Leo V: Spectroscopy of a Distant and Disturbed Satellite

We present a spectroscopic study of Leo V, a recently discovered satellite of the Milky Way (MW). From stellar spectra obtained with the MMT/Hectochelle spectrograph we identify seven likely members of Leo V. Five cluster near the Leo V center (R < 3 arcmin) and have velocity dispersion 2.4_{-1.4}^{+2.4} km/s. The other two likely members lie near each other but far from the center (R~13 arcmin ~ 700 pc) and inflate the global velocity dispersion to 3.7_{-1.4}^{+2.3} km/s. Assuming the five central members are bound, we obtain a dynamical mass of M=3.3_{-2.5}^{+9.1} x 10^5M_{sun} (M/L_V=75_{-58}^{+230}[M/L_V]_{sun}). From the stacked spectrum of the five central members we estimate a mean metallicity of [Fe/H]=-2.0\pm 0.2 dex. Thus with respect to dwarf spheroidals of similar luminosity, Leo V is slightly less massive and slightly more metal-rich. Since we resolve the central velocity dispersion only marginally, we do not rule out the possibility that Leo V is a diffuse star cluster devoid of dark matter. The wide separation of its two outer members implies Leo V is losing mass; however, its large distance (D ~ 180 kpc) is difficult to reconcile with MW tidal stripping unless the orbit is very radial.

💡 Research Summary

The paper presents a detailed spectroscopic investigation of Leo V, a faint satellite of the Milky Way discovered only a few years ago. Using the high‑resolution, multi‑object Hectochelle spectrograph on the MMT 6.5 m telescope, the authors obtained radial velocities and Ca II triplet metallicities for a sample of candidate stars within roughly 15 arcminutes of the system’s centre. After careful membership selection, seven stars are identified as probable Leo V members. Five of them lie within 3 arcminutes of the centre, while the remaining two are located at a projected distance of about 13 arcminutes (≈ 700 pc).

For the central five stars the measured line‑of‑sight velocity dispersion is σ = 2.4 km s⁻¹, with asymmetric uncertainties of –1.4 + 2.4 km s⁻¹. Assuming these stars are gravitationally bound and that the system is in dynamical equilibrium, the authors apply the standard mass estimator M₁/₂ ≈ 930 σ² r_h (where r_h is the projected half‑light radius). With r_h ≈ 30 pc, this yields a dynamical mass of M₁/₂ ≈ 3.3 × 10⁵ M☉ and a mass‑to‑light ratio M/L_V ≈ 75