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 HI gas. Aims. We aim to obtain the higher S-to-N SALT optical spectra of two H II 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 II 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 II region in which a O IIIλ4363 Å line is well detected in order to estimate its parameter 12 + log(O/H). In the west H II region, the line OIIIλ4363 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 II 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 II 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.