

Counting the number and brightness of ionizing radiation sources out to a redshift of z ~ 1.2 will revolutionize our understanding of how the ionizing background is created and sustained by the embedded growth of meta-galactic structures. The sheer number of sparsely separated targets required to efficiently construct redshift binned luminosity functions is industrial in scale, driving the need for low spectral resolution multi-object spectroscopy (MOS) with a short wavelength cut-off ~ 1000 {\AA}, a sensitivity in the far-UV to better than 30 abmag, and an instantaneous field-of-view ~ (2')$^2$. A MOS on Habitable Worlds Observatory is the only instrument that could conceivably carry out such an ambitious observing program. This program will quantify how much of the ionizing radiation produced by galaxies is attenuated by intervening neutral H, He and dust, and how much escapes to maintain the universe in a mostly ionized state.

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arXiv2025

Power-law Emission-line Wings and Radiation-Driven Superwinds in Local Lyman Continuum Emitters

We investigate broad emission-line wings, reaching $\leq 800\rm~km~s^{-1}$, observed in 26 galaxies with Lyman continuum (LyC) observations, primarily from the Low-redshift Lyman Continuum Survey (LzLCS). Using Magellan/MIKE, VLT/X-shooter, and WHT/ISIS high-resolution spectroscopy, we show that this fast gas appears to probe the dominant feedback mechanisms linked to LyC escape. We find that in 14 galaxies, the wings are best fit with power laws of slope $\alpha \sim -3.5 \text{ to } -1.6$, with four others best fit by Gaussians of width $\sigma_{\rm BW} \sim 300~\rm km~s^{-1}$; the remaining eight show ambiguous wing morphologies. Gaussian wings are found only at low $O_{32}$ = $[\rm O~III]\lambda5007/[O~II]\lambda3726,3729$ and high metallicity, while power-law wings span the full range of these parameters. The general evidence suggests a dual-mode paradigm for LyC escape: radiation-driven superwinds traced by power-law wings and supernova-driven feedback traced by Gaussian wings. For the former, the $<3$ Myr-old, pre-supernova stellar population correlates with more luminous, faster winds. The data also show that radiation-driven wind parameters like wind luminosity and power-law slope $\alpha$ depend on the UV luminosity more than the optically thick covering fraction, consistent with ``picket-fence" radiative transfer. Observed $\alpha$ values flatten with both escaping LyC luminosity and higher extinction, while still preserving the anticorrelation between these two quantities. Additionally, the differential between red and blue slopes implies that extinction and dense gas are centrally concentrated relative to the wind emission. Overall, our results show that power-law emission-line wings probe LyC-driven winds and LyC escape in metal-poor starbursts.

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arXiv2025

The Lyman-alpha and Continuum Origins Survey. I. Survey description and Ly$\alpha$ imaging

Understanding the mechanisms driving the escape of ionizing or Lyman continuum (LyC) emission from the interstellar medium of galaxies is necessary to constrain the evolution of reionization, and the sources responsible for it. While progress has been made into identifying the global galaxy properties linked to the escape fraction of ionizing radiation, $f_{esc}^{LyC}$, little is currently known about how spatially resolved galaxy properties impact this parameter. We present Hubble Space Telescope (HST) imaging data obtained as part of the Lyman $\alpha$ and Continuum Origins Survey (LaCOS). LaCOS consists of HST imaging in 5 filters covering rest-frame optical and UV bands for a subsample of 42 galaxies in the Low redshift Lyman Continuum Survey, 22 being Lyman continuum emitters ($f_{esc}^{LyC}=0.01-0.49$). Here, we describe the sample, observations and data reduction, and investigate connections between global and sub-kpc Lyman $\alpha$ (Ly$\alpha$) emission, and $f_{esc}^{LyC}$. We confirm the correlation between $f_{esc}^{LyC}$ and EW$_{Ly\alpha}$ , and the anticorrelation with $r_{50}$, when using values obtained via global photometry. We also find correlations previously found with spectroscopy with global photometric $L_{Ly\alpha}$, $f_{esc}^{Ly\alpha}$, $\Sigma_{SFR}$ , and $f_{esc}^{LyC}$, but with a smaller degree of correlation ($\overline{|\Delta\tau|}\sim0.1$). We find correlations are strongest between $Ly\alpha$ observables ($L_{\rm Ly\alpha}$, EW$_{\rm Ly\alpha}$) and $f_{\rm esc}^{\rm LyC}$ when measured in a small aperture around the brightest UV source in each galaxy. We interpret these results as evidence that LyC photons escaping on the line-of-sight are contributed by a small number of UV-bright compact regions in most galaxies in LaCOS.

