AUTOMATIC UNSUPERVISED CLASSIFICATION OF ALL SLOAN DIGITAL SKY SURVEY DATA RELEASE 7 GALAXY SPECTRA

Abstract

Using the 'k-means' cluster analysis algorithm, we carry out an unsupervised\nclassification of all galaxy spectra in the seventh and final Sloan Digital Sky\nSurvey data release (SDSS/DR7). Except for the shift to restframe wavelengths,\nand the normalization to the g-band flux, no manipulation is applied to the\noriginal spectra. The algorithm guarantees that galaxies with similar spectra\nbelong to the same class. We find that 99 % of the galaxies can be assigned to\nonly 17 major classes, with 11 additional minor classes including the remaining\n1%. The classification is not unique since many galaxies appear in between\nclasses, however, our rendering of the algorithm overcomes this weakness with a\ntool to identify borderline galaxies. Each class is characterized by a template\nspectrum, which is the average of all the spectra of the galaxies in the class.\nThese low noise template spectra vary smoothly and continuously along a\nsequence labeled from 0 to 27, from the reddest class to the bluest class. Our\nAutomatic Spectroscopic K-means-based (ASK) classification separates galaxies\nin colors, with classes characteristic of the red sequence, the blue cloud, as\nwell as the green valley. When red sequence galaxies and green valley galaxies\npresent emission lines, they are characteristic of AGN activity. Blue galaxy\nclasses have emission lines corresponding to star formation regions. We find\nthe expected correlation between spectroscopic class and Hubble type, but this\nrelationship exhibits a high intrinsic scatter. Several potential uses of the\nASK classification are identified and sketched, including fast determination of\nphysical properties by interpolation, classes as templates in redshift\ndeterminations, and target selection in follow-up works (we find classes of\nSeyfert galaxies, green valley galaxies, as well as a significant number of\noutliers). The ASK classification is publicly accessible through various\nwebsites.\n