Centaurus A is the largest radio source in the southern hemisphere. Its optical counterpart is known as NGC 5128, the nearest giant elliptical galaxy to our Milky Way. As it is often the case, the nearest object of a given kind is often a peculiar case. NGC 5128 has had a perturbed life. It has definitely undergone at least one recent merger event, which left a prominent dust lane in the center of the galaxy, but also some neutral cool gas orbiting in the halo at larger distances from the galaxy center.   In the inner regions of the galaxy there is an active galactic nucleus (AGN) with a supermassive black hole in its center. The AGN produces a powerful x-ray and radio jet which extends in particular in the north east halo. The counter-jet in the south western part is much weaker. 

However, in spite of all these peculiarities, which clearly attracted lots of attention but which make up a small fraction of the total mass of the galaxy, NGC 5128 is the closest and brightest example of a large elliptical galaxy. It is not so anomalous, given that we know that many elliptical galaxies (if not all) have a central black hole, and often host an AGN or show evidences of modest amounts of recent star formation (frosting) and dust.  Therefore studying this galaxy in detail can provide insight on the star formation epoch, assembly and evolution history of a quite typical giant elliptical galaxy.

This page summarizes some of the most important results from stellar populations studies in NGC 5128/Centaurus A that I was involved in. For a more comprehensive overview of Centaurus A literature and images see the nice web page maintained by Helmut Steinle (MPE).

Deep ACS Imaging of the Halo of NGC 5128: Reaching the Horizontal Branch

In 2005, in collaboration with Laura Greggio, Bill and Gretchen Harris and Eric Peng, I published the deepest color-magnitude diagram of NGC 5128 halo field observed with the ACS camera on board of HST. The photometric catalogue used in this work is available for download. It is not extinction corrected. Extinction coefficients used in the paper were E(B-V)=0.11, Av=3.1*E(B-V), Ai=0.479*Av. The catalogue contains x, y pixel coordinates based on the deep combined drizzled image containing all V and I band exposures.

The color-magnitude diagram displays a very wide red giant branch (RGB), an asymptotic giant branch (AGB) bump, and the red clump, dominated by an old population with a range of metallicities extending to above Solar metallicity. The metallicity distribution derived from the upper RGB has an average [M/H]=-0.64 and dispersion 0.49 dex. No noticeable population of blue horizontal branch stars is present, confirming previous suggestions that old, very metal-poor population is not ubiquitous in the halo of this galaxy. Combining the metallicity-sensitive colors of the RGB stars with the metallicity- and age-sensitive features of the AGB bump and the red clump, we infer the average age of the halo stars to be ~8 Gyr.

A more in-depth analysis of the age distribution of the halo stars in NGC 5128 has been published in the follow-up paper:

How old are the stars in the halo of NGC 5128 (Centaurus A)?

In this paper we simulated large number of synthetic color-magnitude diagrams assuming different star formation histories and compared them with the observations obtained with the HST. The main results we found are that the observed colour-magnitude diagram can be reproduced satisfactorily only by simulations that have the bulk of the stars with ages in excess of ~10 Gyr, and that the alpha-enhanced models fit the data much better than the solar scaled ones. Data are not consistent with extended star formation over more than 3-4 Gyr. Two burst models, with 70-80% of the stars formed 12±1 Gyr ago and with 20-30% younger contribution with 2-4 Gyr old stars provide the best agreement with the data. The old component spans the whole metallicity range of the models (Z = 0.0001-0.04), while for the young component the best fitting models indicate higher minimum metallicity (~ 1/10-1/4 Z_sun).
This allows us to conclude that the bulk of the halo stars in NGC 5128 must have formed at redshift z~2 or higher and that the chemical enrichment was very fast, reaching solar or even twice-solar metallicity already for the ~11-12 Gyr old population. The minor young component, adding ~20-30% of the stars to the halo, and contributing less than 10% of the mass, may have resulted from a later star formation event ~2-4 Gyr ago.