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arXiv2025

The Parallel Ionizing Emissivity Survey (PIE). I. Survey design and selection of candidate Lyman Continuum leakers at 3.1<z<3.5

We present the survey design and initial results from the Parallel Ionizing Emissivity (PIE) survey. PIE is a large HST survey designed to detect Lyman continuum (LyC) emitting galaxies at 3.1$<$ z $<$3.5 and stack their images in order to measure average LyC escape fractions as a function of galaxy properties. PIE has imaged 37 independent fields in three filters (F336W, F625W and F814W), of which 18 are observed with a fourth band (F475W) from the accompanying PIE+ program. We use photometric colors to select candidate Lyman Break Galaxies (LBGs) at 3.1$<$ z $<$3.5, which can be followed up using ground-based spectrographs to confirm their redshifts. Unlike previous surveys, we use many independent fields to remove biases caused by correlated absorption in the IGM. In this paper, we describe the survey design, photometric measurements, and the use of mock galaxy samples to optimize our color selection. With 3 filters, we can select a galaxy sample of which $\approx$90\% are LBGs and over 30\% lie in the 3.1$<$ z $<$3.5 range for which we can detect uncontaminated LyC emission in F336W. We also use mock IGM sightlines to measure the expected transmission of the IGM, which will allow us to determine escape fractions from our stacked galaxies. We color-select $\approx$1400 galaxies, and predict that this includes $\approx$80 LyC-emitting galaxies and $\approx$500 that we can use in stacking. Finally, we present the Keck/LRIS spectrum of a galaxy at z $\approx$ 2.99, demonstrating that we can confirm the redshifts of z $\sim$ 3 galaxies from the ground.

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arXiv2025

Forming the local starburst galaxy Haro 11 through hydrodynamical merger simulations

Haro 11 is a metal-poor, starburst galaxy believed to be the result of an ongoing merger, which is shaping the properties of the galaxy. In this study, we carry out a large suite of numerical simulations of a merger between two disc galaxies, to study possible origins of Haro 11 and understand under which conditions various features of the galaxy are formed. By varying galaxy parameters describing the orbital configurations, masses, and their inclination, we perform a total of $\sim$500 simulations. We demonstrate that a two-disc galaxy merger reproduces key, observed features of Haro 11, including its morphology, gas kinematics, star formation history, and stellar population ages and masses. In particular, we present a fiducial Haro 11 model that produces the single observed tidal tail, three stellar knots, and inner gas morphology and kinematics. The resulting orbit and galactic morphology are robust against small variations of the initial parameters. By performing mock observations, we compare with the results of observational data and discuss possible origins for various features. Furthermore, we present newly gathered observational data that confirms the presence of a stellar tidal tail with similar length and morphology as our simulations.

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arXiv2024

The Low-Redshift Lyman Continuum Survey: The Roles of Stellar Feedback and ISM Geometry in LyC Escape

One of the fundamental questions of cosmology is the origin and mechanism(s) responsible for the reionization of the Universe beyond $z\sim6$. To address this question, many studies over the past decade have focused on local ($z\sim0.3$) galaxies which leak ionizing radiation (Lyman continuum or LyC). However, line-of-sight effects and data quality have prohibited deeper insight into the nature of LyC escape. To circumvent these limitations, we analyze stacks of a consolidated sample of {\it HST}/COS observations of the LyC in 89 galaxies at $z\sim0.3$. From fitting of the continuum, we obtain information about the underlying stellar populations and neutral ISM geometry. We find that most LyC non-detections are not leaking appreciable LyC ($f_{esc}^{\rm LyC}<1$\%) but also that exceptional cases point to spatial variations in the LyC escape fraction $f_{esc}^{\rm LyC}$. Stellar populations younger than 3 Myr lead to an increase in ionizing feedback, which in turn increases the isotropy of LyC escape. Moreover, mechanical feedback from supernovae in 8-10 Myr stellar populations is important for anisotropic gas distributions needed for LyC escape. While mechanical feedback is necessary for any LyC escape, high $f_{esc}^{\rm LyC}$ ($>5$\%) also requires a confluence of young stars and ionizing feedback. A two-stage burst of star formation could facilitate this optimal LyC escape scenario.

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arXiv2024

Linking Mg II and [O II] spatial distribution to ionizing photon escape in confirmed LyC leakers and non-leakers

