VLT/MUSE view of the highly ionized outflow cones in the nearby starburst ESO338-IG04^⋆

¹ Department of Astronomy, Oskar Klein CentreStockholm University, AlbaNova University Centre, 106 91 Stockholm, Sweden
e-mail: arjan.bik@astro.su.se
² Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille), 13388 Marseille, France

Received: 9 February 2015
Accepted: 27 March 2015

Abstract

Context. The Lyα line is an important diagnostic for star formation at high redshift, but interpreting its flux and line profile is difficult because of the resonance nature of Lyα. Trends between the escape of Lyα photons and dust and properties of the interstellar medium (ISM) have been found, but detailed comparisons between Lyα emission and the properties of the gas in local high-redshift analogs are vital for understanding the relation between Lyα emission and galaxy properties.

Aims. For the first time, we can directly infer the properties of the ionized gas at the same location and similar spatial scales of the extended Lyα halo around the local Lyα emitter and Lyman-break galaxy analog ESO 338-IG04.

Methods. We obtained VLT/MUSE integral field spectra. We used ionization parameter mapping of the [S ii]/[O iii] line ratio and the kinematics of Hα to study the ionization state and kinematics of the ISM of ESO 338-IG04.

Results. The velocity map reveals two outflows, one toward the north, the other toward the south of ESO 338. The ionization parameter mapping shows that the entire central area of the galaxy is highly ionized by photons leaking from the H ii regions around the youngest star clusters. Three highly ionized cones have been identified, of which one is associated with an outflow detected in the Hα. We propose a scenario where the outflows are created by mechanical feedback of the older clusters, while the highly ionized gas is caused by the hard ionizing photons emitted by the youngest clusters. A comparison with the Lyα map shows that the (approximately bipolar) asymmetries observed in the Lyα emission are consistent with the base of the outflows detected in Hα. No clear correlation with the ionization cones is found.

Conclusions. The mechanical and ionization feedback of star clusters significantly changes the state of the ISM by creating ionized cones and outflows. The comparison with Lyα suggests that especially the outflows could facilitate the escape of Lyα photons.