Puzzling Lyman-alpha line profiles in green pea galaxies

¹ Astronomical Institute of the Czech Academy of Sciences, Boční II/1401, 141 00 Praha, Czech Republic
e-mail: ivana.orlitova@asu.cas.cz
² Observatoire de Genève, Université de Genève, Chemin des Maillettes 51, 1290 Versoix, Switzerland
³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore MD 21218, USA
⁴ Minnesota Institute for Astrophysics, University of Minnesota, Minneapolis MN 55455, USA
⁵ Department of Astronomy, Smith College, 44 College Lane, Northampton MA 01063, USA
⁶ Department of Astronomy, University of Massachusetts – Amherst, Amherst MA 01003, USA
⁷ Department of Astronomy, University of Michigan, 311 West Hall, 1085 S. University Ave, Ann Arbor MI 48109-1107, USA
⁸ IRAP/CNRS, 14 Av. E. Belin, 31400 Toulouse, France

Received: 16 December 2017
Accepted: 11 May 2018

Abstract

Context. The Lyman-alpha (Lyα) line of hydrogen is of prime importance for detecting galaxies at high redshift. For a correct data interpretation, numerical radiative transfer models are necessary due to Lyα resonant scattering off neutral hydrogen atoms.

Aims. Recent observations have discovered an escape of ionizing Lyman-continuum radiation from a population of compact, actively star-forming galaxies at redshift z ~ 0.2−0.3, also known as “green peas”. For the potential similarities with high-redshift galaxies and impact on the reionization of the universe, we study the green pea Lyα spectra, which are mostly double-peaked, unlike in any other galaxy sample. If the double peaks are a result of radiative transfer, they can be a useful source of information on the green pea interstellar medium and ionizing radiation escape.

Methods. We select a sample of twelve archival green peas and we apply numerical radiative transfer models to reproduce the observed Lyα 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 Lyα 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.

Results. The peculiar green pea double-peaked Lyα 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⁻¹ than those derived from optical emission lines; 2) the double-peaked Lyα profiles are best reproduced with low-velocity (≲150 km s⁻¹) outflows that contradict the observed ultraviolet absorption lines of low-ionization-state elements with characteristic velocities as large as 300 km s⁻¹; and 3) the models need to consider intrinsic Lyα profiles that are on average three times broader than the observed Balmer lines.

Conclusions. Differences between the modelled and observed velocities are larger for targets with prominent Lyα blue peaks. The blue peak position and flux appear to be connected to low column densities of neutral hydrogen, leading to Lyα and Lyman-continuum escape. This is at odds with the kinematic origin of the blue peak in the homogeneous shell models. Additional modelling is needed to explore alternative geometries such as clumpy media and non-recombination Lyα sources to further constrain the role and significance of the Lyα double peaks.