Planetary nebulae in the elliptical galaxy NGC4649 (M 60): Kinematics and distance redetermination

Abstract

Using a slitless spectroscopy method with (1) the 8.2m Subaru telescope and its FOCAS Cassegrain spectrograph and (2) the ESO Very Large Telescope unit 1 (Antu) and its FORS2 Cassegrain spectrograph, we have detected 326 planetary nebulae (PNs) in the giant Virgo elliptical galaxy NGC4649 (M 60) and measured their radial velocities. After rejecting some PNs more likely to belong to the companion galaxy NGC4647, we have built a catalog with kinematic information for 298PNs in M60. Using these radial velocities, we have concluded that they support the presence of a dark matter halo around M60. Based on an isotropic, two-component Hernquist model, we estimate the dark matter halo mass within 3Re to be 4 × 1011 M⊙, which is almost one-half of the total mass of about 1012 M⊙ within 3Re. This total mass is similar to that estimated from globular cluster, XMM-Newton, and Chandra observations. The dark matter becomes dominant outside. More detailed dynamical modeling of the PN data is being published in a companion paper. We have also measured the m(5007) magnitudes of many of these PNs and built a statistically complete sample of 218PNs. The resulting PN luminosity function (PNLF) was used to estimate a distance modulus of 30.7 ± 0.2mag, equivalent to 14 1Mpc. This confirms an earlier PNLF distance measurement based on a much smaller sample. The PNLF distance modulus remains smaller than the surface brightness fluctuation distance modulus by 0.4mag. © 2011. The American Astronomical Society. All rights reserved.