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.
