A GMOS-N IFU study of the central H II region in the blue compact dwarf galaxy NGC 4449: Kinematics, Nebular Metallicity and Star-Formation

Abstract

We use integral field spectroscopic (IFS) observations from the Gemini North Multi-Object Spectrograph (GMOS-N) to study the central H II region in a nearby blue compact dwarf (BCD) galaxy NGC 4449. The IFS data enable us to explore the variation of physical and chemical conditions of the star-forming region and the surrounding gas on spatial scales as small as 5.5 pc. Our kinematical analysis shows possible signatures of shock ionisation and shell structures in the surroundings of the star-forming region. The metallicity maps of the region, created using direct T${}_e$ and indirect strong line methods (R${}_{23}$, O3N2 and N2), do not show any chemical variation. From the integrated spectrum of the central H II region, we find a metallicity of 12 + log(O/H) = 7.88 $\pm$ 0.14 ($\sim$ 0.15${}_{-0.04}^{+0.06}$ Z${}_{\odot }$) using the direct method. Comparing the central H II region metallicity derived here with those of H II regions throughout this galaxy from previous studies, we find evidence of increasing metallicity with distance from the central nucleus. Such chemical inhomogeneities can be due to several mechanisms, including gas-loss via supernova blowout, galactic winds, or metal-poor gas accretion. However, we find that the localised area of decreased metallicity aligns spatially with the peak of star-forming activity in the galaxy, suggesting that gas-accretion may be at play here. Spatially-resolved IFS data for the entire galaxy is required to confirm the metallicity inhomogeneity found in this study, and determine its possible cause.

22 pages, 18 figures, 4 tables, accepted for publication in MNRAS

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