Large binocular telescope observations of new six compact star-forming galaxies with [Ne v] λ3426 Å emission. Issue 2 (2nd October 2021)

• Record Type: Journal Article
• Authors: Izotov, Y I; Thuan, T X; Guseva, N G
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 508:Issue 2(2021)
• Page Start: 2556
• Page End: 2574
• Publication Date: 2021-10-02
• Subjects: galaxies: abundances -- galaxies: dwarf -- galaxies: ISM -- galaxies: starburst; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stab2798
• Languages: English
• ISSNs: 0035-8711

Abstract

ABSTRACT: We report the discovery of [Ne v ] λ3426 emission, in addition to He ii λ4686 emission, in six compact star-forming galaxies. These observations considerably increase the sample of eight such galaxies discovered earlier by our group. For four of the new galaxies, the optical observations are supplemented by near-infrared spectra. All galaxies, but one, have H ii regions that are dense, with electron number densities of ∼ 300–700 cm −3 . They are all characterized by high H β equivalent widths EW(H β) ∼ 190–520 Å and high O32 = [O iii ] λ5007/[O ii ] λ3727 ratios of 10–30, indicating young starburst ages and the presence of high ionization radiation. All are low-metallicity objects with 12 + logO/H = 7.46–7.88. The spectra of all galaxies show a low-intensity broad component of the H α line and five of six objects show Wolf–Rayet features. Comparison with photoionization models shows that pure stellar ionization radiation from massive stars is not hard enough to produce such strong [Ne v ] and He ii emission in our galaxies. The [Ne v ] λ3426/He ii λ4686 flux ratio of ∼1.2 in J1222+3602 is consistent with some contribution of active galactic nucleus ionizing radiation. However, in the remaining five galaxies, this ratio is considerably lower, $\lesssim$ 0.4. The most plausible models are likely to be non-uniform in density, where He ii and [Ne v ] lines are emitted in low-density channels made by outflows and illuminated by harder ionizing radiation from radiative shocks propagating through these channels, whereas [O iii ] emission originates in denser regions exposed to softer stellar ionizing sources.