  We report the discovery of CFBDS J005910.90-011401.3 (hereafter CFBDS0059),
the coolest brown dwarf identified to date. We found CFBDS0059 using i' and z'
images from the Canada-France-Hawaii Telescope (CFHT), and present optical and
near-infrared photometry, Keck laser guide star adaptive optics imaging, and a
complete near-infrared spectrum, from 1.0 to 2.2 $\mu$m. A side to side
comparison of the near-infrared spectra of CFBDS0059 and ULAS
J003402.77-005206.7 (hereafter ULAS0034), previously the coolest known brown
dwarf, indicates that CFBDS0059 is ~50+/-15K cooler. We estimate a temperature
of Teff ~ 620K and gravity of log g ~ 4.75. Evolutionary models translate these
parameters into an age of 1-5 Gyr and a mass of 15-30 M_Jup. We estimate a
photometric distance of ~13pc, which puts CFBDS0059 within easy reach of
accurate parallax measurements. Its large proper motion suggests membership in
the older population of the thin disk. The spectra of both CFBDS0059 and ULAS
J0034 shows probable absorption by a wide ammonia band on the blue side of the
$H$-band flux peak. If, as we expect, that feature deepens further for still
lower effective temperatures, its appearance will become a natural breakpoint
for the transition between the T spectral class and the new Y spectral type.
CFBDS0059 and ULAS J0034 would then be the first Y0 dwarfs.
