The AMIGA sample of isolated galaxies - II. Morphological refinement

We present a refinement of the optical morphologies for galaxies in the Catalog of Isolated Galaxies that forms the basis of the AMIGA ( Analysis of the interstellar Medium of Isolated GAlaxies) project. Uniform reclassification using the digitized POSS II data benefited from the high resolution and dynamic range of that sky survey. Comparison with independent classifications made for an SDSS overlap sample of more than 200 galaxies confirms the reliability of the early vs. late-type discrimination and the accuracy of spiral subtypes within Delta T = 1 - 2. CCD images taken at the Observatorio de Sierra Nevada were also used to solve ambiguities in early versus late-type classifications. A considerable number of galaxies in the catalog (n = 193) are flagged for the presence of nearby companions or signs of distortion likely due to interaction. This most isolated sample of galaxies in the local Universe is dominated by two populations: 1) 82% are spirals (Sa-Sd) with the bulk being luminous systems with small bulges (63% between types Sb-Sc) and 2) a significant population of early-type E-S0 galaxies (14%). Most of the types later than Sd are low luminosity galaxies concentrated in the local supercluster where isolation is difficult to evaluate. The late-type spiral majority of the sample spans a luminosity range MB-corr = - 18 to - 22 mag. Few of the E/S0 population are more luminous than - 21.0 marking the absence of the often-sought super L* merger (e.g. fossil elliptical) population. The rarity of high luminosity systems results in a fainter derived M* for this population compared to the spiral optical luminosity function (OLF). The E-S0 population is from 0.2 to 0.6 mag fainter depending on how the sample is defined. This marks the AMIGA sample as unique among samples that compare early and late-type OLFs separately. In other samples, which always involve galaxies in higher density environments, M*(E/S0) is almost always 0.3 - 0.5 mag brighter than M-S*, presumably reflecting a stronger correlation between M* and environmental density for early-type galaxies.