Detection of a Supervoid Aligned with the Cold Spot of the Cosmic
  Microwave Background
  We use the WISE-2MASS infrared galaxy catalog matched with Pan-STARRS1 (PS1)
galaxies to search for a supervoid in the direction of the Cosmic Microwave
Background Cold Spot. Our imaging catalog has median redshift $z\simeq 0.14$,
and we obtain photometric redshifts from PS1 optical colours to create a
tomographic map of the galaxy distribution. The radial profile centred on the
Cold Spot shows a large low density region, extending over 10's of degrees.
Motivated by previous Cosmic Microwave Background results, we test for
underdensities within two angular radii, $5^\circ$, and $15^\circ$. The counts
in photometric redshift bins show significantly low densities at high detection
significance, $\gtrsim 5 \sigma$ and $\gtrsim 6 \sigma$, respectively, for the
two fiducial radii. The line-of-sight position of the deepest region of the
void is $z\simeq 0.15-0.25$. Our data, combined with an earlier measurement by
Granett et al. 2010, are consistent with a large $R_{\rm void}=(220 \pm 50)
h^{-1}Mpc $ supervoid with $\delta_{m} \simeq -0.14 \pm 0.04$ centered at
$z=0.22\pm0.03$. Such a supervoid, constituting at least a $\simeq 3.3\sigma$
fluctuation in a Gaussian distribution of the $\Lambda CDM$ model, is a
plausible cause for the Cold Spot.