The geometry of the neutral gas in and around galaxies is a key regulator of the escape of ionizing photons. We present the first statistical study aiming at linking the neutral and ionized gas distributions to the Lyman continuum (LyC) escape fraction (fesc(LyC)) in a sample of 22 confirmed LyC leakers and non-leakers at z~0.35 using the Keck Cosmic Web Imager (Keck/KCWI) and the Low Resolution Spectrograph 2 (HET/LRS2). Our integral field unit data enable the detection of neutral and low-ionization gas, as traced by Mg II, and ionized gas, as traced by [O II], extending beyond the stellar continuum for 7 and 10 objects, respectively. All but one object with extended Mg II emission also shows extended [O II] emission; in this case, Mg II emission is always more extended than [O II] by a factor 1.3 on average. Most of the galaxies with extended emission are non or weak LyC leakers (fesc(LyC) < 5%), but we find a large diversity of neutral gas configurations around these weakly LyC-emitting galaxies. Conversely, the strongest leakers (fesc(LyC) > 10%) appear uniformly compact in both Mg II and [O II] with exponential scale lengths <1 kpc. We also find a trend between fesc(LyC) and the spatial offsets of the nebular gas and the stellar continuum emission. Moreover, we find significant anti-correlations between the spatial extent of the neutral gas and the [O III]/[O II] ratio, and H$\beta$ equivalent width, as well as positive correlations with metallicity and UV size, suggesting that galaxies with more compact neutral gas sizes are more highly ionized. The observations suggest that strong LyC emitters do not have extended neutral gas halos and ionizing photons may be emitted in many directions. Combined with high ionization diagnostics, we propose the Mg II, and potentially [O II], spatial compactness are indirect indicators of LyC emitting galaxies at high-redshift.

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arXiv2023

Tidally offset neutral gas in Lyman continuum emitting galaxy Haro 11

Around 400 million years after the Big Bang, the ultraviolet emission from star-forming galaxies reionized the Universe. Ionizing radiation (Lyman Continuum, LyC) is absorbed by cold neutral hydrogen gas (HI) within galaxies, hindering the escape of LyC photons. Since the HI reservoir of LyC emitters has never been mapped, major uncertainties remain on how LyC photons escape galaxies and ionize the intergalactic medium. We have directly imaged the neutral gas in the nearby reionization-era analog galaxy Haro 11 with the 21cm line to identify the mechanism enabling ionizing radiation escape. We find that merger-driven interactions have caused a bulk offset of the neutral gas by about 6 kpc from the center of the galaxy, where LyC emission production sites are located. This could facilitate the escape of ionizing radiation into our line of sight. Galaxy interactions can cause both elevated LyC production and large-scale displacement of HI from the regions where these photons are produced. They could contribute to the anisotropic escape of LyC radiation from galaxies and the reionization of the Universe. We argue for a systematic assessment of the effect of environment on LyC production and escape.

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arXiv2023

Unveiling the gravitationally unstable disc of a massive star-forming galaxy using NOEMA and MUSE

Using new high-resolution data of CO (2-1), H-alpha and H-beta obtained with the Northern Extended Millimeter Array (NOEMA) and the Multi-Unit Spectroscopic Explorer (MUSE) at the Very Large Telescope, we have performed a Toomre-Q disc stability analysis and studied star formation, gas depletion times and other environmental parameters on sub-kpc scales within the z~0 galaxy SDSS J125013.84+073444.5 (LARS 8). The galaxy hosts a massive, clumpy disc and is a proto-typical analogue of main-sequence galaxies at z~1-2. We show that the massive (molecular) clumps in LARS 8 are the result of an extremely gravitationally unstable gas disc, with large scale instabilities found across the whole extent of the rotating disc, with only the innermost 500 pc being stabilized by its bulgelike structure. The radial profiles further reveal that - contrary to typical disc galaxies - the molecular gas depletion time decreases from more than 1 Gyr in the center to less than ~100 Myr in the outskirts of the disc, supporting the findings of a Toomre-unstable disc. We further identified and analysed 12 individual massive molecular clumps. They are virialized and follow the mass-size relation, indicating that on local (cloud/clump) scales the stars form with efficiencies comparable to those in Milky Way clouds. The observed high star formation rate must thus be the result of triggering of cloud/clump formation over large scales due to disc instability. Our study provides evidence that "in-situ" massive clump formation (as also observed at high redshifts) is very efficiently induced by large-scale instabilities.

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arXiv2023

The Lyman $α$ Reference Sample XIV: Lyman $α$ imaging of 45 low redshift star-forming galaxies and inferences on global emission

We present Ly $α$ 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 using far-ultraviolet (FUV) luminosity and \ha equivalent width criteria, but no constraints on Ly $α$ luminosity. We employ a pixel stellar continuum fitting code to obtain accurate continuum subtracted Ly $α$, H $α$ and H $β$ maps. We find that Ly $α$ is less concentrated than FUV and optical line emission in almost all galaxies with significant Ly $α$ emission. We present global measurements of Ly $α$ and other quantities measured in apertures designed to capture all of the Ly $α$ emission. We then show how the escape fraction of Ly $α$ relates to a number of other measured quantities (mass, metallicity, star formation, ionisation parameter, and extinction). We find that the escape fraction is strongly anti-correlated with nebular and stellar extinction, weakly anti-correlated with stellar mass, but no conclusive evidence for correlations with other quantities. We show that Ly $α$ 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 a star formation rate calibration based on Ly $α$ luminosity, where the equivalent width of Ly $α$ is used to correct for non-unity escape fraction, and show that this relation provides a reasonably accurate SFR estimate. We also show stacked growth curves of Ly $α$ for the galaxies that can be used to find aperture loss fractions at a given physical radius.

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