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

Abstract: The dwarf galaxy Peekaboo was recently identified as a Local Volume (LV) gas-rich and 'eXtremely Metal-Poor' (XMP) dIrr. Its gas metallicity is Z~Zsun/50, with +/-1$\sigma$ uncertainty range of [Zsun/72-Zsun/35]). Its the "Tip of Red Giant Stars" (TRGB)-based distance is of 6.8$\pm$0.7 Mpc. HST data for its individual stars reveal that its older RGB stars comprise a smaller part, while the majority of visible stars have ages of less than one to a few Gyr. Thus, Peekaboo dwarf can be considered as the nearest record-low-Z dwarf. As such, the galaxy deserves a deeper multi-method study, including properties of both, young massive stars and the fainter older population, and its ionised gas and the dominant baryonic component of HI gas. We use the direct (Te) method for the east HII region, in which the [OIII]4363A line is well detected, to estimate its parameter 12+log(O/H). Since in the west HII region the line [OIII]4363A is not detected, its O/H is estimated via the empirical "strong-line" method. The resulting value of O/H is very close to that in the east HII region. The new spectroscopy of Peekaboo dwarf allows us to improve substantially the accuracy of its direct O/H estimate, which appears of 12+log(O/H) = 6.99$\pm$0.06. The new data reveal that emission lines in the E region consist of two components with the velocity difference of ~65 km/s. The fainter, approaching, component can 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 identify tentative O-type and very hot candidate WO stars, which are likely the ionising stars of the studied HII regions. With the new optical spectra, the Peekaboo galaxy is confirmed as the lowest-metallicity dwarf in the Local Volume and the valuable object for indeep multi-method studies.