The Fornax cluster is comparable in distance to the Virgo cluster
(de Vaucouleurs 1975),
but it is found almost opposite to Virgo in the
skies of the southern hemisphere. The Fornax cluster is less rich in
galaxies than Virgo
(Ferguson & Sandage 1988),
but it is also
substantially more compact than its northern counterpart (Figure 30).
As a result of its lower mass, the influence of Fornax on the local
velocity field is less dramatic than that of the Virgo cluster. And
because of its compact nature, questions concerning the membership of
individual galaxies in Fornax are less problematic, while the
back-to-front geometry is far less controversial than any of these
same points raised in the context of the Virgo cluster complex.
Clearly, Fornax is a much more interesting site for a test of the
local expansion rate.
p style="font-family: cursive" align=justify>
Figure 30.A comparison of the distribution
of galaxies as projected on the sky for the Virgo cluster (right panel)
and the Fornax cluster (left panel). M100 and NGC 1365 are each
individually marked by arrows showing their relative disposition with
respect to the main body and cores of their respective clusters.
Units are arcmin.
Although the goals of the Key Project on the Extragalactic Distance Scale (Kennicutt et al. 1995) are far broader than simply investigating the distances to a few nearby clusters, there are several important reasons to secure a distance to the Fornax cluster. It is both a probe of the local expansion velocity field, and Fornax is a major jumping-off point for a variety of secondary distance indicators which can be used to probe a volume of space at least 1,000 times larger. To obtain a distance to the Fornax cluster, the Key Project is configured to monitor three member galaxies; the first of these, discussed here, is the Seyfert 1 galaxy NGC 1365, a strikingly picturesque, two-armed, barred-spiral galaxy with an active galactic nucleus. In the coming year, two additional galaxies, NGC 1425 and NGC 1326A, are slated for imaging with HST.
At least three lines of evidence independently suggest that the first galaxy to be observed, NGC 1365 is a physical member of the Fornax cluster. First, NGC 1365 is almost directly along our line of sight to Fornax. The galaxy is projected only ~ 70 arcmin from the geometric center of the cluster, whereas the diameter of the cluster is ~ 200 arcmin (Ferguson 1989; see Figure 30). In addition, NGC 1365 is also coincident with the Fornax cluster in velocity space. The systemic (heliocentric) velocity and velocity dispersion of the main population of galaxies in Fornax are well defined: 30 spirals/irregular galaxies give V = 1,415 km/sec and = ± 347 km/sec, 70 E/SO galaxies give V = 1,473 km/sec with = ± 335 km/sec, and the combined sample gives = ± 340 km/sec. The observed velocity of NGC 1365 (+1,636 km/sec) is only +181 km/sec larger than the mean velocity of the Fornax cluster as a whole, which based on 100 galaxies is found to be 1,455 ± 34 km/sec (Schroder 1995; Schroder & Richter 1997; Han & Mould 1990; NED); with the mean velocity of the spirals agreeing with the mean for the ellipticals to within 60 km/sec). The velocity off-set of NGC 1365 is only half of the cluster velocity dispersion. Finally, we note that for its rotational velocity, NGC 1365 sits only 0.02 mag from the central ridge line of the apparent Tully-Fisher relation relative to other cluster members defined by recent studies of the Fornax cluster (Bureau et al. 1996; Schroder 1995).
On the other hand, it is often noted that NGC 1365 is impressively large in its angular size, and that it is very bright in apparent luminosity as compared to any other galaxy in the immediate vicinity of the Fornax cluster. However, corrected for an inclination of 44 degrees, the 21cm neutral hydrogen line width of NGC 1365 is found to be ~ 575 km/sec (Bureau et al. 1996; Mathewson et al. 1992). Using the Tully-Fisher relation as a relative guide to intrinsic size and luminosity, this rotation rate places NGC 1365 among the most luminous galaxies in the local Universe; brighter than M31 or M81, and comparable to NGC 4501 in the Virgo cluster or NGC 3992 in the Ursa Major cluster. We therefore conclude that NGC 1365 is in all respects apparently normal, (albeit large and luminous) and that its distance is indicative of the ensemble distance to the other spiral and elliptical galaxies constituting the Fornax cluster proper.