Monitoring broad emission-line components in spectra of the two low-metallicity dwarf compact star-forming galaxies SBS 1420+540 and J1444+4840

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 10^6.24 and 10^6.59 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.