The Peekaboo galaxy: New SALT spectroscopy and implications of archive HST data

Context. The dwarf galaxy Peekaboo (HIPASSJ1131–31) was recently identified as a local volume (LV) gas-rich and extremely metal-poor (XMP) dIrr. Its gas metallicity is Z ∼ Z⊙/50, with a ±1σ uncertainty range of [Z⊙/72–Z⊙/35]). Its ‘tip of the red-giant branch’ distance is 6.8 ± 0.7 Mpc. The Hubble Space Telescope (HST) data for its individual stars revealed that its older red-giant-branch stars comprise a smaller part of the galaxy, while the majority of visible stars have ages of less than one to a few gigayears. Thus, the Peekaboo dwarf can be considered as the nearest record-low Z dwarf. As such, the galaxy deserves a deeper multi-method study that examines the properties of its young massive stars and the fainter older population as well as its ionised gas and the dominant baryonic component of H<ns0:sc>I</ns0:sc> gas. Aims. We aim to obtain the higher S-to-N SALT optical spectra of two H <ns0:sc>II</ns0:sc> regions in Peekaboo in order to improve the accuracy of its gas O/H and to determine abundances of Ne, S, N, and Ar. With archive HST images, we aim to identify the hot massive stars, including exciting the two H <ns0:sc>II</ns0:sc> regions (i.e. east and west), and the XMP supergiants as important targets for follow-up studies of their evolution with upcoming extremely large telescopes. Methods. We used the direct (Te) method for the east H <ns0:sc>II</ns0:sc> region in which a [O <ns0:sc>III</ns0:sc>]λ4363 Å line is well detected in order to estimate its parameter 12 + log(O/H). In the west H <ns0:sc>II</ns0:sc> region, the line [O<ns0:sc>III</ns0:sc>]λ4363 Å is not detected, so we estimated its O/H via the empirical ‘strong-line’ method of Izotov et al. (2019, A&A, 523, A40). The resulting value of O/H is very close to that in the east H <ns0:sc>II</ns0:sc> region. Results. The new spectroscopy of the Peekaboo dwarf allowed us to substantially improve the accuracy of its direct O/H estimate, and we obtained 12 + log(O/H) = 6.99 ± 0.06 dex. The new data reveal that emission lines in the east region consist of two components with a velocity difference of ∼65 km s−1. The fainter approaching component could be related to a fast-moving WR star thrown from a cluster or a binary system. Using the HST V magnitudes and colour V − I, we identified tentative O-type and very hot candidate WO stars, which are likely the ionising stars of the studied H <ns0:sc>II</ns0:sc> regions. Conclusions. With the new optical spectra, the Peekaboo galaxy is confirmed as the lowest-metallicity dwarf in the LV and as a valuable object for in-depth multi-method studies. We separate its most luminous stars for follow-up ground-based brightness monitoring and spectroscopy.