Ten milliparsec-scale structure of the nucleus region in Centaurus A

We present the results of a VLBI Space Observatory Programme (VSOP) observation of the subparsec structure in Centaurus A at 4.9 GHz. Owing to its proximity, our Centaurus A space-VLBI image is one of the highest spatial resolution images of an AGN ever made -- 0.01 pc per beam. The elongated core region is resolved into several components over 10 milli-arcseconds long (0.2 pc) including a compact component of brightness temperature 2.2x10^10K. We analyze the jet geometry in terms of collimation. Assuming the strongest component to be the core, the jet opening angle at ~ 5,000 r_s (Schwarzchild radii) from the core is estimated to be ~ 12 degree, with collimation of the jet to ~ 3 degree continuing out to ~ 20,000 r_s. This result is consistent with previous studies of the jet in M87, which favor MHD disk outflow models. Future space VLBI observations at higher frequencies will probably be able to image the collimation region, within 1,000 r_s of the center of Centaurus A, together with the accretion disk itself.Comment: 12 pages, 6 figures, accepted for publication in PASJ, Vol.57 No.6, VSOP special issu

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

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.Comment: Accepted for publication in A&A. 11 pages, including 14 figures and 1 tabl