MMT/Hectospec spectra of ACO 1489

The caustic technique applied to the redshift survey gives the dynamical parameters M_200_=(1.25+/-0.09)x10^15^M_{sun}_, R_200_=1.97+/-0.05Mpc, and a cluster line-of-sight velocity dispersion of 1150+/-72km/s within R_200_. These parameters are very similar to those of other strong lensing systems with comparably large Einstein radii.

We use the spectroscopy and deep photometry to demonstrate that A1489 is probably dynamically active; its four brightest cluster galaxies have remarkably different rest-frame radial velocities. Like other massive strong lensing clusters, the velocity dispersion function for members of A1489 shows an excess for dispersions >~250km/s. The central dispersions also provide enhanced constraints on future lensing models.