Discussion

While extensive experience has been garnered in the application of maximum likelihood to stellar photometry with HST (see for example Stetson 1994), comparatively little has been done for galaxy images (see Schade and Elson 1994). What we present here is only a first step, albeit a successful one. The real test of the method comes from its application to actual data.

In the application to real HST observations, we generally do achieve convergence on galaxy parameters to the magnitude limits given in Table 1. However, significant ambiguity remains in the interpretation of the results. Real galaxy images do not follow the simple models used here: spiral arms, bars, double nuclei, dust lanes, and bright knots of star formation all add complications that cannot be properly modeled with the limited information available in typical images. The only recourse in our exploratory runs has been to estimate a best fit using a simple galaxy profile and to inspect the residual distribution for additional visual clarification. More detailed results will be reported in Griffiths et al. (1994) and Ratnatunga, Griffiths, and Casertano (1994).

We should also point out that a key ingredient in achieving our limited success on actual data has been the improved calibration of WFC images, optimized for the quantitative analysis of faint images. The details are reported in Ratnatunga et al. (1994).

Acknowledgments

This work is based on observations taken with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Associations of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Coordination and analysis of data for the Medium-Deep Survey is funded by STScI grants GO 2684.0X.87A and GO 3917.0X.91A. We acknowledge the helpful comments from the full HST Medium Deep Survey team.