A unique isolated dwarf spheroidal galaxy at D=1.9 Mpc

TL;DR

This study provides a detailed photometric and spectroscopic analysis of the isolated dwarf spheroidal galaxy KKR25, revealing its stellar populations, star formation history, and the discovery of the first planetary nebula in such a galaxy outside the Local Group.

Contribution

It presents the first detection of a planetary nebula in an isolated dwarf spheroidal galaxy outside the Local Group and offers comprehensive insights into its stellar populations and structure.

Findings

Main star formation occurred 12.6-13.7 Gyr ago with metal-poor stars.

Intermediate age stars formed 1-4 Gyr ago without significant metal enrichment.

Discovered a planetary nebula with oxygen abundance 12+log(O/H)=7.60.

Abstract

We present a photometric and spectroscopic study of the unique isolated nearby dSph galaxy KKR25. The galaxy was resolved into stars with HST/WFPC2 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 function of time. The main star formation activity period occurred about 12.6 to 13.7 Gyr ago. These stars are mostly metal-poor, with a mean metallicity [Fe/H]\sim -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 KKR25 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 KKR25. This is the first known PN in a dwarf spheroidal galaxy outside the Local Group. We have measured its oxygen abundance 12+log(O/H)=7.60+-0.07 dex and a radial velocity Vh=-79 km/s. 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 KKR25, which belongs to a rare class of very isolated dwarf galaxies with spheroidal morphology.