NGC 55: a disc galaxy with flat abundance gradients

TL;DR

This study presents spectroscopic analysis of NGC 55 revealing a homogenous chemical composition across its disc with no radial abundance gradients, suggesting strong gas mixing likely due to its surface density profile.

Contribution

It provides detailed chemical abundance measurements of NGC 55's HII regions and compares its flat metallicity gradient with NGC 300, proposing a link to their surface density profiles.

Findings

Homogeneous chemical composition across NGC 55's disc.

No detectable radial abundance gradients in NGC 55.

Flat metallicity gradients consistent with previous studies.

Abstract

We present new spectroscopic observations obtained with GMOS@Gemini-S of a sample of 25 hii regions located in NGC 55, a late-type galaxy in the nearby Sculptor group. We derive physical conditions and chemical composition through the te-method for 18 hii regions, and strong-line abundances for 22 hii regions. We provide abundances of He, O, N, Ne, S, Ar, finding a substantially homogenous composition in the ionised gas of the disc of NGC 55, with no trace of radial gradients. The oxygen abundances, both derived with \te- and strong-line methods, have similar mean values and similarly small dispersion: 12+$\log$(O/H)=8.13$\pm$0.18~dex with the former and 12+$\log$(O/H)=8.17$\pm$0.13~dex with the latter. The average metallicities and the flat gradients agree with previous studies of smaller samples of \hii\ regions and there is a qualitative agreement with the blue supergiant radial gradient as well. We investigate the origin of such flat gradients comparing NGC 55 with NGC 300, its companion galaxy, which is also twin of NGC 55 in terms of mass and luminosity. We suggest that the differences in the metal distributions in the two galaxies might be related to the differences in their K-band surface density profile. The flatter profile of NGC 55 probably causes in this galaxy higher infall/outflow rates than in similar galaxies. This likely provokes a strong mixing of gas and a re-distribution of metals.