Discovery of a Solitary Dwarf Galaxy in the APPLES Survey

During the APPLES parallel campaign, the Hubble Space Telescope (HST) Advanced Camera for Surveys has resolved a distant stellar system, which appears to be an isolated dwarf galaxy. It is characterized by a circularly symmetric distribution of stars with an integrated magnitude mF775W = 20.13 ± 0.02, a central surface brightness μF775W ≃ 21.33 ± 0.18 mag arcsec-2, and a half-light radius of ≃1.″8. The ACS and VLT spectra show no evidence of ionized gas and appear to be dominated by a 3 Gyr old stellar population. The OB spectral type derived for two resolved stars in the grism data and the systemic radial velocity (Vhel ≃ 670 km s-1) measured from the VLT data give a fiducial distance of ≃9 ± 2 Mpc. These findings, with the support of the spatial morphology, would classify the system among the dwarf spheroidal (dSph) galaxies. Following IAU rules, we have named this newly discovered galaxy APPLES 1. An intriguing peculiarity of APPLES 1 is that the properties (age and metallicity) of the stellar content so far detected are similar to those of dSph galaxies in the Local Group, where star formation is thought to be driven by galaxy interactions and mergers. Yet, APPLES 1 seems not to be associated with a major group or cluster of galaxies. Therefore, APPLES 1 could be the first example of a field dSph galaxy with self-sustained and regulated star formation, and therefore would make an interesting test case for studies of the formation and evolution of unperturbed dSph galaxies.