2. DISTANCE AND MEMBERSHIP OF THE FORNAX CLUSTER

The distance to NGC 1365 has recently been estimated from Cepheid variables discovered in an outlying field of the galaxy by the Hubble Space Telescope (HST). This work is done as part of the HST Key Project on the Extragalactic Distance Scale. From 34 such variables with light curves of high quality and a period-luminosity relation calibrated against Cepheids in the Large Magellanic Cloud (Silbermann et al. 1998) a distance modulus (m - M)₀ = 31^m.35 ± 0^m.07 was derived (Madore et al. 1998a). This corresponds to a distance of 18.6 ± 0.6 Mpc. A more complete discussion of the errors (Madore et al. 1998b) gives 18.6 ± 1.9 (statistical error) ± 1.9 Mpc (systematic error). We will adopt this distance for the present review. Using the visual absorption A_V = 0^m.33 given by Madore et al. (1998a), a reddening law A_B / A_V = 1.337 of Cardelli et al. (1989) and the apparent B magnitude quoted in Section 1, the total absolute magnitude will be M_B = -21^m.5. NGC 1365, together with M 101, proves to be one of the two largest spiral galaxies in a local sample consisting of 11 spiral galaxies with distances from Cepheid variables (Goodwin et al. 1997). Its isophotal diameter at B = 25^m arcsec^-2 of 11'.2 corresponds to 61 kpc. Thus, NGC 1365 is a true supergiant galaxy.

The question whether NGC 1365 really is a member of the Fornax cluster or not is of some fundamental importance. This nearby cluster of galaxies with a small, dense central concentration of early type galaxies dominated by the cD galaxy NGC 1399 is one of the cornerstones in the establishment of the cosmic distance scale and the determination of the Hubble constant. Ferguson (1989) lists 340 likely cluster members and the projected distance of NGC 1365 from the geometric centre of the cluster is 70' as compared to the cluster radius of ~ 200'. If a member, NGC 1365 has probably the best determined distance of all the galaxies in the cluster. Further, if a member, NGC 1365 is one of the two brightest galaxies in the cluster field and establishes the outermost bright tail of the galaxy luminosity function in the cluster.

There are a number of published distance estimates for the Fornax cluster:

• McMillan et al. (1993) detected 224 planetary nebula candidates in three galaxies in the cluster, including NGC 1399 and Fornax A. Using a planetary nebula luminosity function the authors derive a distance modulus of the cluster of 31^m.14 ± 0^m.14. Correcting to a recent M 31 distance scale, 0^m.06 should be added (Jacoby 1997) and the corresponding distance will be 17.4 ± 1.1 Mpc.

• In a review of the globular clusters as distance indicators Whitmore (1997) discusses the application to the Fornax cluster. From measurements of globular cluster luminosity functions in seven cluster galaxies he derives a weighted mean distance of 15.9 ± 2.0 Mpc. The major part of the error comes from the uncertainty of the absolute magnitude at the turnover of the luminosity function.

• Bureau et al. (1996) use the infrared Tully-Fisher relation, which is the relation between the extinction corrected infrared absolute magnitude and the inclination corrected velocity width of the radio 21 cm emission line, and derive a distance modulus for the Fornax cluster of 30^m.94 ± 0^m.33. However, Shanks (1997), using spiral galaxies with HST Cepheid or Type Ia supernova distances, concludes that Tully-Fisher distance moduli are too small by 0^m.43 ± 0^m.11. Adding this correction gives a distance for the Fornax cluster of 18.8 ± 3.0 Mpc.

• Measuring surface brightness fluctuations in a number of galaxies in the Fornax cluster Tonry (1997) finds an average distance of 17.6 ± 0.5 Mpc. The values given by this method have, however, varied substantially over the years and the method has been criticized (Tammann & Federspiel 1997). In view of this, the quoted error seems underestimated.

The distance for NGC 1365 determined from Cepheids falls within the range of estimated cluster distances. The systemic radial velocity of NGC 1365 as given by Jörsäter and van Moorsel (1995) is +1 632 km s^-1. This value is rather central within the fairly dispersed velocity distribution in the cluster as judged from the velocities listed by Ferguson (1989). Finally, NGC 1365 fits well the upper end of the infrared Tully-Fisher relation for the cluster galaxies as presented by Bureau et al. (1996).

As a conclusion it seems that NGC 1365 with reasonable probability is a member of the Fornax cluster.

Table 1. Data for NGC 1365

(1950) = 3h 31m 41s.80	(1)
(1950) = -36° 18' 26s.6	(1)
(J2000) = 3h 33m 36s.37
(J2000) = -36° 08' 25".4
Galactic longitude l = 237°.96
Galactic latitude b = -54°.60
Total apparent magnitude BT = 10m.32	(2)
Far-infrared flux FIR = 5.07 x 10-12 W m-2	(6)
Systemic radial velocity = +1 632 km s-1	(3, 5)
PA line of nodes = 220°	(3)
Inclination i = 40°	(3)
Distance = 18.6 Mpc &	(4)
Scale: 1" = 90 pc &
Isophotal radius (at B = 25m arcsec-2) = 337" (30 kpc)	(2)
Total absolute magnitude MB = -21m.5	(7)
Far-infrared luminosity L(FIR) = 5.4 x 1010 L
Total mass 36 x 1010 M	(3*)
H I mass = 1.3 x 1010 M	(3*)
H2 mass = 1.7 x 1010 M	(5*)
Mass-luminosity ratio M/LB 5.8
(1) P.O. Lindblad et al. 1996a
(2) de Vaucouleurs et al. 1991
(3) Jörsäter & van Moorsel 1995
(4) Madore et al. 1998a
(5) Sandqvist et al. 1995
(6) Rice et al. 1988
(7) see text
(*) Recomputed for a distance of 18.6 Mpc