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

TL;DR

This paper presents azimuthally resolved catalogs of H II regions in NGC 628 and NGC 6946 using narrow-band imaging of Hα, Hβ, [O II], and [O III], enabling 2D metallicity maps via Delaunay triangulation. By applying two empirical calibrations (PT05 and KK04) to derive oxygen abundances, it confirms radial abundance gradients consistent with prior studies while highlighting substantial calibration-dependent offsets (~0.6 dex). The 2D metallicity maps uncover arm-associated high-metallicity knots in NGC 628 and a pronounced azimuthal metallicity arm in NGC 6946, which may reflect variations in the ionizing-cluster temperature and possibly the IMF between arm and interarm regions. Overall, the work demonstrates the value of 2D metallicity reconstructions from imaging data for understanding star-formation processes and density-wave effects in spiral galaxies.

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 Hα, H\b{eta}, [O II]λ3727, and [O III]λλ4959, 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 Hα 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 Hα 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.

Figures (21)

• Figure 1: Continuum substracted images for NGC 628. Top left, H$\alpha$; top right, H$\beta$; botton left, [OII]; and bottom right [OIII]. North is at top and East at the left for all images. Scale as indicated.
• Figure 2: Continuum substracted images for NGC 6946. Top left, H$\alpha$; top right, H$\beta$; botton left, [OII]; and bottom right [OIII]. North is at top and East at the left for all images. Scale as indicated.
• Figure 3: Positions of the H II regions detected for NGC 628 (left panel) and NGC 6946 (right panel). The arms, as determined in Cedrés et al. (2012), are indicated by the continuous lines. North is at the top and East is at the left.
• Figure 4: Histograms of the areas of the H II regions in kpc, for NGC 628 (left panel) and NGC 6946 (right panel).
• Figure 5: Histograms of the extinctions of the H II regions in magnitudes, for NGC 628 (left panel) and NGC 6946 (right panel).