Title:The morphology distribution of 800,000 SDSS galaxies based on colours and 
concentration index.

Abstract:
Using a sample of 2253 galaxies from the SDSS-DR3, Fukugita et al. (2007) 
assembled a catalogue of galaxies with a visual morphology classification. They 
correlated these morphological classes with the u-g, g-r, r-i and i-z colours, 
as obtained form the SDSS. Shimasaku et al. (2001) correlated the same visual 
morphology classes with an index measuring the concentration of light of the 
galaxies, defined as the ratio of the Petrosian radii containing 50% and 90% of 
their total Petrosian flux, r_50/r_90. Combining both studies, we have a set of 
five photometric characteristics of these galaxies, as obtained from the SDSS, 
that relate with their visual morphology.

Since it is not practical to obtain a homogeneous, single-person visual 
morphology classification of galaxy samples larger than tens of thousands, a 
different approach to infer the distribution of these morphology classes for 
such a large sample must be taken. In order to achieve this, we have reversely 
applied the relations described in the previous paragraph to infer the 
morphology class of ~ 800K galaxies in the SDSS-DR7 spectroscopic catalogue 
from their photometric colours and concentration index. We have found that, in 
a galaxy-by-galaxy comparison, our inferred classes are similar to the visual 
morphology classes to the same degree as the visual morphology of a single 
galaxy varies when based on the classification by different individuals.

Despite that with this result we consider our inference as successful, we 
conservatively conclude that our inference is best suited to assess the 
distribution of the morphology of large samples of SDSS galaxies, since the 
galaxy-to-galaxy differences are less significant with a larger sample size. We 
intend to use our inference to study the morphology-local density relation of 
large samples of galaxies in different environments. We also compare our 
inference with the results of Lintott et al. (2011, Galaxy Zoo).