Star-forming regions and the metallicity gradients in the tidal tails: the case of NGC 92

Abstract

We present new Gemini/GMOS spectroscopic and archival imaging data of the interacting galaxy NGC 92, which is part of a compact group and displays an extended tidal tail.We have studied the physical properties of 20 star-forming complexes in this system. We found that the star-forming regions located in the tidal tail of NGC 92 have ages younger than ∼8Myr, which suggests that these objects were formed in situ. The spectroscopic data reveal that these regions have slightly sub-solar metallicities, suggesting that they were formed from pre-enriched material. Using the oxygen abundances derived for each system, we found that the extended tidal tail of NGC 92 has a flat metallicity distribution. Although this scenario is consistent with N-body simulations of interacting systems, where there is gas mixing triggered by the interaction, archival Hα Fabry–Perot data cubes of NGC 92 have not shown a velocity gradient along the tail of this galaxy, which under certain assumptions could be interpreted as a lack of gas flow in the tail. Our results suggest that a fraction of the enriched gas that was originally located in the centre of the galaxy was expelled into the tidal tail when the interacting process that formed the tail happened. However, we cannot exclude the scenario in which the star formation in the tail has increased its original oxygen abundance