4. Extended objects and H distribution
The luminosities of point sources and slightly extended objects in and around NGC 1427A were determined by PSF fitting in the B, V, and I images. The colors were measured in apertures of diameter assuming constant sky values. We present in the following foremost results using the colors, because the resolution in the B image is not as good as in the V and I images due to the larger pixel scale and the worse seeing. About 30% of the objects we found in V and I could not be measured in B. Fig. 2 shows in the upper panel the color magnitude diagram (CMD) of all objects within a radius of about 8 kpc () from the geometrical center of NGC 1427A (which is the center of the faintest isophote). In the lower panel the CMD for a two times larger control field located north of NGC 1427A is plotted. Objects redder than mag in both panels most probably are resolved as well as unresolved background galaxies and foreground stars. The slight excess of extended objects in the control field compared to the target field can be explained by statistical fluctuations. Comparison of the two CMDs shows clearly that NGC 1427A sample is dominated by very blue objects.
Fig. 3 shows the V image of NGC 1427A with H contours overplotted. Our H image is not deep enough to search for diffuse filaments and shells similar the supergiant shell around Shapley Constellation III in the LMC (e.g. Meaburn 1980, Kennicutt et al. 1995). The aperture and colors of the most prominent blue knots are indicated. Almost all regions that are associated with H emission show very blue components with colors in the range mag (or mag). In some cases the color is negative, whereas the color is positive. This can only be explained by the strong contribution of emission lines in the V filter. The most prominent emission lines of H II regions in the V band are the O III lines at 4959 and 5007 Å. A comparison with the HST image shows that most of the bright blue knots are multiple objects. Thus, from their color and sizes these objects can be regarded as OB associations and H II regions.
© European Southern Observatory (ESO) 1997
Online publication: April 6, 1998