DECISION TREE CLASSIFIERS FOR STAR/GALAXY SEPARATION

We study the star/galaxy classification efficiency of 13 different decision tree algorithms applied to photometric objects in the Sloan Digital Sky Survey Data Release Seven (SDSS-DR7). Each algorithm is defined by a set of parameters which, when varied, produce different final classification trees. We extensively explore the parameter space of each algorithm, using the set of 884,126 SDSS objects with spectroscopic data as the training set. The efficiency of star-galaxy separation is measured using the completeness function. We find that the Functional Tree algorithm (FT) yields the best results as measured by the mean completeness in two magnitude intervals: 14 <= r <= 21 (85.2%) and r >= 19 (82.1%). We compare the performance of the tree generated with the optimal FT configuration to the classifications provided by the SDSS parametric classifier, 2DPHOT, and Ball et al. We find that our FT classifier is comparable to or better in completeness over the full magnitude range 15 <= r <= 21, with much lower contamination than all but the Ball et al. classifier. At the faintest magnitudes (r > 19), our classifier is the only one that maintains high completeness (> 80%) while simultaneously achieving low contamination (similar to 2.5%). We also examine the SDSS parametric classifier (psfMag - modelMag) to see if the dividing line between stars and galaxies can be adjusted to improve the classifier. We find that currently stars in close pairs are often misclassified as galaxies, and suggest a new cut to improve the classifier. Finally, we apply our FT classifier to separate stars from galaxies in the full set of 69,545,326 SDSS photometric objects in the magnitude range 14 <= r <= 21.

Alfred P. Sloan Foundation

National Science Foundation (NSF)

U.S. Department of Energy (DOE)

NASA National Aeronautics and Space Administration

Japanese Monbukagakusho (MEXT)

Max Planck Society

Higher Education Funding Council for England

American Museum of Natural History

Astrophysical Institute Potsdam

University of Basel

University of Cambridge

Case Western Reserve University

University of Chicago

Drexel University

Fermilab

Institute for Advanced Study

Japan Participation Group

Johns Hopkins University

Joint Institute for Nuclear Astrophysics (JINA)

Kavli Institute for Particle Astrophysics and Cosmology

Korean Scientist Group

CAS of China - Chinese Academy of Sciences (LAMOST)

Los Alamos National Laboratory

Max-Planck-Institute for Astronomy (MPIA)

Max-Planck-Institute for Astrophysics (MPA)

New Mexico State University

Ohio State University

University of Pittsburgh

University of Portsmouth

Princeton University

United States Naval Observatory

University of Washington