arXiv2025
Ly$\alpha$ emission is the strongest tracer of recombining ionized hydrogen in young, star-forming galaxies, but its origin is still debated. Ly$\alpha$ arises when emitted photons scatter in neutral hydrogen and, so far, observational efforts have mostly focused on the Ly$\alpha$ 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$\alpha$ degree of polarization $1\sigma$ upper limit of $4.6\%$. We develop new Ly$\alpha$ 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 $\theta_{o,Wind}\sim 30^\circ$ for line-of-sight angles $\theta_{LOS} \leq 20^\circ$, $\theta_{o,Wind}\sim 45^\circ$ for $\theta_{LOS}\leq 20^\circ$, $\theta_{o,Wind}\sim 60^\circ$ for $\theta_{LOS}\leq 20^\circ$, and $\theta_{o,Wind}\sim 75^\circ$ for $\theta_{LOS}\leq 40^\circ$, where $\theta_{LOS}=0^\circ$ 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$\alpha$ 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$\alpha$ origin mechanisms.