Red giants in the outer halo of the elliptical galaxy NGC 5128/Centaurus A
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abstract
We used VIMOS on VLT to perform $V$ and $I$ band imaging of the outermost
halo of NGC 5128 / Centaurus A ($(m-M)_0=27.91\pm0.08$), 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.
