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

• Published in: Monthly notices of the Royal Astronomical Society 2015-11, Vol.453 (3), p.2808-2823
• Main Authors: Olave-Rojas, D., Torres-Flores, S., Carrasco, E. R., Mendes de Oliveira, C., de Mello, D. F., Scarano, S.
• Format: Article
• Language: English
• Subjects: Astronomy; Displays; Flats; Flattening; Galaxies; Metallicity; Oxygen; Oxygen enrichment; Spectra; Star & galaxy formation; Star formation
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stv1798

We have obtained Gemini/Gemini Multi-Object Spectrograph 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 12 + log(O/H) ∼ 8.5 and a flat gas-phase metallicity gradient (β = 0.002 ± 0.004 dex kpc−1) out to ∼4R 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.