| Dynamics of the globular cluster system associated with M87 (NGC 4486). II. Analysis (2001) | |||||||||||||||
Abstract | |||||||||||||||
| Also archived in: arXiv:astro-ph/0106005 v1 31 May 2001. We present a dynamical analysis of the globular cluster system associated with M87 (= NGC 4486), the cD galaxy near the dynamical center of the Virgo cluster. The analysis utilizes a new spectroscopic and photometric database which is described in a companion paper (Hanes et al. 2001). Using a sample of 278 globular clusters with measured radial velocities and metallicities, and new surface density profiles based on wide-field Washington photometry, we study the dynamics of the M87 globular cluster system both globally — for the entire cluster sample — and separately — for the metal-rich and metal-poor globular cluster samples. This constitutes the largest sample of radial velocities for pure Population II tracers yet assembled for any galaxy. Our principal findings are ummarized as follows: (1) Surface density profiles constructed from our Washington photometry reveal the metal-poor cluster system to be more spatially extended than its metal-rich counterpart, consistent with earlier findings based on HST imaging in the central regions of the galaxy. Beyond a radius of R ≃ 1.5Re (10 kpc), the metal-poor component dominates the total globular cluster system. (2) When considered in their entirety, each of the combined, metal-poor and metal-rich globular cluster samples (278, 161 and 117 clusters, respectively) appear to rotate, with similar rotation amplitudes, about axes whose position angles are indistinguishable from that of the photometric minor axis, 0 = 65◦. (3) The one-dimensional rotation curve (i.e., binned in circular annuli) for the metal-rich cluster system has a roughly constant mean amplitude of R = 160+120 −99 km s−1. The metal-rich clusters appear to be rotating, at all radii, about the photometric minor axis of the galaxy. However, a smoothed, two-dimensional map of the line-of-sight velocity residuals suggests that the rotation field for the metal-rich clusters is non-cylindrical in nature. Instead, it exhibits a “double-lobed” pattern, with maxima at R ∼ 3.5-4Re (25-30 kpc) along the approximate photometric major axis of the galaxy. (4) The one-dimensional rotation curve of the metal-poor cluster system has mean amplitude of R = 172+51 −108 km s−1. The two-dimensional map of the rotation field for the metal-poor clusters shows some evidence for solid-body rotation or, alternatively, for a “shear” in the line-of-sight velocity. This shear is similar in size and orientation to that observed for Virgo galaxies within two degrees of M87, and is consistent with a scenario, previously suggested on the basis of dwarf galaxy kinematics and x-ray imaging, in which material is gradually infalling onto M87 along the so-called “principal axis” of the Virgo cluster. (5) Beyond a radius of R ≃ 2Re (15 kpc), the approximate onset of the galaxy’s cD envelope, the metalpoor globular cluster system rotates about the photometric minor axis — similar to its metal-rich counterpart. Inside this radius, however, the metal-poor clusters appear to rotate around the photometric major axis. (6) The complete sample of 278 globular clusters has an almost perfectly isotropic velocity ellipsoid, with cl = 1 − 2 / 2 r ≃ 0. (7) When considered separately, the metal-poor cluster system shows a modest but significant tan- gential bias of cl ≃ −0.4, while the velocity ellipsoid of the metal-rich cluster system is radially biased, with cl ≃ +0.4. Taken together, these results demonstrate that the dual nature of the M87 globular cluster system — first identified on the basis of its bimodal metallicity distribution — also extends to its dynamical properties. We discuss the implications of these findings for the various formation scenarios proposed for giant elliptical galaxies and their globular cluster systems (Refer to PDF file for exact formulas).. PC gratefully acknowledges support provided by the Sherman M. Fairchild Foundation during the course of this work. DEM acknowledges support from NASA through grant number HF-1097.01-97A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA under contract NAS5-26555. DG acknowledges financial support for this project received from CONICYT through Fondecyt grant 8000002, and by the Universidad de Concepcion through research grant No. 99.011.025-1.0. The research of DAH and GLHH is supported through grants from the Natural Sciences and Engineering Research Council of Canada. | |||||||||||||||
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