The structure of the Fornax dwarf elliptical galaxy. II - Two-dimensional structure

The two-dimensional structure of the Fornax dwarf elliptical galaxy has been investigated using automated starcounts from United Kingdom Schmidt Telescope Unit (UKSTU) plates. The overall shape of the 2-D contours is similar to that obtained by Hodge (1961b). In particular, the well-known globular clusters are recovered, as is the apparent asymmetry in the major-axis profile noted by Hodge (1961b). In order to investigate the extent of the nonsymmetric structures best-fit contours, which are constrained to be concentric and elliptical, have been constructed from the starcount data. These fits have been subtracted from the actual data, revealing evidence for significant density excesses at the ~35% level above the best symmetric fit. The scale size of these structures is ~170 pc. Given the velocity dispersion of the Fornax globular clusters, the crossing times for these features are on the order of ~10^8^ yr. The assumption that they are marginally bound yields a similar timescale. As Fornax appears to have a significant population of stars as young as ~3 Gyr, these density anomalies may be the result of localized bursts of intermediate-age star formation. Although first-epoch star groups are expected to survive no longer than ~10^9^ yr (Hodge and Michie 1969), if these features result from groups that formed after the general field in Fornax had relaxed, they could be expected to survive group-group encounters for ~10^11^ yr. Estimates of the ratio of kinetic to potential energy for the clumps seem to argue for their current dispersal. Calculations following arguments by Tremaine (1981) indicate that clumps such as these can be expected to survive the tidal effect of the Fornax core for the order of a Hubble time. However, these last calculations are based on a number of assumptions as to the physical state of the clumps, and should not be trusted without further observational study.