Spatially resolved integral field spectroscopy of the ionized gas in IZw18

We present a detailed 2D study of the ionized ionized interstellar medium (ISM) of IZw18 using new Potsdam Multi-Aperture Spectrophotometer-integral field unit (PMAS-IFU) optical observations. IZw18 is a high-ionization galaxy which is among the most metal-poor starbursts in the local Universe. This makes IZw18 a local benchmark for understanding the properties most closely resembling those prevailing at distant starbursts. Our IFU aperture (∼1.4 × 1.4 kpc2) samples the entire IZw18 main body and an extended region of its ionized gas. Maps of relevant emission lines and emission line ratios show that higher-excitation gas is preferentially located close to the north-west knot and thereabouts. We detect a Wolf–Rayet feature near the north-west knot. We derive spatially resolved and integrated physical–chemical properties for the ionized gas in IZw18. We find no dependence between the metallicity indicator R23 and the ionization parameter (as traced by [O iii]/[O ii]) across IZw18. Over ∼0.30 kpc2, using the [O iii] λ4363 line, we compute T e[O iii] values (∼15 000–25 000 K), and oxygen abundances are derived from the direct determinations of T e[O iii]. More than 70 per cent of the higher-T e[O iii] (≳22 000 K) spaxels are He iiλ4686-emitting spaxels too. From a statistical analysis, we study the presence of variations in the ISM physical–chemical properties. A galaxy-wide homogeneity, across hundreds of parsecs, is seen in O/H. Based on spaxel-by-spaxel measurements, the error-weighted mean of 12 + log(O/H) = 7.11 ± 0.01 is taken as the representative O/H for IZw18. Aperture effects on the derivation of O/H are discussed. Using our IFU data we obtain, for the first time, the IZw18 integrated spectrum.