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3.1. Overview

With V = 6.98 mag (de Vaucouleurs et al., 1976 - RC2), NGC 5128 is the fifth brightest galaxy in the sky, immediately after the Local Group members M 31, M 33, LMC and SMC. Images of relatively short exposure, limited to a surface brightness in B of about 22 mag per arcsec2, show an almost circular appearance, which has led to the classification S0p or E0p (Fig. 1). However, at lower surface-brightness levels (Fig. 8), the shape of the galaxy becomes increasingly noncircular. At about 25 mag per arcsec2, the axial ratio has increased to 1.3, after which elliptical symmetry is lost (cf. Haynes et al. 1983). A more appropriate classification appears to be E2 (Dufour et al. 1979; McElroy & Humphreys 1982; Haynes et al. 1983; Ebneter & Balick 1983), with the photometric major axis of the galaxy at position angle 35°. The light distribution closely follows the r1/4 de Vaucouleurs law (van den Bergh 1976) characteristic of elliptical galaxies. The inner parts of NGC 5128 have a roughly constant total mass-to-light ratio inccreasing with radius, probably due to the presence of dark matter (Sect. 6.2).

Of particular interest are the deep and specially processed optical images presented by Malin et al. (1983 - see Fig. 6) and Haynes et al. (1983 - see Fig. 8). The former reveals an extensive system of shells of old disk stars within the extended elliptical galaxy. The system is most regular on the northeastern side, and more fragmented on the southwestern side. The outermost shell is found at 18' from the nucleus. Atomic neutral hydrogen emission with a total mass of 1.5 × 108 M$\scriptstyle \odot$ is present just outside several of the outer shells (Schiminovich et al. 1994), with a rotation axis at position angle 285°, somewhat offset from the position angle of the minor axis of the E2 galaxy and the dust band (Fig. 7).

Figure 6

FIgure 6. Deep negative B-band image of NGC 5128 shows the system of optical shells at the edges of the galaxy. This system of shells is one of the clearest indications that NGC 5128 has undergone mergers in its past. Also note optical jet features at a position angle of about 45°, inside and outside the northeastern shells respectively. Courtesy D. Malin, Ango-Australian Observatory)

Figure 7

Figure 7. Contours of total HI superposed on a schematic image of the shells of NGC 5128. Contours reach from 1 to 40 × 1020 cm-2. Thick lines mark the position of sharp (solid) and diffuse (dashed) shells. Shaded regions represent dust patches. Thin contours marked An and As represent the radio continuum emission from the inner lobes and jets discussed in Sects. 2.3 and 2.4. Most of the neutral hydrogen is associated with the warped disk discussed in Sect. 4, but some of it is associated with the shells (Sect. 3.1). From Schiminovich et al. 1994; image courtesy J.M. van der Hulst.

The image presented by Haynes et al (1983; see also Cannon 1981) shows diffuse extensions emanating from the elliptical galaxy at position angle $ \approx$ 30° almost perpendicular to the major axis of the dusty disk in the centre (Fig. 8). The northeastern extension is narrower, longer and better-defined than the one in the southwest. It is just west from the northern middle radio lobe. The faint extensions most likely consist of stars, or possibly of dust reflecting light from stars further in (Cannon 1981; Haynes et al. 1983).

Figure 8

Figure 8. Very deep, amplified positive B-band image of NGC 5128 shows the elongated structure of the galaxy, as well as faint emission extending roughly along the direction of the inner radio jets. The "traditional" negative image of NGC 5128 has been superposed for comparison. The dark band is clearly inside the galaxy. From Haynes et al. 1983; image courtesy D. Malin, Ango-Australian Observatory).

The appearance of NGC 5128 departs strikingly from that of a normal elliptical galaxy because of its broad and patchy equatorial dark band. Bisecting the almost circular bright central part of the galaxy, it is oriented along the minor axis of the elongated shape seen in deep images (Fig. 8). Consequently, NGC 5128 is sometimes also classified as a polar ring galaxy although it fails to share several of the characteristics common to these (see Richter et al. 1994). The dark band is associated with young stellar objects (Dufour et al. 1979 - see also Fig. 6). Modelling has shown that it is in fact a thin, strongly warped disk embedded in the host galaxy that creates the superficial appearance of a broad band (Sects. 4 and 6). Kinematically, the galaxy and the dark band represent different entities. The elliptical system and its globular cluster retinue have low rotational velocities, whereas the dust disk exhibits much higher rotational velocities (Sect. 6.1).

