Comments on a faint old stellar system at 150 kpc
Department of Earth and Ocean Sciences, School of Applied Sciences, National Defense Academy of Japan, Yokosuka 239-8686, Japan ( Department of Astronomy, School of Science, Kyoto University, Sakyo-Ku, Kyoto 606-8502, Japan, before 2007 April.) e-mail: email@example.com
Accepted: 2 June 2007
Context.Recent study of SDSS J1257+3419 has revealed that this stellar system is either a faint and small dwarf galaxy or a faint and widely extended globular cluster.
Aims.In this short note, the author suggests that this system is one of the smallest dwarf spheroidals (dSphs) in the Milky Way.
Methods.We re-examine some observational quantities of this object and check whether it can be bound system.
Results. As a result, we find the mass of SDSS J1257+3419 is the lowest of dSphs in the Milky Way, and its mass density is typical of dSphs. Important is that the tidal radius of SDSS J1257+3419 is much larger than its half-light radius. That is, this very small dSph can be bound by its own gravity.
Key words: Galaxy: halo / galaxies: dwarf
© ESO, 2007