The halos of elliptical galaxies are faint and difficult to explore , but they contain vital clues to both structure and formation .
We present the results of an imaging and spectroscopic survey for planetary nebulae ( PNe ) in the nearby elliptical NGC 5128 .
We extend the work of Hui et al .
( 1995 ) well into the halo of the galaxy—out to distances of 100 and 50 kpc along the major and minor axes .
We now know of 1141 PNe in NGC 5128 , 780 of which are confirmed .
Of these 780 PNe , 349 are new from this survey , and 148 are at radii beyond 20 kpc .
PNe exist at distances up to 80 kpc ( \sim 15 r _ { e } ) , showing that the stellar halo extends to the limit of our data .
This study represents by far the largest kinematic study of an elliptical galaxy to date , both in the number of velocity tracers and in radial extent .
We confirm the large rotation of the PNe along the major axis , and show that it extends in a disk-like feature into the halo .
The rotation curve of the stars flattens at \sim 100 km s ^ { -1 } with V / \sigma between 1 and 1.5 , and with the velocity dispersion of the PNe falling gradually at larger radii .
The two-dimensional velocity field exhibits a zero-velocity contour with a pronounced twist , showing that the galaxy potential is likely triaxial in shape , tending toward prolate .
The total dynamical mass of the galaxy within 80 kpc is \sim 5 \times 10 ^ { 11 } M _ { \sun } , with M / L _ { B } \sim 13 .
This mass-to-light ratio is much lower than what is typically expected for elliptical galaxies .