Deep Narrowband Photometry of the M101 Group: Strong-Line Abundances of 720 HII Regions

TL;DR

This study provides a comprehensive analysis of oxygen abundances in the M101 galaxy group using deep narrowband imaging, revealing strong abundance gradients in M101 and minimal gradients in its satellites, with evidence of past interactions affecting the distribution.

Contribution

First to include the [OII] emission line in narrowband imaging of the M101 Group, enabling complete spatial coverage and detailed abundance mapping of 720 HII regions.

Findings

M101 exhibits a strong radial abundance gradient.

Satellite galaxies show little to no abundance gradient.

Evidence suggests a flattening of M101's gradient beyond 14 kpc.

Abstract

We present deep, narrowband imaging of the nearby spiral galaxy M101 and its satellites to analyze the oxygen abundances of their HII regions. Using CWRU's Burrell Schmidt telescope, we add to the narrowband dataset of the M101 Group, consisting of H$\alpha$, H$\beta$, and [OIII] emission lines, the blue [OII]$\lambda$3727 emission line for the first time. This allows for complete spatial coverage of the oxygen abundance of the entire M101 Group. We used the strong-line ratio $R_{23}$ to estimate oxygen abundances for the HII regions in our sample, utilizing three different calibration techniques to provide a baseline estimate of the oxygen abundances. This results in ~650 HII regions for M101, 10 HII regions for NGC 5477, and ~60 HII regions for NGC 5474, the largest sample for this Group to date. M101 shows a strong abundance gradient while the satellite galaxies present little or no gradient. There is some evidence for a flattening of the gradient in M101 beyond $R \sim 14 \text{ kpc}$. Additionally, M101 shows signs of azimuthal abundance variations to the west and southwest. The radial and azimuthal abundance variations in M101 are likely explained by an interaction it had with its most massive satellite NGC 5474 ~300 Myr ago combined with internal dynamical effects such as corotation.