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

I. The Sagittarius dwarf irregular 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: a.andradevalenzuela@uandresbello.edu

Received: 3 January 2025
Accepted: 22 May 2025

Abstract

Context. Studying metal-poor galaxies is crucial for understanding physical mechanisms that drive the formation and evolution of galaxies, such as internal dynamics, star formation history, and chemical enrichment. Most of the observational studies on dwarf galaxies employ integral field spectroscopy to investigate gas physics in the entire body of galaxies. However, these past studies have not investigated the detailed spatially resolved properties of individual extragalactic H II regions.

Aims. We study the only known H II region in the Sagittarius dwarf irregular galaxy, a metal-poor galaxy of the local Universe, using integral field unit (IFU) VIMOS/VLT and long-slit EFOSC2/NTT archival data. We explore the spatially resolved gaseous structure by using optical emission lines, to (i) provide insights into the physical processes that are shaping the evolution of this H II region, and (ii) relate these mechanisms to the metal-poor, gas-phase component in extragalactic H II regions.

Methods. We probe optical emission line structures of the H II region, fully covered within the 27″×27″ field of view of VIMOS. The oxygen abundances were estimated by applying the T_e-sensitive method, by using the auroral [O III]λ4363 emission line detectable at S/N > 3 integrating the spectral fibres of the data cube.

Results. From the emission line maps, the O⁺⁺ emission is concentrated towards the centre, in comparison to the low-ionised species such as O⁺ and H⁺. The Hβ maps reveal that the H II shows two prominent clumps, showing a biconic-like shape aligned along the same axis. Radial flux-density profiles reveal that those clumps are similar in terms of size (∼8″) and flux distribution in Hβ and [O III]λ5007. Comparing stellar populations from HST photometry in the gaseous structure, we find that old stellar populations (>1 Gyr) are uniformly distributed across the H II region, whereas the young stellar populations (⪅700 Myr) are found closer to the edges of the Hβ clumps and distributed in filamentary configurations. We estimate T_e = 17 683±1254 K for the gaseous structure. The T_e-based oxygen abundance of the SagDIG H II region is 12+log(O/H) = 7.23±0.04, which is in agreement with empirical estimations of the literature, and is also in line with the low-mass end of the mass-metallicity relation (MZR). Considering corrections on T_e fluctuations, we estimate 12+log(O/H) = 7.50±0.08.

Conclusions. The stratified composition of the H II region is a signature that this gaseous structure is expanding. This feature, together with SagDIG falling in the low-mass end of the MZR, suggests that the evolution of this H II region is sustained by ionisation from massive stars, stellar winds, and supernovae explosions expanding the gas structure. The filamentary configuration of young stars is likely produced by the interaction between atomic and ionised gas, in line with many galactic H II regions and those found in the Large Magellanic Cloud. If this proposed scenario is confirmed with multi-wavelength data and data cubes with better spectral coverage and spatial resolution, it could imply that H II regions in metal-poor dwarf galaxies are subject to the same physics as H II regions in the Milky Way.