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

• Published in: Monthly notices of the Royal Astronomical Society Vol. 428; no. 1; pp. 86 - 102
• Main Authors: James, B. L., Tsamis, Y. G., Barlow, M. J., Walsh, J. R., Westmoquette, M. S.
• Format: Journal Article
• Language: English
• Published: London Oxford University Press 2013
• Subjects: Correlation analysis; Electrons; Morphology; Stars & galaxies; Temperature; Velocity; galaxies: dwarf; stars: Wolf-Rayet; galaxies: kinematics and dynamics; galaxies: abundances; galaxies: individual: UM 448; galaxies: interactions
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/sts004

Using Very Large Telescope/Fibre Large Array Multi Element Spectrograph optical integral field unit 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. New Technology Telescope/Superb-Seeing Imager images reveal a morphology suggestive of a merger of two systems of contrasting colour, whilst our Hα  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 most lines consisting of a narrow [full width at half-maximum (FWHM) 100 km s−1], central component, an underlying broad component (FWHM ∼ 150-300 km s−1) and a third, narrow blueshifted component. Radial velocity maps of all three components 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, based on the [O iii] λλ4363, 4959, [N ii] λ6584, [S ii] λλ6716, 6731 and [Ne iii] λ3868 line maps, is presented. Whilst the eastern tail of UM 448 has electron temperatures (T
e) that are typical of BCGs, we find a region within the main body of the galaxy where the narrow and broad [O iii] λ4363 line components trace temperatures differing by 5000 K 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 that they do not agree, possibly due the flux weighting of T
e  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 kpc2 region within the main body of the galaxy. Contrary to previous studies, however, we do not find evidence for a large Wolf-Rayet (WR) 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 suggest that interaction-induced inflow of metal-poor gas may be responsible.