Spatially resolved integral field spectroscopy of the ionized gas in IZw18

Abstract

We present a detailed 2D study of the ionized ISM of IZw18 using new 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 kpc 2 ) 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 NW knot and thereabouts. We detect a Wolf-Rayet feature near the NW knot. We derive spatially resolved and integrated physical-chemical properties for the ionized gas in IZw18. We find no dependence between the metallicity-indicator R 23 and the ionization parameter (as traced by [OIII]/[OII]) across IZw18. Over ∼ 0.30 kpc 2 , using the [OIII]λ4363 line, we compute T e [OIII] values (∼ 15000 -25000 K), and oxygen abundances are derived from the direct determinations of T e [OIII]. More than 70% of the higher-T e [OIII] ( 22000 K) spaxels are HeIIλ4686-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.

Neutral gas heating by X-rays in primitive galaxies: Infrared observations of the blue compact dwarf I Zw 18 with Herschel

“…3.2) and given the location of the X-ray source within NW, it can be safely surmised that the model more closely reflects (average) properties of the NW region. -The ionization structure and metallicity do not vary much between NW and SE (Legrand et al 2000;Skillman & Kennicutt 1993;Kehrig et al 2016). Since our line measurements are normalized to Hβ, this means that line constraints should not be significantly affected by SE.…”

The interstellar medium of dwarf galaxies: new insights from Machine Learning analysis of emission-line spectra

“…15. The blue and green shaded histograms report the results obtained in this work and in Kehrig et al (2016), respectively. electron temperature, 12+log(O/H) = 7.11 ± 0.01 (∼ 1/40 solar) (see Fig.…”

“…This implies the presence of a hard ionizing field (i.e., photon energies ≥ 54.4 eV) in this region. This field has been ascribed in the literature (Kehrig et al 2015(Kehrig et al , 2016 to the presence of stars with a spectral energy distribution typical of PopIII models (Schaerer 2002;Raiter et al 2010). In fact, it has been shown (Kehrig et al 2015) that the HeII-ionizing energy budget cannot be explained by conventional excitation sources (i.e., WRs, shocks, X-ray binaries).…”

“…It is located at a distance of 18.2 Mpc (Aloisi et al 2007) with a corresponding angular scale of 88 pc/ . It is considered an analogue of high-redshift galaxies (Lebouteiller et al 2013, and references therein) and after the first studies (Sargent & Searle 1970;Searle & Sargent 1972;Searle et al 1973), it continues to keep attraction thanks to its extremely low metallicity: Z/Z ∼ 0.026 (Kunth & Sargent 1986;Pagel et al 1992;Vílchez & Iglesias-Páramo 1998;Lebouteiller et al 2013;Kehrig et al 2016). Since the metallicity is homogeneously distributed (Kehrig et al 2016), IZw18 should host an underlying old population of intermediate/low-mass stars distributed over the whole area of the galaxy (Vílchez & Iglesias-Páramo 1998), although older/fainter ones are more easily detectable only at the system's periphery (Contreras Ramos et al 2011).…”

“…In this section, we present the ISM physical properties in IZw18 inferred by applying game to IFU data taken from Kehrig et al (2016). IZw18 data were collected by the Potsdam Multi-Aperture Spectrophotometer (PMAS; Roth et al 2005, 2010) on the 3.5 m telescope at the Calar Alto Observatory.…”