VLT/GIRAFFE spectroscopic observations of the metal-poor blue compact dwarf galaxy SBS 0335–052E

¹ Main Astronomical Observatory, Ukrainian National Academy of Sciences, Zabolotnoho 27, Kyiv 03680, Ukraine e-mail: izotov@mao.kiev.ua
² Observatoire de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland
³ Laboratoire d'Astrophysique Toulouse-Tarbes, UMR 5572, 14 Av. E. Belin, 31400 Toulouse, France
⁴ GEPI, CNRS UMR 8111, Observatoire de Paris, 5 place Janssen, 92195 Meudon Cedex, France
⁵ Laboratoire d'Astrophysique, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire, 1290 Sauverny, Switzerland

Received: 17 May 2006
Accepted: 24 July 2006

Abstract

Aims.We present two-dimensional spectroscopy of the extremely metal-deficient blue compact dwarf (BCD) galaxy SBS 0335–052E to study physical conditions, element abundances and kinematical properties of the ionised gas in this galaxy.

Methods.Observations were obtained in the spectral range λ3620–9400 Å  with the imaging spectrograph GIRAFFE on the UT2 of the Very Large Telescope (VLT). These observations are the first ones carried out so far with GIRAFFE in the ARGUS mode which allows one to simultaneously obtain 308 spectra covering a 11473 region.

Results.We produced images of SBS 0335–052E in the continuum and in emission lines of different stages of excitation. While the maximum of emission in the majority of lines, including the strong lines Hβ 4861 Å, Hα 6563 Å, [O iii] 4363,5007 Å, [O ii] 3726,3729 Å, coincides with the youngest south-eastern star clusters 1 and 2, the emission of He II 4686 Å line is offset to the more evolved north-west clusters 4, 5. This suggests that hard ionising radiation responsible for the He II λ4686 Å  emission is not related to the most massive youngest stars, but rather is related to fast radiative shocks. This conclusion is supported by the kinematical properties of the ionised gas from the different emission lines as the velocity dispersion in the He II λ4686 Å line is systematically higher, by ~50–100%, than that in other lines. The variations of the emission line profiles suggest the presence of an ionised gas outflow in the direction perpendicular to the galaxy disk. We find a relatively high electron number density N_e of several hundred cm^-3 in the brightest part of SBS 0335–052E. There is a small gradient of the electron temperature T_e and oxygen abundance from the East to the West with systematically higher T_e and lower 12+log O/H in the western part of the galaxy. The oxygen abundances for the whole H II region and its brightest part are 12 + log O/H = and , respectively. We derive the He mass fraction taking into account all systematic effects. The He mass fraction , derived from the emission of the whole H II region, is consistent with the primordial value predicted by the standard Big Bang nucleosynthesis model. We confirm the presence of Wolf-Rayet stars in cluster 3.