Galaxy Zoo: the fraction of merging galaxies in the SDSS and their morphologies

Daniel W. Darg, Sugata Kaviraj, Chris J. Lintott, Kevin Schawinski, Marc Sarzi, Steven P. Bamford, Joseph Silk, Richard Proctor, Dan Andreescu, Phil Murray, Robert C. Nichol, M. Jordan Raddick, Anze Slosar, Alexander S. Szalay, Daniel Thomas, Jan Vandenberg

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Abstract

We present the largest, most homogeneous catalogue of merging galaxies in the nearby Universe obtained through the Galaxy Zoo project – an interface on the World Wide Web enabling large-scale morphological classification of galaxies through visual inspection of images from the Sloan Digital Sky Survey (SDSS). The method converts a set of visually inspected classifications for each galaxy into a single parameter (the ‘weighted-merger-vote fraction,’fm) which describes our confidence that the system is part of an ongoing merger. We describe how fm is used to create a catalogue of 3003 visually selected pairs of merging galaxies from the SDSS in the redshift range 0.005 < z < 0.1. We use our merger sample and values of fm applied to the SDSS Main Galaxy Spectral sample to estimate that the fraction of volume-limited (Mr < −20.55) major mergers (1/3 < M*1/M*2 < 3) in the nearby Universe is 1–3 ×C per cent, where C∼ 1.5 is a correction factor for spectroscopic incompleteness. Having visually classified the morphologies of the constituent galaxies in our mergers, we find that the spiral-to-elliptical ratio of galaxies in mergers is higher by a factor of ∼2 relative to the global population. In a companion paper, we examine the internal properties of these merging galaxies and conclude that this high spiral-to-elliptical ratio in mergers is due to a longer time-scale over which mergers with spirals are detectable compared to mergers with ellipticals.
Original languageEnglish
Pages (from-to)1043-1056
JournalMonthly Notices of the Royal Astronomical Society
Volume401
Issue number2
Early online date11 Jan 2010
DOIs
Publication statusPublished - 2010

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