Letter to the Editor

New KiDS in town

Sextans II: A new stellar system on the outskirts of the Milky Way

¹ INAF-Osservatorio Astronomico di Capodimonte, Via Moiariello, 16, 80131 Napoli, Italy
e-mail: massimiliano.gatto@inaf.it
² INAF-Osservatorio di Astrofisica e Scienza dello Spazio, Via Gobetti, 93/3, 40129 Bologna, Italy
³ Physics Department, University of Pisa, Largo Bruno Pontecorvo 3, 56127 Pisa, Italy
⁴ INFN-Largo Bruno Pontecorvo 3, 56127 Pisa, Italy
⁵ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁶ Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute (AIRUB), German Centre for Cosmological Lensing, 44780 Bochum, Germany
⁷ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
⁸ School of Physics and Astronomy, Sun Yat-sen University, Guangzhou, 519082 Zhuhai Campus, PR China
⁹ Department of Physics and Astronomy, University of Victoria, Victoria, BC V8P 1A1, Canada

Received: 10 November 2023
Accepted: 22 December 2023

Abstract

We report the discovery of a significant and compact over-density of old and metal-poor stars in the fourth data release of the KiDS survey (DR4). The discovery is confirmed by deeper HSC-SSP data revealing the old main sequence turn-off of a stellar system located at a distance from the sun of D_⊙ = 145_-13⁺¹⁴ kpc in the direction of the Sextans constellation. The system has an absolute integrated magnitude (M_V = −3.9 ± 0.4), half-light radius (r_h = 193_-46⁺⁶¹ pc), and ellipticity (e = 0.46_-0.15^+0.11) values that are typical of ultra-faint dwarf galaxies (UFDs). The central surface brightness is near the lower limits of known local dwarf galaxies of a similar integrated luminosity, as expected for stellar systems that have escaped detection until now. The distance of the newly found system suggests that it is likely to be a satellite of our own Milky Way and we have thus tentatively named it Sextans II (KiDS-UFD-1).