The merging dwarf galaxy UM 448: chemodynamics of the ionized gas from VLT integral field spectroscopy

TL;DR

This study uses VLT integral field spectroscopy to analyze the chemodynamics of the ionized gas in the merging dwarf galaxy UM 448, revealing complex gas motions, temperature variations, and localized nitrogen enrichment without evidence of Wolf-Rayet stars.

Contribution

It provides the first spatially-resolved chemodynamical analysis of UM 448, highlighting the impact of galaxy interactions on gas properties and chemical abundances.

Findings

Detected complex emission line profiles indicating multiple gas components.

Identified a nitrogen-enriched region likely caused by gas inflow from interactions.

Showed that integrated spectra can misrepresent local abundances due to flux-weighting.

Abstract

Using VLT/FLAMES optical IFU observations, we present a detailed study of UM 448, a nearby Blue Compact Galaxy (BCG) previously reported to have an anomalously high N/O abundance ratio. NTT/SuSI2 images reveal a morphology suggestive of a merger of two systems of contrasting colour, whilst our H-alpha emission maps resolve UM 448 into three separate regions that do not coincide with the stellar continuum peaks. UM 448 exhibits complex emission line profiles, with lines consisting of a narrow, central component, an underlying broad component and a third, narrow blue-shifted component. Radial velocity maps show signs of solid body rotation across UM 448, with a projected rotation axis that correlates with the continuum morphology of the galaxy. A spatially-resolved, chemodynamical analysis is presented. Whilst the eastern tail of UM 448 has electron temperatures (Te) that are typical of BCGs, we find a region within the main body of the galaxy where the narrow and broad [O III] 4363 line components trace temperatures differing by 5000K and oxygen abundances differing by 0.4 dex. We measure spatially resolved and integrated ionic and elemental abundances for O, N, S and Ne throughout UM 448, and find they do not agree, possibly due the flux-weighting of Te from the integrated spectrum. This has significant implications for abundances derived from long-slit and integrated spectra of star-forming galaxies in the nearby and distant universe. A region of enhanced N/O ratio is indeed found, extended over a ~0.6 kpc^2 region within the main body of the galaxy. Contrary to previous studies, however, we do not find evidence for a large Wolf-Rayet population, and conclude that WR stars alone cannot be responsible for producing the observed N/O excess. Instead, the location and disturbed morphology of the N-enriched region suggests that interaction-induced inflow of metal-poor gas may be responsible.