Globular Clusters in NGC 1399 and Background Galaxies

NGC 1399, the central galaxy of the Fornax cluster, is known to have an overabundance of globular clusters (Hanes and Harris, 1986). In order to determine the luminosity function of these globular clusters, two fields near NGC 1399 were observed in the B, V, and R band. One field was chosen near the center of NGC 1399 and the second field 20′ east of the galaxy center. The second field is supposed to contain no globular clusters and is used for comparison. Both fields were reduced with DAOPHOT (Stetson, 1987) to determine the number-magnitude relation for all detected objects (see Figure 1). The differential number counts give log N (m) = 0.84 mB for the comparison field while the differential number counts for the central field (background galaxies and globular clusters) cannot be described by this simple law. Figure 2 shows the number-magnitude relation of the excess objects in the central field (difference between all counts in the central and in the comparison field). The differential number counts N (m) of the excess objects can be represented by the superposition of three populations, each following approximately a Gaussian distribution. The population with the brightest peak luminosity (mB = 22.1 mag) is due to a galaxy cluster with an estimated redshift z = 0.3–0.4. Figure 1.The differential number counts of the central field (upper line) and the comparison field (lower line) are displayed for the band until 0.5 mag short of the limiting magnitude. The differential number counts of the comparison field can be described by a simple exponential law; this function is displayed as a dotted line. The differential number counts of the central field cannot be approximated by a simple exponential function.Figure 2.The difference among all detected objects between both fields (Ncent(m) - Ncomp(m)) is shown for the B band. The excess objects in the central field can be represented by the superposition of three populations. This three-component Gaussian model is indicated by the dotted line. The most abundant population (N > 250) with peak luminosity mB = 24.8 mag is identified with globular clusters belonging to NGC 1399.