Two-dimensional metallicity distribution of the ionized gas in NGC 628 and NGC 6946

TL;DR

This study provides detailed two-dimensional metallicity maps of ionized gas in NGC 628 and NGC 6946, revealing high metallicity regions linked to spiral arms and offering insights into star formation and chemical distribution in these galaxies.

Contribution

The paper introduces a method to create 2D metallicity distributions in spiral galaxies using Delaunay triangulation, expanding the spatial resolution and detail compared to previous studies.

Findings

High metallicity knots are found in NGC 628 linked to spiral arms.

A high metallicity azimuthal structure is discovered in NGC 6946.

The metallicity gradients agree with previous studies, but with more detailed spatial resolution.

Abstract

We present here two H II region catalogues with azimuthal resolution for the two grand design galaxies NGC 628 and NGC 6946. With the help of these catalogues, we study several properties of the star-forming processes occurring in spiral galaxies. We obtained direct imaging in the narrow-band filters centred at H{\alpha}, H\b{eta}, [O II]{\lambda}3727, and [O III]{\lambda}{\lambda}4959, 5007 and their respective continua. After the calibration and correction of the data, we obtained for each H II region the de-reddened fluxes in the aforementioned lines, the size, the H{\alpha} equivalent width, and, using two different empirical calibrations, the metallicity. Employing a method based on the Delaunay triangulation, a two-dimensional (2D) representation of the metallicity was obtained. Data for 209 H II regions of NGC 628 and 226 H II regions of NGC 6946 are obtained. The radial behaviours of the H{\alpha} equivalent width, the excitation, and the oxygen abundance are derived. Two-dimensional representations of the metallicity and the excitation are calculated for the galaxies in the study. The two empirical calibrations of the metallicity are compared. The oxygen abundance gradients obtained in this study agree with previously published values. However, more regions were examined than in previous studies. We find a difference of about 0.6 dex between the two empirical calibrations employed. Finally, the 2D representations of the metallicity reveal high metallicity knots in NGC 628, and for NGC 6946 a high metallicity azimuthal structure is discovered. These high metallicity regions seem to be linked to the arms of the galaxies and are probably produced by an increase in the temperature of the ionizing clusters in the H II regions, which may be linked to variations in the initial mass functions of the galaxies between the arm and interarm regions.