2000ApJ...531..137K

Using wide-field, two-color Hubble Space Telescope (HST) imaging of the cluster Cl 1358+62 (z=0.33), we derive structural parameters for a large, magnitude-limited sample of cluster members. These structural parameters are combined with accurate velocity dispersions in another paper to investigate the fundamental plane in the cluster. We fit integrated r^1/4 laws to the integrated surface brightness profiles and fit two-dimensional r^1/4-law model galaxies to the images directly. A comparison of the results from the two methods shows that the derived half-light radii, r_e, agree very well, with an rms scatter of only 13%. The half-light radii range from approximately 1 to 20 kpc with a median of about 3 kpc (H₀=65 km.s^–1.Mpc^–1, q₀=0.1).

We investigated the stability of r_e by comparing the r^1/4-law fits to the half-light radii derived using other commonly used surface brightness profiles. In particular, we fit Sersic r^1/n laws (finding the best-fit n in the range n=1-6) and superpositions of r^1/4-law bulges with exponential disks. The half-light radii derived from the best-fit Sersic profiles varied with respect to r_e from the r^1/4-law fits by only 1% in the median, but with a standard deviation of more than 40% in r_e. For the bulge+disk fits, the derived half-light radii were offset from r_e of the r^1/4-law fits by 10% in the mean, also showing a large standard deviation of more than 40%. By comparing the fitted half-light radii from the Sersic laws with those derived from the bulge+disk fitting, one also finds a large scatter of 30%. Based on these tests, we conclude that, in general, half-light radii are only measured with a typical accuracy of 30%-40%.

While there are large uncertainties in half-light radii, these do not impact the subsequent fundamental plane analysis because the combination r<I>^0.76, which enters the fundamental plane, is extremely stable. The rms scatter in r<I>^0.76 is less than 3%, regardless of the form of the profile fit to the galaxies.

By fitting bulge+disk profiles, we find that the median bulge fraction of the sample is 84% and that the few E+A galaxies in this sample have disks which make up about 10%-35% of their total light. These results are consistent with the residuals from fitting two-dimensional r^1/4-law models directly to the galaxies, in which disklike structures are revealed in about half of the sample. Two of the three E+A galaxies show spiral arm structure.