The ionization structure and chemical history in isolated H ii regions of dwarf galaxies with VIMOS/IFU II. The Leo A galaxy

Abstract

Study the ionized gas in metal-poor environments is key to understanding the mechanisms regulating galaxy evolution. However, most of the previous studies of extragalactic HII regions rely on unresolved observations of gaseous structures. We study the south-western, spatially resolved, HII region of Leo A, one of the most studied isolated dwarf galaxies in the Local Group. Using archival VIMOS-IFU/VLT data, we explored its gaseous structure through optical emission lines to gain insights into the present-day drivers of gas physics in this dIrr, and we place constraints on the chemical evolution scenario responsible for this low chemical enrichment. The emission line maps reveal that the strongest emission comes from the south-west region. A stratified distribution of ionic species was detected, likely powered by the young star cluster at the nebular centre. HST/ACS data show that the brightest star is in the centre of both the HII region and the star cluster. Photoionization production rates derived indicate that this star can sustain the ionization budget to power the HII region, although subject to the assumed electron density. Using the direct method, we derived a metallicity of $12+\log(\mathrm{O/H})=7.29\pm0.06$ dex, increasing to $7.46\pm0.09$ dex after correcting for temperature fluctuations, placing Leo A in the low-mass end of the MZR. Chemical evolution models suggest that, under constant accretion, the stellar mass growth and metal enrichment over the last 10 Gyr are successfully reproduced by both leaky-box and gas-regulator models. Those results are similar to those found in SagDIG, supporting a picture in which the present-day evolution of Leo A is dominated by stellar feedback processes. The combination of mass loss mechanisms and accretion events reproduces its chemical evolution, suggesting that Leo A has evolved under a gas equilibrium regime across its lifetime.

Figures (14)

• Figure 1: Subaru Suprime-Cam H$\alpha$ frame from stonkute14 (stonkute14, stonkute19). The Hii region studied in this work is shown inside the yellow square, representing the VIMOS-IFU FoV.
• Figure 2: Integrated VIMOS-IFU spectrum of the Leo A Hii region normalised by the flux at the peak of the H$\beta$ line. Left panel: spectral window showing H$\gamma$ and $[\ion{O}{iii}]\lambda4363$ detection, from left to right, respectively. Middle panel: spectral window presenting the H$\beta$, and the $[\ion{O}{iii}]\lambda\lambda 4959,5007$ detection, from left to right, respectively. Right panel: spatial distribution of fibres selected to generate the integrated spectrum of the Leo Hii region with $[\ion{O}{iii}]\lambda4363$ detection. The colour code represents the jump value of each selected fibre. The grey contours represent the H$\beta$ emission of the nebula as a reference.
• Figure 3: Emission line maps of the Leo A Hii region. Left panel: H$\beta$ emission line map. Middle panel: $[\ion{O}{iii}]\lambda5007$ emission line map. Right panel: H$\beta/$H$\gamma$ map, where the H$\beta$ map contours are superimposed as reference. In all panels, the colour code represents the flux of the emission lines per spectral fibre acquired by fitting Gaussian curves. The grey dashed lines are circles of increasing radius of $1.34"$ (2 px) units up to $13.4"$ (20 px). Black dotted lines show the angles that separate the south-west emission clump and the extended arc.
• Figure 4: Flux density profiles for the south-west clump and the extended arc, with red and blue colours, respectively. The top panel shows the radial H$\beta$ flux density profiles for both structures. The upper-middle panel shows the radial $[\ion{O}{iii}]\lambda5007$ flux density profile for both structures. The bottom-middle panel shows the H$\beta$ and $[\ion{O}{iii}]\lambda5007$ flux density profile, with purple and green colours, respectively, for the complete nebula integrated over the entire angular range. The bottom panel shows the H$\beta$ /$[\ion{O}{iii}]\lambda5007$ ratio with black colour for the complete nebula integrated over the entire angular range.
• Figure 5: Comparison of the Leo A Hii region with H$\beta$ emission (grey contours) with HST/ACS photometry. The left panel shows the location of young MS stars (magenta triangles) and old stars (cyan squares). A 10 Myr PARSEC isochrones is shown as the black curve, in order to get a proxy of the stellar masses of the young stars. The middle panel shows the spatial distribution of old stars in the Hii region, whereas the right panel shows the distribution of young MS stars in the Hii region. The most massive star is shown with the orange triangle, and those with masses $8< M_{\odot}<10$ are shown with green triangles