Structure and Dynamics of the Globular Cluster Palomar 13

Details Structure and Dynamics of the Globular Cluster Palomar 13 Structure and Dynamics of the Globular Cluster Palomar 13

Jan 2012

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2012aas...21915116b&link_type=abstract

American Astronomical Society, AAS Meeting #219, #151.16

Astronomy and Astrophysics

Astronomy

Scientific paper

We present Keck/DEIMOS spectroscopy and CFHT/MegaCam photometry for the Milky Way globular cluster Palomar 13. We triple the number of spectroscopically confirmed members, including many repeat velocity measurements. Palomar 13 is the only known globular cluster with possible evidence for dark matter, based on a Keck/HIRES 21 star velocity dispersion of σ=2.2+/-0.4 km/s. We reproduce this measurement, but demonstrate that it is inflated by unresolved binary stars. For our sample of 61 stars, the velocity dispersion is σ=0.7(+0.6/-0.5) km/s. Combining our DEIMOS data with literature values, our final velocity dispersion is σ=0.4(+0.4/-0.3) km/s. We determine a spectroscopic metallicity of [Fe/H]=-1.6+/-0.1 dex, placing a 1-σ upper limit of σ[Fe/H] 0.2 dex on any internal metallicity spread. We determine Palomar 13's total luminosity to be MV=-2.8+/-0.4, making it among the least luminous known globular clusters. The photometric isophotes are regular out to the half-light radius and mildly irregular outside this radius. The outer surface brightness profile slope is shallower than typical globular clusters (η=-2.8+/-0.3). Thus at large radius, tidal debris is likely affecting the appearance of Palomar 13. Combining our luminosity with the intrinsic velocity dispersion, we find a dynamical mass of of M1/2=1.3(+2.7/-1.3)x103 Msun and a mass-to-light ratio of M/LV=2.4(+5.0/-2.4) Msun/Lsun. Within our measurement errors, the mass-to-light ratio agrees with the theoretical predictions for a single stellar population. We conclude that, while there is some evidence for tidal stripping at large radius, the dynamical mass of Palomar 13 is consistent with its stellar mass and neither significant dark matter, nor extreme tidal heating, is required to explain the cluster dynamics.

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