The ionisation structure and chemical history in isolated HII regions of dwarf galaxies with integral field unit

II. The Leo A galaxy^★

¹ Universidad Andres Bello, Facultad de Ciencias Exactas, Departamento de Física y Astronomía – Instituto de Astrofísica, Fernández Concha 700, Las Condes, Santiago, Chile
² European Southern Observatory, Alonso de Cordova 3107, Vitacura, Casilla, 19001 Santiago de Chile 19, Chile
³ Universidad Andres Bello, Facultad de Ciencias Exactas, Departamento de Física y Astronomía – Instituto de Astrofísica, Autopista Concepción-Talcahuano, 7100 Talcahuano, Chile
⁴ INAF – Osservatorio Astronomico di Trieste, Via G. B. Tiepolo 11, 34143 Trieste, Italy
⁵ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, I-35122 Padova, Italy

^★★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 4 April 2025
Accepted: 15 December 2025

Abstract

Context. Examining the ionised gas in metal-poor environments is key to understanding the physical mechanisms regulating galaxy evolution. However, most of the previous studies of extragalactic H II regions rely on unresolved observations of gaseous structures.

Aims. We aim to study the south-western, spatially resolved H II region of Leo A, one of the most studied gas-rich isolated galaxies in the Local Group. Using archival VIMOS-IFU/VLT data, we investigate 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 its low chemical enrichment.

Methods. We mapped the Hβ and [O III]λ5007 flux distributions of the H II region, fully covered within the 27″ × 27″ VIMOS field of view. Oxygen abundances were derived with the T_e-sensitive method, using the auroral [O III]λ4363 emission-line detection, obtained by integrating spectral fibres of the data cube.

Results. The emission-line maps reveal that the strongest emission comes from the south-west region. Differences between the H⁺ and O⁺⁺ distributions indicate a stratified distribution of ionic species, likely powered by the young star cluster at the nebular centre. HST/ACS photometry shows that the brightest star (∼15 M_⊙) is in the centre of both the H II region and the young star cluster. Photoionisation production rates derived indicate that this star is able to sustain most of the ionisation budget to power the H II region, although subject to the assumed electron density. We derive an oxygen abundance of 12 + log(O/H) = 7.29 ± 0.06 dex, increasing to 7.46 ± 0.09 dex after correcting for temperature fluctuations. These values place Leo A on the low-mass end of the mass-metallicity relation. Chemical-evolution models indicate that, under constant accretion, the stellar-mass growth and metal enrichment over the last 10 Gyr are successfully reproduced by both the gas-regulator and leaky-box models.

Conclusions. The distribution of young stars in this H II region reveals similar features to those of the H II region in the Sagittarius dIrr (SagDIG), supporting a picture in which the present-day evolution of Leo A is dominated by stellar feedback processes, associated with young stars in the cluster ionising the H II region studied in this work. The combination of mass-loss mechanisms and accretion events efficiently reproduces its chemical evolution, suggesting Leo A has evolved under a gas equilibrium regime across its lifetime.