The Atacama Cosmology Telescope: Cross-Correlation of CMB Lensing and Quasars

TL;DR

This paper reports the first detection of the cross-correlation between CMB lensing maps from the Atacama Cosmology Telescope and quasar maps from SDSS, confirming that quasars trace high-redshift mass distribution and demonstrating the potential of this method for astrophysical studies.

Contribution

It presents the first measurement of the CMB lensing-quasar cross-power spectrum and uses it to estimate quasar bias at high redshift, showing the method's promise for future research.

Findings

Detected CMB lensing-quasar cross-correlation at 3.8 sigma significance.

Measured quasar bias at z ~ 1.4 as b = 2.5 +/- 0.6.

Confirmed quasars trace mass distribution at high redshifts.

Abstract

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.