A comparative study of morphological classifications of APM galaxies

Abstract

We investigate the consistency of visual morphological classifications of\ngalaxies by comparing classifications for 831 galaxies from six independent\nobservers. The galaxies were classified on laser print copy images or on\ncomputer screen produced from scans with the Automated Plate Measuring (APM)\nmachine. Classifications are compared using the Revised Hubble numerical type\nindex T. We find that individual observers agree with one another with rms\ncombined dispersions of between 1.3 and 2.3 type units, typically about 1.8\nunits. The dispersions tend to decrease slightly with increasing angular\ndiameter and, in some cases, with increasing axial ratio $(b/a)$. The agreement\nbetween independent observers is reasonably good but the scatter is\nnon-negligible. In spite of the scatter the Revised Hubble T system can be used\nto train an automated galaxy classifier, e.g. an Artificial Neural Network, to\nhandle the large number of galaxy images that are being compiled in the APM and\nother surveys.\n