2016ApJ...823..157I

We present Subaru/Faint Object Camera and Spectrograph and Keck/Deep Imaging Multi-Object Spectrometer medium-resolution spectroscopy of a tidally disrupting Milky Way (MW) globular cluster Palomar 5 (Pal 5) and its tidal stream. The observed fields are located to cover an angular extent of ∼17° along the stream, providing an opportunity to investigate a trend in line-of-sight velocities (V_los) along the stream, which is essential to constrain its orbit and underlying gravitational potential of the MW's dark matter halo. A spectral fitting technique is applied to the observed spectra to obtain stellar parameters and metallicities ([Fe/H]) of the target stars. The 19 stars most likely belonging to the central Pal 5 cluster have a mean V_los of -58.1±0.7 km/s and metallicity [Fe/H]=-1.35±0.06 dex, both of which are in good agreement with those derived in previous high-resolution spectroscopic studies. Assuming that the stream stars have the same [Fe/H] as the progenitor cluster, the derived [Fe/H] and {V}_{los} values are used to estimate the possible V_los range of the member stars at each location along the stream. Because of the heavy contamination of the field MW stars, the estimated V_los range depends on prior assumptions about the stream's {V}_{los} , which highlights the importance of more definitely identifying the member stars using proper motion and chemical abundances to obtain unbiased information of V_los in the outer part of the Pal 5 stream. The models for the gravitational potential of the MW's dark matter halo that are compatible with the estimated V_los range are discussed.