arXiv subjects

S. -J. Chang

Publications and source records attributed to S. -J. Chang.

2 recordsLinked to original sources

arXiv2026

The gradual decline of Ly$α$ visibility in the CANDELS fields: evidence for the combined effects of galaxy evolution and reionization

We investigate the evolution of Ly$α$ visibility and the physical properties of Ly$α$ emitters (LAEs) across the five CANDELS fields using publicly available JWST/NIRSpec PRISM spectroscopy. Our catalog comprises 3446 spectroscopically confirmed sources at 4 $\leq $ z < 14.2, including 3361 star-forming galaxies (SFGs), of which 539 are robust (S/N > 3) LAEs. We measure the fraction of LAEs with EW$_0$ > 25A (X$_{\mathrm{Lyα}}$) and trace its redshift evolution in two UV luminosity bins, namely -20.25 < M$_{\mathrm{UV}}$ < -18.75 and -21.75 < M$_{\mathrm{UV}}$ < -20.25. Within the fainter-UV range, X$_{\mathrm{Lyα}}$ increases from z = 5 to z = 6 at 3$σ$ significance and subsequently declines toward higher redshifts with a significant monotonic trend at z > 6. We also investigate the physical properties of both LAEs and the full SFG population. We find that the stellar mass, UV slope $β$, stellar reddening, SFR, metallicity, sSFR, and burstiness of LAEs remain approximately stable with redshift. The only exception is the mass-weighted age which decreases with increased redshift. Conversely, the properties of the full SFG population evolve significantly, progressively approaching the region of galaxy-property space occupied by LAEs as redshift increases. This suggests that galaxy evolution may enhance the intrinsic production and escape of Ly$α$ photons toward earlier epochs. We argue that this effect should be accounted when inferring the evolution of the neutral hydrogen content of the IGM from the observed visibility of Ly$α$ emission. To this end, we employ a physically motivated framework based on the Attenuation-Free Model, jointly accounting for galaxy evolution and IGM attenuation. Our observations favor reionization histories that proceed gradually over scenarios characterized by a rapid increase in the cosmic neutral hydrogen fraction.

astro-ph.GA↗

arXiv2025

Constraining the geometry of the gas surrounding a typical galaxy at $z = 3.4$ with Ly$α$ polarization

Ly$α$ emission is the strongest tracer of recombining ionized hydrogen in young, star-forming galaxies, but its origin is still debated. Ly$α$ arises when emitted photons scatter in neutral hydrogen and, so far, observational efforts have mostly focused on the Ly$α$ surface brightness and spectral profile, which depend on the neutral hydrogen column density, geometry, kinematics, powering mechanism and on the region from which the photons are emitted. Different processes produce similar spectra, but have different degrees of polarization, that we can use to discriminate between them. In this paper, we present the first spectropolarimetric observations of a typical star-forming galaxy at $z\sim 3.4$, strongly lensed by the cluster of galaxies Abell 2895, taken with the PMOS mode of the VLT/FORS2 instrument. We measure a Ly$α$ degree of polarization $1σ$ upper limit of $4.6\%$. We develop new Ly$α$ radiative transfer models to reproduce the observations, that can be explained by assuming the star-forming galaxy being embedded in a CGM with a biconical outflow geometry, with an opening angle of the wind $θ_{o,Wind}\sim 30^\circ$ for line-of-sight angles $θ_{LOS} \leq 20^\circ$, $θ_{o,Wind}\sim 45^\circ$ for $θ_{LOS}\leq 20^\circ$, $θ_{o,Wind}\sim 60^\circ$ for $θ_{LOS}\leq 20^\circ$, and $θ_{o,Wind}\sim 75^\circ$ for $θ_{LOS}\leq 40^\circ$, where $θ_{LOS}=0^\circ$ means observing in the direction of the outflow. We notice that the constraints from polarization are complementary to those from the spectral line profile. This study shows the potential of adding measurements of the Ly$α$ degree of polarization to constrain the geometry of the gas surrounding typical star-forming galaxies and paves the way to spatially resolved studies that will allow us to disentangle between different Ly$α$ origin mechanisms.

astro-ph.GA↗