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

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,' $f_m$) which describes our confidence that the system is part of an ongoing merger. We describe how $f_m$ 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 $f_m$ applied to the SDSS Main Galaxy Spectral sample (MGS) to estimate that the fraction of volume-limited ($M_r < -20.55$) major mergers ($1/3 < {M}^*_1/{M}^*_2 < 3$) in the nearby universe is $1 - 3 \times C%$ where $C \sim 1.5$ 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 $\sim 2$ 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.