3.2. Globular clusters and metallicity

The galactic foreground confusion caused by the low galactic latitude (b = +19°) of NGC 5128 for a long time impeded attempts to identify its globular clusters, notwithstanding its proximity. However, after the first identifications were finally made by Graham & Phillips (1980) and van den Bergh, Hesser & Harris (1981), the number of confirmed globular clusters rose steadily from 20 (Hesser et al. 1984) to 35 (Hesser, Harris & Harris 1996) to 87 (Harris et al. 1992) while Minitti et al. (1996) added another 26 globular clusters in the inner 3 kpc of the galaxy. Analyzing image counts, Harris et al. (1984a) have estimated a total cluster population of 1550 ± 350. Globular cluster studies have been used primarily to extract information on the galaxy's metallicity and dynamics, as well as its distance (Sect. 1.2). In terms of both numbers and metallicities, the globular cluster system of NGC 5128 appears to be normal for large elliptical galaxies (Harris et al., 1984b, 1992) or indeed for spheroidal components of galaxies in general (Jablonka et al. 1996).

At least at radii beyond 4', the surface density of globular clusters appears to follow the same r1/4 law as found by van den Bergh (1976) for the spheroidal halo light. In addition, the distribution of globular clusters on the sky hints at a preferential orientation, with a major axis aligned with the major axis of the outer isophotes of NGC 5128 (Hesser et al. 1984). Although the first determinations of very high metallicity by Frogel (1984) turned out to be overestimates, the clusters in NGC 5128 do seem to have somewhat greater metallicities (-0.6 leq [Fe/H] leq +0.1) than their Milky Way counterparts which they otherwise resemble (Harris et al. 1992; Jablonka et al. 1996; Minniti et al. 1996; Alonso & Minniti 1996). Rather similar metallicities were derived for red giant branch stars by Soria et al. (1996). The innermost clusters are on average more metal-rich, implying the presence of a metallicity gradient $ \Delta$[Fe/H]/$ \Delta$R = -0.08 dex kpc-1; this gradient is only apparent, and in fact caused by different concentrations of metal-poor and metal-rich clusters (Alonso & Minniti 1996, but see Jablonka et al. 1996).

Half a dozen of the innermost clusters have magnitudes and colours (but undetermined metallicities) suggesting that they are intermediate-age clusters such as found in the Magellanic Clouds (Minniti et al. 1996; Alonso & Minniti 1996). Hui et al. (1995) showed that the metal-poor globular cluster ensemble lacks significant rotation whereas the metal-rich ensemble rotates more rapidly. A puzzling, but still uncertain result was obtained by Hesser et al. (1986). They found that the metal-rich innermost cluster ensemble has a mean velocity about a 100 km s-1 (4$ \sigma$) higher than the systemic velocity of NGC 5128, in contrast to the outermost clusters that conform to the systemic velocity.

The bimodal globular cluster population of NGC 5128 is of great interest in view of the proposed merger-nature of the galaxy, particularly as a bimodal metallicity distribution with metal-rich clusters more concentrated than metal-poor clusters has been identified by Zepf & Ashman (1993) as the natural consequence of galaxy mergers.

3.3. Late-type objects

A survey of [OIII] $ \lambda$5007 emission from planetary nebulae in NGC 5128 by Hui et al. (1993a), extending over 20 kpc along the major axis and fully covering the central 10 kpc, yielded 785 detections (Hui et al. 1993b). The high-luminosity cut-off of the resulting PN luminosity function was used to derive the galaxy distance in Sect. 1.2. Hui et al. (1995) measured radial velocities for 433 PN's, which were used to study the kinematics and dynamics of the galaxy (Sect. 6.2).

