PNe and globular clusters (GCs) are both used as probes of galaxy halos. While PNe sample the field star population, and have emission lines that are more conducive to identification and radial velocity measurements, GCs are old star clusters for which we also can obtain metallicity and size information. Together, PNe and GCs are complementary tools for studying the structure and evolutionary histories of early-type galaxies.
At a distance of 3.5 Mpc (Hui et al. 1993; H93b), Centaurus A (NGC 5128) is the nearest massive elliptical galaxy, making it an excellent target for PNe and GC studies. Previous PNe surveys (H93b, Hui et al. 1995; H95) identified 785 PNe out to 20 Kpc along the major axis and 10 Kpc along the minor axis, and resulted in velocities for 433 PNe. We recently completed an extended survey that reaches projected radii of 80 and 40 Kpc along the major and minor axes, respectively. This brings the total number of PNe to 1140, of which 736 have velocities. In addition, we conducted a UBVRI survey with spectroscopic follow-up out to 50 and 30 Kpc to find new GCs and study the faint halo light.
In total, there are now 188 GCs with measured velocities in Cen A, 125 of which are newly discovered from our survey.
The smoothed PN velocity field shown in Figure 1 is the most detailed and extensive stellar velocity field in an elliptical galaxy to date. The rotation in the halo, discovered at 20 Kpc in previous work (H95), is now evident to the 80 Kpc limit of our survey. Most striking, however, is the previously undetected twist in the zero-velocity contour. Earlier work detected kinematic misalignments in Cen A and in other ellipticals (H95; Franx, Illingworth, & Heckman 1989), but this is some of the first compelling evidence for a twist in the kinematic axis. Misalignment and twisting of the kinematic axis are predicted for triaxial galaxies seen in projection (Statler 1991), and also are seen in merger simulations (e.g. Bendo & Barnes 2000). This program is an excellent advertisement for the potential of future PNe surveys.
Figure 1. Cen A PN and GC Smoothed Velocity Fields. The x/y-axes are the photometric major/minor axes. The dotted ellipses are 1-4 re isophotes for the old stellar light. Each point is a confirmed PN or GC. The mean velocity field at each object is the radial velocity average of all objects within r 3 Kpc. In sparse regions, the velocities are averaged with nearby neighbors. Velocities greater/less than systemic are represented by an "x"/box, with a point size proportional to the absolute deviation from systemic (up to ~ ± 150 km/s). We assume point symmetry (valid in a triaxial potential), and reflect each object through the origin while reversing the sign of its velocity wrt the galaxy. a) (top) The PN velocity field. Overplotted are the outlines of our survey (large boxes), and the zero velocity contour. Points appear to exist outside our survey area because of the assumed point symmetry, creating a 1472 particle field. The zero-velocity contour has a strong twist in the inner regions. b) (bottom left) The Red GC Velocity Field. There are 86 GCs redder than (V-I)0 = 0.98, the dip in the bimodal color distribution. These GCs are more centrally concentrated than the blue GCs, and rotate in a sense similar to the PNe. c) (bottom right) The Velocity Field for 99 Blue GCs. These are more extended than the red GCs, and have kinematics less similar to the PNe.
Globular cluster candidates were identified from UBVRI CCD imaging based on morphological and color criteria. Radial velocities confirm membership in the Cen A GC system (Vsystemic = 541 km s-1), and provide kinematic information. We constructed the velocity fields for the GCs using our observations of new and known GCs, combined with 29 velocities from Harris et al. (1992). The V-I color distribution of GCs is confirmed to be bimodal. Using the V-I color, a proxy for metallicity in old stellar systems, we can separate the GCs into blue (metal-poor) and red (metal-rich) populations. Figure 1 shows the spatial distribution and velocity fields of the two GC populations. The red GCs are more centrally concentrated, and more rotationally supported than the blue GCs. The kinematics of the PNe are more like those of the red GCs than the blue. This is consistent with the field stars in Cen A being predominantly metal-rich (Harris & Harris 2001), and with the red clusters and PNe having a common formation history.
This research has made use of NASA's Astrophysics Data System Abstract Service and the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. The authors wish to thank the CTIO staff for their excellent help during the observing runs on the CTIO 4-m telescope. This research was supported in part by NASA grant NAG5-7697 and NSF grant AST-0098566.