8 Color distribution

In the color magnitude diagram (Fig. 8) all galaxies brighter than V = 23.5 mag are shown. Most of the objects have colors of about (V-I) = 1.2 mag similar to the average color of the GCSs of the ellipticals in Fornax (Kissler-Patig et al. 1997). Assuming $E(B-V) \simeq 0$ mag towards Fornax, all galaxies redder than V-I = 1.8 mag are most likely background galaxies (since no redder stellar populations are expected from any theoretical stellar evolution model, e.g. Worthey 1994). The definite cluster dwarf galaxies (circles) are located at the blue side of the color distribution. Since most of them are early-type dwarfs and show no indications of star formation, their blue colors can only be explained by low metallicities. The dEs and dE, Ns follow a sequence in the sense that fainter galaxies are bluer (see dashed line in Fig. 8). This trend was already noticed by Caldwell & Bothun (1987, who measured UBV magnitudes of 30 relatively bright Fornax cluster dwarf ellipticals) and continues for dwarfs with low surface brightness (Bothun et al. 1991; Cellone et al. 1994). Held & Mould (1994) have shown for 10 nucleated dE, Ns in Fornax that their colors are correlated with their metallicities derived from line strengths ($-1.5 < [{\rm Fe/H}] < -0.8$). A color - magnitude relation for dwarf galaxies is also seen in other galaxy clusters, as for example in Virgo (Caldwell 1983; Caldwell & Bothun 1987) or Coma (Secker 1996; Secker & Harris 1997).

  

Figure 8: The plot shows the color magnitude diagram of all galaxies in our CCD fields. The (V - I) colors were measured in apertures with diameters of 3$^{\prime\prime}$. Mean errors are shown on the right. The Fornax dwarf galaxies are encircled. The dashed line is a fit to 8 dEs and dE, Ns from Table 2, excluding the bluest and reddest dwarf. A clear trend is visible in the sense that fainter dEs are bluer, as it is also seen in other clusters (e.g. in Coma, see Secker & Harris 1997). Crosses indicate definite background galaxies according to their radial velocities (see Paper II)

No color dependence on the projected distance to the center of NGC 1399 is seen for the dwarfs. Galaxies with actual or recent star formation activity which would stand out by very blue colors cannot be identified. However, some moderately blue galaxies of $V-I \simeq 0.5$ are present. Most of them are located in the background cluster behind the Fornax center (see Paper II) and most likely represent irregular galaxies surrounding ellipticals in this distant cluster. In the center of the Fornax cluster a prominent example of an irregular with this color is NGC 1427A (Hilker et al. 1997).