The two brightest ellipticals in the Virgo Cluster, M87 (=NGC 4486) and M49 (=NGC 4472), had their extensive GC systems studied kinematically (e.g., Côté et al. 2001, 2003). Both these and X-ray studies (Schindler et al. 1999; Matsushita et al. 2002) found massive dark halos. Yet no stellar nor PN kinematical studies have been done in these galaxies' halos for comparison.
We have pursued kinematic follow-up studies of the large PN samples discovered around these two galaxies with narrow-band imaging surveys (Ciardullo et al. 1998; X. Hui et al., unpublished). PN kinematic studies have till now been sporadically fruitful because of the low efficiency of spectroscopic follow-up, which seems to be largely due to astrometric difficulties. We used the WYFFOS/AF2 multi-fiber spectrograph on the 4.2-m William Herschel Telescope (WHT) on La Palma in May 2000 to observe PNe in M49, giving special attention to the astrometric solution. Even with the 2.7" diameter fibers, it appeared that our recovery rate was not optimal, and we found velocities for ∼ 25 objects.
As a solution to these difficulties, we devised a new technique, masked counter-dispersed imaging, which is a combination of traditional multi-slit spectroscopy and slitless counter-dispersed imaging (see next section). Using the FORS2+MXU spectrograph at the ESO 8-m UT2 telescope in April 2001, we cut standard masks but with the slit sizes increased to 4" × 4", ensuring object recovery. We took two complementary images of each field with the dispersion direction reversed, allowing us to cancel out the astrometric errors and derive the velocities to within ∼ 15 km s−1. Completing this program for M87, we found ∼ 200 PN velocities; for M49, clouds limited our return to ∼ 100 PNe.