Line-of-sight velocity and metallicity measurements of the Palomar 5 tidal stream

Abstract

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 $\sim 17$\arcdeg along the stream, providing an opportunity to investigate a trend in line-of-sight velocities ($V_{\rm los}$) 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 $V_{\rm los}$ 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 $V_{\rm los}$ values are used to estimate the possible $V_{\rm los}$ range of the member stars at each location along the stream. Because of the heavy contamination of the field MW stars, estimated $V_{\rm los}$ range depends on prior assumptions about the stream's $V_{\rm los}$, which highlights the importance of more definitely identifying the member stars using proper motion and chemical abundances to obtain an unbiased information of $V_{\rm los}$ 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 $V_{\rm los}$ range are discussed.