The distance to NGC 5128 , the central galaxy of the Centaurus group and the nearest giant elliptical to us , has been determined using two independent distance indicators : the Mira period-luminosity ( PL ) relation and the luminosity of the tip of the red giant branch ( RGB ) .
The data were taken at two different locations in the halo of NGC 5128 with the ISAAC near-IR array on ESO VLT .
From more than 20 hours of observations with ISAAC a very deep K _ { s } -band luminosity function was constructed .
The tip of the RGB is detected at \mathrm { K _ { s } } = 21.24 \pm 0.05 mag .
Using an empirical calibration of the K -band RGB tip magnitude , and assuming a mean metallicity of [ \mathrm { M } / \mathrm { H } ] = -0.4 dex and reddening of \mathrm { E } ( B - V ) = 0.11 , a distance modulus of NGC 5128 of ( \mathrm { m } - \mathrm { M } ) _ { 0 } = 27.87 \pm 0.16 was derived .
The comparison of the H -band RGB tip magnitude in NGC 5128 and the Galactic Bulge implies a distance modulus of NGC 5128 of ( \mathrm { m } - \mathrm { M } ) _ { 0 } = 27.9 \pm 0.2 in good agreement with the K -band RGB tip measurement .
The inner halo field has larger photometric errors , brighter completeness limits and a larger number of blends .
Thus the RGB tip feature is not as sharp as in the outer halo field .
The population of stars above the tip of the RGB amounts to 2176 stars in the outer halo field ( Field 1 ) and 6072 stars in the inner halo field ( Field 2 ) .
The large majority of these sources belong to the asymptotic giant branch ( AGB ) population in NGC 5128 with numerous long period variables .
Mira variables were used to determine the distance of NGC 5128 from a period-luminosity relation calibrated using the Hipparcos parallaxes and LMC Mira period-luminosity relation in the K -band .
This is the first Mira period-luminosity relation outside the Local Group .
A distance modulus of 27.96 \pm 0.11 was derived , adopting the LMC distance modulus of 18.50 \pm 0.04 .
The mean of the two methods yields a distance modulus to NGC 5128 of 27.92 \pm 0.19 corresponding to D = 3.84 \pm 0.35 Mpc .