Integral field spectroscopy of HII region complexes: the outer disc of NGC6946

RUBÉN GARCÍA-BENITO; ÁNGELES I. DÍAZ; GUILLERMO F. HÄGELE; ENRIQUE PÉREZ-MONTERO; JESÚS LÓPEZ; J. M. VÍLCHEZ; ENRIQUE PÉREZ; ELENA TERLEVICH; ROBERTO TERLEVICH; DANIEL ROSA-GONZÁLEZ

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY

WILEY-BLACKWELL PUBLISHING, INC

Año: 2010 vol. 408 p. 2234 - 2255

0035-8711

Integral Field Spectroscopy obtained with PPak and the 3.5m telescope at the Calar Alto Observatory has been used to study an outer Hii region complex in the well studied galaxy NGC 6946. This technique provides detailed maps of the region in dierent emission lines yielding spatially resolved information about the physical properties of the gas. The configuration was chosen to cover the whole spectrum from 3600 up to 10000 Å, allowing the measurement of the near infrared [Siii] lines. We selected four luminous knots, to perform a detailed integrated spectroscopic analysis of these structures and of the whole PPak field-of-view (FOV). For all the knots the electron density has been found to be very similar and below 100 cm^-3. The [OIII] electron temperature was measured in knots A, B, C and in the integrated PPak-field, and was found to be around 8000 K. The temperatures of [OII] and [SIII] were estimated in the four cases. The elemental abundances computed from the "direct method" are typical of high metallicity disk Hii regions, with a mean value of 12+log(O/H)=8.65, comparable to what has been found in this galaxy by other authors for regions at similar galactocentric distance. Therefore, a remarkable abundance uniformity isfound despite the different excitations found throughout the nebula. However, due to the quality of the data, the electron temperatures and metallicities obtained, have associated errors comparable to the typical dispersion found in empirical calibrations.Wolf-Rayet features have been detected in three of the knots, leading to a derived total number of WR stars of 125, 22 and 5, for knots A, C and B, respectively. The ratios of the number of WR to O stars are consistent with the prediction of Starburst99 for individual bursts with an age about 4 Myr. Knot D, with no WR features, shows weak H emission, low excitation, and the lowest H equivalent width, all of which points to a more evolved state.The integrated spectrum of the whole PPak FOV shows high excitation and a relatively evolved age which does not correspond to the individual knot evolutionary stages. Some effects associated to the loss of spatial resolution could also be evidenced by the higher ionising temperature that is deduced from the eta´ parameter measured in the integrated PPak spectrum with respect to that of the individual knots.