We present Subaru/FOCAS and Keck/DEIMOS medium-resolution spectroscopy of a tidally disrupting Milky Way ( MW ) globular cluster Palomar 5 and its tidal stream .
The observed fields are located to cover an angular extent of \sim 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 Milky Way ’ 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 \pm 0.7 km s ^ { -1 } and metallicity [ Fe/H ] = -1.35 \pm 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 , 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 an 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 .