Comments on a faint old stellar system at 150 kpc

TL;DR

This paper argues that SDSS J1257+3419 is one of the smallest bound dwarf spheroidal galaxies in the Milky Way, with a low mass but typical density, challenging previous classifications.

Contribution

It re-examines observational data to demonstrate that SDSS J1257+3419 is a small, bound dwarf spheroidal galaxy with a larger tidal radius than its half-light radius.

Findings

SDSS J1257+3419 has the lowest mass among Milky Way dSphs.

Its mass density is typical of dwarf spheroidals.

The tidal radius exceeds its half-light radius, indicating it is gravitationally bound.

Abstract

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 (see also arXiv:0709.0327 [astro-ph]). 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.