MAPPING THE DARK SIDE WITH DEIMOS: GLOBULAR CLUSTERS, X-RAY GAS, AND DARK MATTER IN THE NGC 1407 GROUP

Abstract

NGC 1407 is the central elliptical in a nearby evolved galaxy group apparently destined to become a cluster core. We use the kinematics of globular clusters to probe the dynamics and mass profile of the group's center, out to 60 kpc (~10 R_eff) -- the most extended data set to date around an early-type galaxy. This sample consists of 172 GC velocities, most of them newly obtained using Keck/DEIMOS, with a few additional objects identified as DGTOs or as IGCs. We find weak rotation in the GC system's outer parts, with the metal-poor and metal-rich GCs misaligned. The RMS velocity profile declines rapidly to a radius of ~20 kpc, and then becomes flat or rising to ~60 kpc. There is evidence that the GC orbits have a tangential bias that is strongest for the metal-poor GCs -- possibly contradicting theoretical expectations. We construct cosmologically-motivated galaxy+dark halo dynamical models and infer a mass within 60 kpc of ~3x10^12 M_Sun, extrapolating to a virial mass of ~6x10^13 M_Sun for a typical LCDM halo -- in agreement with results from group galaxy kinematics. We present an independent Chandra-based analysis, whose relatively high mass at ~20 kpc disagrees strongly with the GC-based result unless the GCs are assumed to have a peculiar orbit distribution, and we discuss some general comparisons between X-ray and optical results. The group's B-band mass-to-light ratio of ~800 is extreme even for a rich galaxy cluster, much less a poor group -- placing it among the most dark matter dominated systems in the universe, and also suggesting a massive reservoir of baryons lurking in an unseen phase, in addition to the nonbaryonic dark matter. We compare the kinematical and mass properties of the NGC 1407 group to other nearby groups and clusters, and discuss some implications of this system for structure formation.