Extragalactic Chemical Abundances: Do H Ii Regions and Young Stars Tell the Same Story? The Case of the Spiral Galaxy NGC 300

Abstract

We have obtained new spectrophotometric data for 28 H II regions in the spiral galaxy NGC 300, a member of the nearby Sculptor Group. The detection of several auroral lines, including [O III] λ4363, [S III] λ6312 and [N II] λ5755, has allowed us to measure electron temperatures and direct chemical abundances for the whole sample. We determine for the first time in this galaxy a radial gas-phase oxygen abundance gradient based solely on auroral lines, and obtain the following least-square solution: 12 + log(O/H) = 8.57 (±0.02) − 0.41 (±0.03) R/R 25 , where the galactocentric distance is expressed in terms of the isophotal radius R 25 . The characteristic oxygen abundance, measured at 0.4×R 25 , is 12 + log(O/H) = 8.41. The gradient corresponds to −0.077 ± 0.006 dex kpc −1 , and agrees very well with the galactocentric trend in metallicity obtained for 29 B and A supergiants in the same galaxy, −0.081 ± 0.011 dex kpc −1 . The intercept of the regression for the nebular data virtually coincides with the intercept obtained from the stellar data, which is 8.59 (±0.05). This allows little room for depletion of nebular oxygen onto dust grains, although in this kind of comparison we are somewhat limited by systematic uncertainties, such as those related to the atomic parameters used to derive the chemical compositions.We discuss the implications of our result with regard to strong-line abundance indicators commonly used to estimate the chemical compositions of star-forming galaxies, such as R 23 . By applying a few popular calibrations of these indices based on grids of photoionization models on the NGC 300 H II region fluxes we find metallicities that are higher by 0.3 dex (a factor of two) or more relative to our nebular (T e -based) and stellar ones.We detect Wolf-Rayet stellar emission features in ∼1/3 of our H II region spectra, and find that in one of the nebulae hosting these hot stars the ionizing field has a particularly hard spectrum, as gauged by the 'softness'We suggest that this is related to the presence of an early WN star. By considering a larger sample of extragalactic H II regions we confirm, using direct abundance measurements, previous findings of a metallicity dependence of η, in the sense that softer stellar continua are found at high metallicity.

The slow flow model of dust efflux in local star-forming galaxies

“…Bresolin et al (2009a) show that the metallicities inferred from temperature sensitive [OIII]λ4363 line and those determined from B and A supergiants by Kudritzki et al (2008) agree well in NGC 300. If coulomb interactions are responsible for exchange of momentum between dust and gas, then a preferential loss of more ionized species may occur in dustier HII regions.…”

NGC 55: a disc galaxy with flat abundance gradients

“…The continuous line is the fit to the H ii regions with Te determinations as in Figure 3, whereas the dotted line is the fit to the whole sample of supergiant abundances. Lower panel: NGC 300 H ii regions (Bresolin et al 2009;Stasińska et al 2013, filled circles from) and supergiants from Kudritzki et al (2008) (empty stars) radial distribution. low metallicity.…”

Gemini GMOS spectroscopy of HeII nebulae in M 33

“…We calculated the N/O and Ne/O abundance ratios assuming that N/O = N + /O + and Ne/O = Ne ++ /O ++ , based on the similarity of the ionization potentials of the ions involved (Peimbert & Costero 1969). This ICF scheme has been used in previous work (e.g., López-Sánchez et al 2007;Hägele et al 2008;Bresolin et al 2009) and is a good approximation for high ionization degree objects. In our spectra, neon is seen via the [NeIII] emission line at 3869 Å.…”