A unique isolated dwarf spheroidal galaxy at D = 1.9 Mpc

• Published in: Monthly notices of the Royal Astronomical Society Vol. 425; no. 1; pp. 709 - 719
• Main Authors: Makarov, Dmitry, Makarova, Lidia, Sharina, Margarita, Uklein, Roman, Tikhonov, Anton, Guhathakurta, Puragra, Kirby, Evan, Terekhova, Natalya
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Science Ltd 01.09.2012
• Subjects: galaxies: distances and redshifts; galaxies: dwarf; galaxies: individual: KKR 25; galaxies: stellar content; galaxies: dwarf; galaxies: individual: KKR 25; galaxies: distances and redshifts; galaxies: stellar content
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1111/j.1365-2966.2012.21581.x

Abstract We present a photometric and spectroscopic study of the unique isolated nearby dwarf spheroidal (dSph) galaxy KKR 25. The galaxy was resolved into stars with Hubble Space Telescope/Wide Field Planetary Camera 2 including old red giant branch and red clump. We have constructed a model of the resolved stellar populations and measured the star formation rate and metallicity as a function of time. The main star formation activity period occurred about 12.6-13.7 Gyr ago. These stars are mostly metal poor, with a mean metallicity [Fe/H] ∼ −1 to −1.6 dex. About 60 per cent of the total stellar mass was formed during this event. There are indications of intermediate-age star formation in KKR 25 between 1 and 4 Gyr with no significant signs of metal enrichment for these stars. Long-slit spectroscopy was carried out using the Russian 6-m telescope of the integrated starlight and bright individual objects in the galaxy. We have discovered a planetary nebula (PN) in KKR 25. This is the first known PN in a dSph galaxy outside the Local Group. We have measured its oxygen abundance 12 + log (O/H) = 7.60 ± 0.07 dex and a radial velocity V h = −79 km s−1. We have analysed the stellar density distribution in the galaxy body. The galaxy has an exponential surface brightness profile with a central light depression. We discuss the evolutionary status of KKR 25, which belongs to a rare class of very isolated dwarf galaxies with spheroidal morphology.