When most people look at NGC 5128 they see a peculiar galaxy dominated by a prominent dust lane and ignore the rest. What I see is a nearby E galaxy whose oldest stars can tell us a great deal about its history. A quick look at the literature over the past 60 years shows a growing body of research mostly on the radio jets, dust lane, Xray sources and much less on the dominant baryonic component which is the E galaxy beneath. But, particularly in the past twenty years, we are now beginning to understand the stellar history of NGC 5128 as well.
2.1. Galaxy Type
Because NGC 5128 is associated with a strong radio source and its physical appearance in the center is dominated by a wide absorption band, this galaxy continues to be perceived as one whose fundamental properties are likely to be atypical. But as early as the mid-1950s studies indicated that, aside from the dust lane, the global, structural properties of NGC 5128 were consistent with those of an E galaxy. Baade & Minkowski (1954) called it "an unresolved E0 nebula with an unusually strong and wide central absorption band" and concluded that "the two nebulae...form a close pair in a state of strong gravitational interaction, perhaps actually in collision". Based on data from photographic plates in three wavelength bands, Sérsic (1958) found isophotal countours which were remarkably similar in all colours and fitted a deVaucouleurs profile out to 10' (~ 10 kpc) from the centre of the galaxy.
In their review Ebneter & Balick (1983) argue that "Cen A has a probably undeserved reputation for being one of the most peculiar galaxies in the sky" and point out that many of its characteristics would not be as visible or obvious if the system were more distant. And in his study of both the halo light distribution and the dynamics of the dust lane region, Graham (1979) concludes the "the main body of NGC 5128 resembles in many respects a normal giant elliptical galaxy" and "the main radio characteristics of NGC 5128 and the energetic phenomena...are a consequence rather than the cause of the unusual structural features of the galaxy as a whole".
In spite of this, there is still no clear consensus on what type to use for NGC 5128 (cf. Table 1); the "vote" is clearly split between S0p and E0p, presumably because of the dust lane. Morgan (1958) suggested that the S0 classification is applied to galaxies which are often very different from each other and van den Bergh (1990) agrees that "the S0 classification type comprises a number of physically quite distinct types of objects that exhibit only superficial morphological similarities". More recently he indicates that "NGC 5128 is an object that does not find a natural home in the Hubble classification scheme" (van den Bergh 2009, private communication). In this discussion I will consider NGC 5128 an Ep.
|like NGC 3379 (E1)||Sérsic (1958)|
|(E0 + Sb?)||Sandage (1961)|
|E0p||van den Bergh (1976)|
|E + spiral||Dufour et al. (1979)|
|S0||Sandage & Tammann (1981)|
|S0p||NED database (2009)|
It may seem a bit odd that several decades passed before we could determine a reliable distance to a galaxy not far beyond the boundaries of the Local Group. But NGC 5128 was (until recently) too distant to apply most stellar candles, and is too close to use the Hubble Law with confidence or to observe as we do other large E galaxies. But in the past two decades we have been able to derive a distance based on the properties of its stellar component.
As can be seen in Figure 1, distance estimates before ~ 1980 ranged from 2 to 9 Mpc, a factor of > 4! And this apparent dichotomy between large and small distances often led to the use of an "average" value of ~ 5 Mpc. But since the late 1980s it has been possible to use a variety of stellar distance indicators: luminosity of the red giant branch tip (TRGB), planetary luminosity function (PNLF), long period variables (LPV), surface brightness fluctuations (SBF), and Cepheid variables. The quality of the data and the calibration of these methods now support a distance of 3.8 ± 0.1 Mpc for NGC 5128, and a full review is given in this volume (Harris et al. 2010).
Figure 1. Distance to NGC 5128 vs. year of publication: H II regions (magenta stars), redshift (open black circles), globular clusters (red squares), SN 1986g (blue chevron), TRGB (red circles), SBF (cyan circles), PNLF (green stars), LPV (filled blue triangle), and Cepheid variables (empty blue triangle). Error bars are plotted when available. For sources see Israel (1998), Rejkuba (2004), Harris et al. (2010) and references given therein. The horizontal line is the current best value of d = 3.8 ± 0.1 (Harris et al. 2010).