Galaxy Clusters from the DESI Legacy Imaging Surveys. I. Cluster
  Detection
  Based on the photometric redshift catalog of Zou H. et al. (2019), we apply a
fast clustering algorithm to identify 540,432 galaxy clusters at $z\lesssim1$
in the DESI legacy imaging surveys, which cover a sky area of about 20,000
deg$^2$. Monte-Carlo simulations indicate that the false detection rate of our
detecting method is about 3.1\%. The total masses of galaxy clusters are
derived using a calibrated richness--mass relation that are based on the
observations of X-ray emission and Sunyaev \& Zel'dovich effect. The median
redshift and mass of our detected clusters are about 0.53 and
$1.23\times10^{14} M_\odot$, respectively. Comparing with previous clusters
identified using the data of the Sloan Digital Sky Survey (SDSS), we can
recognize most of them, especially those with high richness. Our catalog will
be used for further statistical studies on galaxy clusters and environmental
effects on the galaxy evolution, etc.
