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Internal Stellar Kinematics of M32 from the SPLASH Survey: Dark Halo Constraints

Howley, K. M. and Guhathakurta, P. and van der Marel, R. and Geha, M. and Kalirai, J. and Yniguez, B. and Kirby, E. and Cuillandre, J.-C. and Gilbert, K. (2013) Internal Stellar Kinematics of M32 from the SPLASH Survey: Dark Halo Constraints. Astrophysical Journal, 765 (1). Art. No. 65. ISSN 0004-637X.

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As part of the SPLASH survey of the Andromeda (M31) system, we have obtained Keck/DEIMOS spectra of the compact elliptical (cE) satellite M32. This is the first resolved-star kinematical study of any cE galaxy. In contrast to most previous kinematical studies that extended out to r≾30" ~ 1 r^(eff) I ~ 100 pc, we measure the rotation curve and velocity dispersion profile out to r ~ 250" and higher order Gauss-Hermite moments out to r ~ 70". We achieve this by combining integrated-light spectroscopy at small radii (where crowding/blending are severe) with resolved stellar spectroscopy at larger radii, using spatial and kinematical information to account statistically for M31 contamination. The rotation curve and velocity dispersion profile extend well beyond the radius (r ~ 150") where the isophotes are distorted. Unlike NGC 205, another close dwarf companion of M31, M32's kinematics appear regular and symmetric and do not show obvious sharp gradients across the region of isophotal elongation and twists. We interpret M31's kinematics using three-integral axisymmetric dynamical equilibrium models constructed using Schwarzschild's orbit superposition technique. Models with a constant mass-to-light ratio can fit the data remarkably well. However, since such a model requires an increasing tangential anisotropy with radius, invoking the presence of an extended dark halo may be more plausible. Such an extended dark halo is definitely required to bind a half-dozen fast-moving stars observed at the largest radii, but these stars may not be an equilibrium component of M32.

Item Type:Article
Related URLs:
URLURL TypeDescription
Guhathakurta, P.0000-0001-8867-4234
van der Marel, R.0000-0001-7827-7825
Geha, M.0000-0002-7007-9725
Kalirai, J.0000-0001-9690-4159
Kirby, E.0000-0001-6196-5162
Gilbert, K.0000-0003-0394-8377
Additional Information:© 2013 American Astronomical Society. Received 2012 February 10; accepted 2012 November 19; published 2013 February 15. Data herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. K.M.H. was supported in part by the Lawrence Scholars Program at Lawrence Livermore National Laboratory (LLNL-JRNL-496754). E.K. and K.G. were supported through Hubble Fellowship grants 51256.01 and 51273.01, respectively, from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS 5-26555. P.G., K.M.H., and B.Y. acknowledge support from NSF grants AST-0607852 and AST-1010039. J.S.K.’s research is supported in part by a grant from the STScI Director’s Discretionary Research Fund. K.M.H. thanks STScI and Yale University for their hospitality during her visits to carry out some of this work. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. Facility: Keck:II (DEIMOS)
Funding AgencyGrant Number
Lawrence Livermore National LaboratoryLLNL-JRNL-496754
NASA Hubble Fellowship51256.01
NASA Hubble Fellowship51273.01
NASANAS 5-26555
Space Telescope Science InstituteUNSPECIFIED
Subject Keywords:galaxies: dwarf; galaxies: individual (M32, NGC 221); galaxies: kinematics and dynamics; Local Group; techniques: radial velocities; techniques: spectroscopic
Issue or Number:1
Record Number:CaltechAUTHORS:20130402-133929316
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Official Citation:Internal Stellar Kinematics of M32 from the SPLASH Survey: Dark Halo Constraints K. M. Howley et al. 2013 ApJ 765 65
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:37726
Deposited By: Ruth Sustaita
Deposited On:02 Apr 2013 21:04
Last Modified:06 Nov 2019 22:32

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