Warm ionized gas in the blue compact galaxy Haro 14 viewed by MUSE^⋆

The diverse ionization mechanisms acting in low-mass starbursts

¹ Institut für Astrophysik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
e-mail: luzma@astro.physik.uni-goettingen.de
² Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
³ Leibniz-Institut für Astrophysik (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁴ Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany

Received: 1 January 2022
Accepted: 25 April 2022

Abstract

We investigate the warm ionized gas in the blue compact galaxy (BCG) Haro 14 by means of integral field spectroscopic observations taken with the Multi Unit Spectroscopic Explorer (MUSE) at the Very Large Telescope. The large field of view of MUSE and its unprecedented sensitivity enable observations of the galaxy nebular emission up to large galactocentric distances, even in the important but very faint [O I] λ6300 diagnostic line. This allowed us to trace the ionized gas morphology and ionization structure of Haro 14 up to kiloparsec scales and, for the first time, to accurately investigate the excitation mechanism operating in the outskirts of a typical BCG. The intensity and diagnostic maps reveal at least two highly distinct components of ionized gas: the bright central regions, mostly made of individual clumps, and a faint component which extends up to kiloparsec scales and consists of widespread diffuse emission, well-delineated filamentary structures, and faint knots. Noteworthy are the two curvilinear filaments extending up to 2 and 2.3 kpc southwest, which likely trace the edges of supergiant expanding bubbles driven by galactic outflows. We find that while the central clumps in Haro 14 are H II-region complexes, the morphology and line ratios of the whole low-surface-brightness component are not compatible with star formation photoionization. In the spatially resolved emission-line-ratio diagnostic diagrams, spaxels above the maximum starburst line form the majority (∼75% and ∼50% in the diagnostic diagrams involving [O I] and [S II] respectively). Moreover, our findings suggest that more than one alternative mechanism is ionizing the outer galaxy regions. The properties of the diffuse component are consistent with ionization by diluted radiation and the large filaments and shells are most probably shocked areas at the edge of bubbles. The mechanism responsible for the ionization of the faint individual clumps observed in the galaxy periphery is more difficult to assess. These clumps could be the shocked debris of fragmented shells or regions where star formation is proceeding under extreme conditions.