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Globular Cluster Streams as Galactic High-Precision Scales

Date: Wed. November 19th, 2014, 11:00 am-1:00 pm
Location: Sears 552

Globular Cluster Streams as Galactic High-Precision Scales

Andreas Küpper, Columbia University

Tidal streams are promising probes of the gravitational potential of the Milky Way and of the clumsiness of its dark-matter halo. We model the tidal stream of the Milky Way globular cluster Palomar 5 (Pal 5), and show that the unique geometry of the problem yields powerful constraints on the model parameters characterizing the Local Standard of Rest (LSR), the Milky Way and Pal 5 itself. Using only SDSS data and a few radial velocities from the literature, we find that the distance of the Sun from the Galactic Center is 8.30+/-0.25 kpc, and the LSR transverse velocity is 242+/-16 km/s. Assuming that the dark halo of the Galaxy follows a NFW density profile, we fit it with a virial mass of (1.6+/-0.4) 10^12 M_sun, a virial radius of 195+/-19 kpc, and hence a rather low concentration of 5+/-2. Moreover, we find it with a flattening of q_z = 0.95^{+0.16}_{-0.12} to be essentially spherical – at least within the inner 25 kpc, which are effectively probed by Pal 5. We also determine Pal 5’s mass, distance and proper motions independently from other methods, which enables us to perform vital cross-checks for these methods. We conclude that finding more globular cluster streams is essential for mapping out the structure of the halo of our Galaxy to high precision. Finally, we point out that all our best-fit models yield similar substructure patterns as the ones observed in the Pal 5 stream within about 5 kpc of the cluster. The origin of these substructures is epicylic motion of stars along the stream. Such epicylic substructures have to be taken into account when searching tidal streams for signs of past encounters with dark-matter subhalos.

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