The Atacama Cosmology Telescope: Cross-Correlation of CMB Lensing and
  Quasars
  We measure the cross-correlation of Atacama Cosmology Telescope CMB lensing
convergence maps with quasar maps made from the Sloan Digital Sky Survey DR8
SDSS-XDQSO photometric catalog. The CMB lensing-quasar cross-power spectrum is
detected for the first time at a significance of 3.8 sigma, which directly
confirms that the quasar distribution traces the mass distribution at high
redshifts z>1. Our detection passes a number of null tests and systematic
checks. Using this cross-power spectrum, we measure the amplitude of the linear
quasar bias assuming a template for its redshift dependence, and find the
amplitude to be consistent with an earlier measurement from clustering; at
redshift z ~ 1.4, the peak of the distribution of quasars in our maps, our
measurement corresponds to a bias of b = 2.5 +/- 0.6. With the signal-to-noise
ratio on CMB lensing measurements likely to improve by an order of magnitude
over the next few years, our results demonstrate the potential of CMB lensing
cross-correlations to probe astrophysics at high redshifts.
