NGC 6845: metallicity gradients and star formation in a complex compact group

TL;DR

This study investigates the metallicity gradients and star formation in the interacting galaxy group NGC 6845, revealing flatter metallicity profiles and recent star formation in tidal tails, indicating interaction-driven chemical mixing.

Contribution

It provides the first detailed metallicity gradient analysis across NGC 6845 and its tidal tails, showing interaction effects on chemical distribution and star formation.

Findings

Metallicity gradients are shallower than in isolated galaxies.

Central metallicity of NGC 6845A is lower than expected for its mass.

Star formation in tidal tails is recent and inefficient at increasing metallicity.

Abstract

We have obtained Gemini/GMOS spectra of 28 regions located across the interacting group NGC 6845, spanning from the inner regions of the four major galaxies (NGC 6845A, B, C, D) to the tidal tails of NGC 6845A. All regions in the tails are star-forming objects with ages younger than 10 Myr. We derived the gas-phase metallicity gradients across NGC 6845A and its two tails and we find that these are shallower than those for isolated galaxies. NGC 6845A has a gas-phase oxygen central metallicity of \mbox{12+log(O/H)$\sim$8.5} and a flat gas-phase metallicity gradient ($\beta$=0.002$\pm$0.004 dex kpc$^{-1}$) out to $\sim$4 $\times$ R$_{25}$ (to the end of the longest tidal tail). Considering the mass-metallicity relation, the central region of NGC 6845A displays a lower oxygen abundance than the expected for its mass. Taking into account this fact and considering the flat oxygen distribution measured along the eastern tidal tail, we suggest that an interaction event has produced a dilution in the central metallicity of this galaxy and the observed flattening in its metal distribution. We found that the star formation process along the eastern tidal structure has not been efficient enough to increase the oxygen abundances in this place, suggesting that this structure was formed from enriched material.