2025A&A...695A.119B

Lyman-α (Lyα) emission is the intrinsically strongest tracer of recombining ionized hydrogen in young star-forming galaxies, but its origin is still debated. Lyα arises when emitted photons scatter in neutral hydrogen, with each scattering event changing their propagation direction and frequency. So far, observational efforts have mostly focused on the Lyα 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. Although different processes produce similar spectra, they have different degrees of polarization that can be used to discriminate between them and to put stringent constraints on the geometry of the galaxy and its circumgalactic medium (CGM) where Lyα photons scatter and on their emission mechanism. In this paper, we present the first deep spectropolarimetric observations of a typical clumpy star-forming galaxy at z ∼ 3.4, strongly lensed by the cluster of galaxies Abell 2895, taken with the Polarimetric Multi Object Spectroscopy (PMOS) mode of the VLT/FORS2 instrument. We measure a Lyα degree of polarization 1σ upper limit of 4.6%. We develop new Lyα radiative transfer models assuming a biconical outflow geometry to reproduce the observations. We find that they can be explained by assuming the star-forming galaxy is embedded in a CGM with a biconical outflow geometry with an opening angle of the wind θ_o, Wind_ ∼ 30° for line-of-sight angles θ_LOS ≤ 20°, θ_o, Wind_ ∼ 45° for θ_LOS ≤ 20°, θ_o, Wind_ ∼ 60° for θ_LOS ≤ 20°, and θ_o, Wind_ ∼ 75° for θ_LOS ≤ 40°, where θ_LOS = 0° means observing in the direction of the outflow. We notice that the constraints from polarization are complementary to those from the spectral line profile, and the joint analysis allows us to break the degeneracies that affect them individually. This study shows the potential of including measurements of the Lyα degree of polarization to constrain the symmetry of the gas surrounding typical star-forming galaxies at the cosmic noon and paves the way to deep spatially resolved studies of this kind that will allow the different mechanisms that can originate the Lyα emission to be disentangled.