Red giants in the outer halo of the elliptical galaxy NGC 5128/Centaurus A

¹ Tuorla ObservatoryDepartment of Physics and Astronomy, University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
e-mail: sarah.bird@utu.fi
² FINCA – Finnish Centre for Astronomy with ESO, University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
³ Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn VIC, 3122, Australia
⁴ Department of Physics and Astronomy, McMaster University, Hamilton ON, L8S 4M1, Canada

Received: 14 January 2014
Accepted: 14 October 2014

Abstract

We used VIMOS on VLT to perform V and I band imaging of the outermost halo of NGC 5128/Centaurus A ((m − M)₀ = 27.91±0.08), 65 kpc from the galaxy’s center and along the major axis. The stellar population has been resolved to I₀ ≈ 27 with a 50% completeness limit of I₀ = 24.7, well below the tip of the red-giant branch (TRGB), which is seen at I₀ ≈ 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 ≈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 ≈8 kpc out to ≈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.