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We report the discovery of broad components with P-Cygni profiles of the hydrogen and helium emission lines in the two low-redshift low-metallicity dwarf compact star-forming galaxies SBS 1420+540 and J1444+4840. We found small stellar masses of 106.24 and 106.59 M⊙, low oxygen abundances 12 + log O/H of 7.75 and 7.45, high velocity dispersions reaching σ ∼ 700 and ∼1200 km s−1, high terminal velocities of the stellar wind of ∼1000 and ∼1000–1700 km s−1, respectively, and large EW(H β) of ∼300 Å for both. For SBS 1420+540, we succeeded in capturing an eruption phase by monitoring the variations of the broad-to-narrow component flux ratio. We observe a sharp increase of that ratio by a factor of 4 in 2017 and a decrease by about an order of magnitude in 2023. The peak luminosity of ∼1040 erg s−1 of the broad component in L(H α) lasted for about 6 yr out of a three-decades monitoring. This leads us to conclude that there is probably a luminous blue variable candidate (LBVc) in this galaxy. As for J1444+4840, its very high L(H α) of about 1041 ergs s−1, close to values observed in active galactic nuclei (AGNs) and Type IIn supernovae (SNe), and the variability of no more than 20 per cent of the broad-to-narrow flux ratio of the hydrogen and helium emission lines over a 8 yr monitoring do not allow us to definitively conclude that it contains an LBVc. On the other hand, the possibility that the line variations are due to a long-lived stellar transient of type AGN/SN IIn cannot be ruled out.

We present observations with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope of nine most metal-deficient compact star-forming galaxies with oxygen abundances 12  + log(O/H)  = 6.97–7.23, redshifts z  = 0.02811–0.13320, and stellar masses M⋆ ≤ 107 M⊙. We aim to study the properties of Ly α emission in these extremely metal-deficient objects. We find that all nine galaxies are Ly α emitters (LAEs). We examine various relations between the Ly α escape fraction fesc(Ly α) and other characteristics – such as absolute UV magnitude, oxygen abundance, O32 ratio, stellar mass, Lyman-alpha luminosity, and equivalent width EW(Ly α), ionizing photon production efficiency ξion and velocity separation Vsep between the two peaks of the Ly α profile – of a large sample of LAEs, including our lowest metallicity galaxies and other objects from the literature. We find a relatively tight correlation between fesc(Ly α) and two characteristics, EW(Ly α) and Vsep, whereas no correlation is found between fesc(Ly α) and the oxygen abundance. We also find a relatively tight relation between the Ly α and LyC escape fractions. We propose to use the latter relation to estimate indirectly the escaping ionizing radiation in LAEs, when direct measurements of LyC emission are not possible. We show that the global properties of low-z LAEs are very similar to those of z > 6 galaxies. They are thus ideal local proxies for studying physical processes during the epoch of reionization of the Universe.

Recently, Kojima and co-authors have reported a record low oxygen abundance, 12 + logO/H = 6.90 ± 0.03, or 1.6 per cent of solar metallicity, in the low-mass star-forming galaxy HSC J1631 + 4426. This exceptionally low oxygen abundance was obtained by the direct method, using the [O iii]λ4363Å emission line. However, using the strong-line method by Izotov et al. (2019b), these authors have derived a significantly higher metallicity 12 + logO/H = 7.175 ± 0.005. To clarify the situation, we have obtained new observations of HSC J1631 + 4426 with the Large Binocular Telescope (LBT)/Multi-Object Dual Spectrograph (MODS). We have derived a higher oxygen abundance, 12 + logO/H = 7.14 ± 0.03, using the direct method, a value similar to the oxygen abundance obtained by the strong-line method. Thus, HSC J1631 + 4426 has a metallicity close to that of the well known blue compact dwarf galaxy I Zw 18.

We have studied the spectral time variations of candidate luminous blue variable (cLBV) stars in two low-metallicity star-forming galaxies, DDO 68 and PHL 293B. The LBV in DDO 68, located in H ii region #3, shows an outburst, with an increase of more than 1000 times in H α luminosity during the period 2008–2010. The broad emission of the H i and He i lines display a P Cygni profile, with a relatively constant terminal velocity of ∼800 km s−1, reaching a maximum luminosity L(H α) of ∼2 × 1038 erg s−1, with a full width at half-maximum (FWHM) of ∼1000–1200 km s−1. On the other hand, since the discovery of a cLBV in 2001 in PHL 293B, the fluxes of the broad components and the broad-to-narrow flux ratios of the H i and He i emission lines in this galaxy have remained nearly constant over 16 yr, with small variations. The luminosity of the broad H α component varies between ∼2 × 1038 erg s−1 and ∼1039 erg s−1, with the FWHM varying in the range ∼500–1500 km s−1. Unusually persistent P Cygni features are clearly visible until the end of 2020 despite a decrease of the broad-to-narrow flux ratio in the most recent years. A terminal velocity of ∼800 km s−1 is measured from the P Cygni profile, similar to the one in DDO 68, although the latter is 3.7 more metal-deficient than PHL 293B. The relative constancy of the broad H α luminosity in PHL 293B suggests that it is due to a long-lived stellar transient of type LBV/SN IIn.