Centaurus A = NGC 5128
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. A
colour composite image of Centaurus A,
reveals the lobes and jets visible in 870-micron submillimetre data from LABOCA on APEX (orange)
and X-ray data from the Chandra X-ray Observatory (blue) over the visible light from the
WFI on the MPG/ESO 2.2m telescope at La Silla, Chile, that shows the stars and the galaxy's dust
lane in close to "true colour"
(credit: eso0903a ESO/WFI(Optical); MPIfR/ESO/APEX/A.Weiss et al.
(Submillimetre); NASA/CXC/CfA/R. Kraft et al. (X-ray)).
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. During my PhD I studied the resolved stellar populations, both old and young stars in NGC 5128 halo, as well as its globular cluster population. The multi-epoch near-IR monitoring of two fields with ISAAC instrument on the ESO VLT uncovered more than 1500 Mira variable stars in NGC 5128, which was then the subject of an ESO press release. These Mira variables can be used to measure distance to Centaurus A/NGC 5128 through their period-luminosity relation. In addition to Mira variables many other methods have been used to derive distance to this galaxy, and the average distance using the most reliable methods yields 3.8 Mpc (Harris, Rejkuba & Harris, 2010).
At the first conference dedicated to CenA/NGC 5128, organized in Sydney, Australia, in 2009, I gave a talk on the stellar halo in CenA/NGC 5128. In July 2014 Hubble/ESA Science Release Hubble traces the halo of a galaxy more accurately than ever before - An in-depth look at the giant elliptical galaxy Centaurus A was coordinated with the publication of the Astrophysical Journal article that revealed an extended and elongated stellar halo around the galaxy. For a more comprehensive list of press-releases and images of Centaurus A see the nice web page maintained by Helmut Steinle (MPE). Here I summarize some of my latest results.
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.
Tracing the Outer Halo in a Giant Elliptical to 25 Reff
Hubble Space Telescope Cycle 20 observations with the Advanced Camera for Surveys and Wide Field Camera 3 covered 5 remote locations in the halo of NGC 5128 out to a projected distance of 140 kpc (corresponding to 25 effective radii Reff) along the major axis and 90 kpc (16 Reff) along the minor axis. This dataset, obtained in 2013, provided an unprecedented radial coverage of the stellar halo properties in any gE galaxy, and showed that the red giant branch stars belonging to the stellar halo of NGC 5128 are present even in the most distant fields probed. The star counts demonstrated an increasing flattening of the outer halo, which is elongated along the major axis.