Abstract
From observations with the GMOS multislit spectrograph on the Gemini North telescope, we have obtained spectra for 39 globular cluster (GC) candidates in the Virgo giant elliptical galaxy NGC 4649 (M60), of which 38 are confirmed GCs. The clusters extend out to a radius of 260 arcsec (3.5 effective radii). We find no rotation of the GC system, with an upper limit of v/σ < 0.6 at a confidence level of 95 per cent. The GC velocity dispersion is constant with radius, within the uncertainties. We fit isotropic models to the GC and stellar kinematics; these models yield an (M/L)V around 16 at 200 arcsec radius (16 kpc), an increase of a factor of 2 from the central M/L. We also use the mass profile as derived from X-rays to determine the orbital structure. Using axisymmetric orbit-based models and the X-ray mass profile, we find the orbital distribution is close to isotropic within 100 arcsec, and becomes tangentially biased beyond. Furthermore, when using the X-ray profile, we find a better fit to the kinematics compared to using a constant M/L model. Thus, both isotropic and axisymmetric orbit-based models give support for the presence of a dark matter halo in NGC 4649.
Highlights
Globular clusters (GCs hereafter) are an excellent way to study the formation, evolution and dark matter (DM) content of galax ies of all types (e.g. Ashman & Zepf 1998; Harris 2001)
Integrated stellar spectroscopy has the ad vantage of large sample sizes and high signal-to-noise ratio (S/N), which allow the derivation of higher order moments of the velocity distribution, but the faintness of the stellar halo restricts measure
This paper presents spectra for 38 confirmed NGC 4649 globular cluster (GC), and discusses the GC kinematics and DM content of NGC 4649 (Section 3.2)
Summary
Globular clusters (GCs hereafter) are an excellent way to study the formation, evolution and dark matter (DM) content of galax ies of all types (e.g. Ashman & Zepf 1998; Harris 2001). Globular clusters (GCs hereafter) are an excellent way to study the formation, evolution and dark matter (DM) content of galax ies of all types Ashman & Zepf 1998; Harris 2001) They are well suited for dynamical studies of early-type galax ies, since they are found in substantial numbers out to large radii around such galaxies and are bright enough to be seen at large dis tances. X-ray data can probe further out, and recent high-quality data from ROSAT, ASCA and Chandra have shown that extended dark haloes are a common feature of all lumi nous ellipticals (Mathews & Brighenti 2003; Humphrey et al 2006). Only massive ellipticals have hot diffuse gas haloes for mass estimates
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