We used VIMOS on VLT to perform V and I band imaging of the outermost halo of \object NGC 5128 / \object Centaurus A ( ( m - M ) _ { 0 } = 27.91 \pm 0.08 ) , 65 kpc from the galaxy ’ s center and along the major axis .
The stellar population has been resolved to I _ { 0 } \approx 27 with a 50 % completeness limit of I _ { 0 } = 24.7 , well below the tip of the red-giant branch ( TRGB ) , which is seen at I _ { 0 } \approx 23.9 .
The surface density of NGC 5128 halo stars in our fields was sufficiently low that dim , unresolved background galaxies were a major contaminant in the source counts .
We isolated a clean sample of red-giant-branch ( RGB ) stars extending to \approx 0.8 mag below the TRGB through conservative magnitude and color cuts , to remove the ( predominantly blue ) unresolved background galaxies .
We derived stellar metallicities from colors of the stars via isochrones and measured the density falloff of the halo as a function of metallicity by combining our observations with HST imaging taken of NGC 5128 halo fields closer to the galaxy center .
We found both metal-rich and metal-poor stellar populations and found that the falloff of the two follows the same de Vaucouleurs ’ law profiles from \approx 8 kpc out to \approx 70 kpc .
The metallicity distribution function ( MDF ) and the density falloff agree with the results of two recent studies of similar outermost halo fields in NGC 5128 .
We found no evidence of a “ transition ” in the radial profile of the halo , in which the metal-rich halo density would drop rapidly , leaving the underlying metal-poor halo to dominate by default out to greater radial extent , as has been seen in the outer halo of two other large galaxies .
If NGC 5128 has such a transition , it must lie at larger galactocentric distances .