A V-I colour-magnitude diagram of 10000 red giant branch (RGB) stars in the halo of NGC 5128 was constructed by Soria et al. (1996) from HST/WFPC2 images, the first time that individual stars were resolved in a spheroid system beyond the Local Group. Analysis of the results yielded the distance and metallicity values already mentioned (Sects. 1.2 and 3.2). The I-luminosity function of these stars is very similar to that of the RGB stars in the M 31 dwarf elliptical companion NGC 185. About 200 stars were found to be brighter than the tip of the RGB; most of them are probably upper asymptotic giant branch (AGB) stars although confusion with unresolved multiple stars is still a problem. The luminosity functions of these AGB stars suggest the presence of an intermediate-age population of about 5 gigayear, but making up at most 10 % of the total halo stellar population.

Over the five-year period 1985-1989, Ciardullo et al. (1990) monitored NGC 5128 for the appearance of novae. They detected 16 novae, twelve of which formed a statistically complete and homogeneous sample unaffected by the dust lane. Normalized to the near-infrared K-band luminosity, the derived nova rate of 4.2 yr-1 per 1010 LK is virtually identical to that derived for the bulge of M 31. They further argued that the normalized nova-rate of galaxies is largely independent of their luminosity, colour or Hubble type.

The galaxy was the host of the type Ia supernova 1986g (Evans 1986), which reached a maximum at B = 12.45 ± 0.05 (Cristiani et al. 1992). With AB = 4.4 mag and the currently best value for the distance modulus, this translates into an absolute magnitude MB = -19.6 ± 0.5. However, the reddening of SN 1986g, mostly caused by dust internal to NGC 5128, is variously given as E(B - V) = 0.9 (Phillips et al. 1987), 1.1 (Cristiani et at. 1992), 0.6 (Phillips 1993) and 1.6 (Hough et al. 1987). The latter also argued from their polarization measurements that near SN 1986g the ratio of total to selective extinction R is 2.4, rather than 3.1. The resulting AV values thus range from 1.9 to 3.8 mag, so that the extinction correction remains a major source of uncertainty. This could be resolved by assuming an SNIa mean peak absolute magnitude. Unfortunately, estimated values range from MB = -19.75 (Cristiani et al. 1992), -19.4 (Branch et al. 1996) to -18.2 (Phillips 1993), the major uncertainty this time being galaxy distance moduli. A further discussion of the use of SNIa's as standard candles is outside the scope of this review; for recent contributions see e.g. Riess, Press & Kirshner (1996) and Branch, Romanishin & Baron (1996).

3.4. Colours and reddening

The integrated colour of NGC 5128 as given in the RC2 is (B - V)T = +0.98. This is identical to the colour van den Bergh (1976) finds at distances of 150" to the dark band centerline, where also (U - B) = +0.49. The foreground reddening is E(B - V) = 0.11 ± 0.02 (Newell et al. 1969; Frogel 1984; Harris et al. 1992), so that the intrinsic colours become (B - V)0 = 0.88 and (U - B)0 = 0.41, bluer by 0.08 mag resp. 0.15 mag than normal for elliptical galaxies (van den Bergh 1976). The photometry by van den Bergh shows increasing blueness approaching the dark band; at its edge, 40" from the centerline, the colours abruptly spread over a great range +0.4 leq (B - V) leq +1.2 and -0.6 leq (U - B) leq +0.5, whereas in the dark band only red colours are found (+1.0 leq (B - V) leq +1.6; +0.3 leq (U - B) leq +0.9). The excess blue emission has been explained as due to optical synchrotron emission from the inner lobes (Dufour et al. 1979), a metal-poor population, or a young population of hot, blue stars. The first explanation is unlikely (Ebneter & Balick 1983), as is the second in view of the metallicity results given in Sect. 3.2, particularly those by Soria et al. (1996). The last explanation is supported by the presence of a substantial number of HII regions in the dust band itself (Dufour et al. 1979; Hodge & Kennicutt 1983), and even more so by colour images of the galaxy that show substantial very blue stellar populations at the northwestern and southeastern edges of the dust band (cf. Fig. 1).

A much less sharply delineated and less extreme increase of blue colours occurs farther out, in the inner halo, and probably reflects the increasing presence of metal-poor objects (van den Bergh 1976).

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