The interplay between ionized gas and massive stars in the HII galaxy IIZw70: integral field spectroscopy with PMAS

¹ Instituto de Astrofísica de Andalucía (CSIC), Apartado 3004, 18080 Granada, Spain e-mail: [kehrig;jvm]@iaa.es
² Observatório Nacional, José Cristino 77, 20.921-400 Rio de Janeiro - RJ, Brazil e-mail: [kehrig;etelles]@on.br
³ Centro Astronómico Hispano Alemán, Calar Alto, CSIC-MPG, C/ Jesús Durbán Remón 2, 04004 Almeria, Spain e-mail: sanchez@caha.es
⁴ Laboratoire d'Astrophysique de Toulouse et Tarbes (LATT - UMR 5572), Observatoire Midi-Pyrnes, 14 avenue E. Belin, 31400 Toulouse, France e-mail: enrique.perez@ast.obs-mip.fr

Received: 31 May 2007
Accepted: 7 November 2007

Abstract

Aims.We performed an integral field spectroscopic study for the HII galaxy IIZw70 to investigate the interplay between its ionized interstellar medium (ISM) and the massive star formation.

Methods.Observations were taken in the optical spectral range from λ3700 Å-6800 Å with the Potsdam Multi-Aperture Spectrophotometer (PMAS) attached to the 3.5 m telescope at Calar Alto Observatory. We created and analyzed maps of spatially distributed emission-lines (at different stages of excitation), continuum emission, and properties of the ionized ISM (ionization structure indicators, physical-chemical conditions, dust extinction, kinematics). We investigated the relation of these properties to the spatial distribution and evolutionary stage of the massive stars.

Results.For the first time we have detected Wolf-Rayet (WR) stars in this galaxy. The peak of the ionized gas emission coincides with both the location of the maximum of the stellar continuum emission and the WR bump. The region of the galaxy with lower dust extinction corresponds to the region that shows the lowest values of velocity dispersion and radial velocity. The overall picture suggests that the ISM of this region is being disrupted via photoionization and stellar winds, leading to a spatial decoupling between gas+stars and dust clouds. The bulk of dust appears to be located at the boundaries of the region occupied by the probable ionizing cluster. We also found that this region is associated to the nebular emission in HeIIλ4686 and to the intensity maximum of most emission lines. This indicates that the hard ionizing radiation responsible for the HeIIλ4686 nebular emission can be related to the youngest stars. Within ~0.4  0.3 kpc² in the central burst, we derived oxygen abundances using direct determinations of T_e[OIII]. We found abundances in the range 12 + log(O/H) = 7.65-8.05, yielding an error-weighted mean of 12 + log(O/H) = 7.86 ± 0.05 that has been taken as the representative oxygen abundance for IIZw70.