Puzzling Lyman-alpha line profiles in green pea galaxies

Recent observations have discovered an escape of ionizing Lyman-continuum radiation from a population of compact, actively star-forming galaxies at redshift z~0.3, also known as "green peas". We here study the peculiar green pea Lyman-alpha (Lya) spectra, which are mostly double-peaked, unlike in any other galaxy sample. We select a sample of twelve archival green peas and we apply numerical radiative transfer models to reproduce the observed Lya spectral profiles, using the geometry of expanding, homogeneous spherical shells. We use ancillary optical and ultraviolet data to constrain the model parameters, and we evaluate the match between the models and the observed Lya spectra. As a second step, we allow all the fitting parameters to be free, and examine the agreement between the interstellar medium parameters derived from the models and those from ancillary data. The green pea double-peaked Lya line profiles are not correctly reproduced by the constrained shell models. Conversely, unconstrained models fit the spectra, but parameters derived from the best-fitting models are not in agreement with the ancillary data. In particular: 1) the best-fit systemic redshifts are larger by 10 - 250 km/s than those derived from optical emission lines, 2) the double-peaked Lya profiles are best reproduced with low-velocity (<150 km/s) outflows that contradict the observed ultraviolet absorption lines of low-ionization-state elements with characteristic velocities as large as 300 km/s, and 3) the models need to consider intrinsic Lya profiles that are on average three times broader than the observed Balmer lines. Differences between the modelled and observed velocities are larger for targets with prominent Lya blue peaks. The blue peak position and flux appear to be connected to low column densities of neutral hydrogen, leading to Lya and Lyman-continuum escape.