Speaker
Description
Weak gravitational lensing has flourished into one of the most competitive fields in cosmology, based on two main pillars, namely cosmic shear and galaxy-galaxy lensing. However, there is a third pillar that is usually overlooked as an observable in itself: cosmic magnification, the statistical correlation in the sky between the position of foreground galaxies and the number counts of background sources. In particular, the study of galaxy-quasar correlations in the sky due to weak lensing magnification has been abandoned for 20 years owing to the success of galaxy-galaxy lensing. In this talk, we present out work aiming to revive this field by leveraging the much larger amount of high-quality data available at present.
In particular, we work with lenses from the Sloan Digital Sky Survey, both from spectroscopic large-scale structure catalogs like the CMASS and LOWZ samples and from photometric samples with reliable redshifts. Regarding background quasars, we employ both the spectroscopic all-sky Quaia catalog and a photometric one with reliable redshifts from a cross-match between the Pan-STARRS1 (PS1) and WISE surveys. All cross-correlation possibilities are studied to assess the impact of the number density of objects as well as the reliability of the redshift determinations. The quasar samples are binned in magnitude to separate the positive and negative contributions to cosmic magnification.
The most robust signal is clearly measured using the photometric lens sample, highlighting the importance of a high number density of lenses. Both background quasar catalogs agree on the common magnitude ranges, but the PS1-WISE sample allows us to go to much deeper magnitude and leverage the negative-correlation regime, which involves a much higher number density of quasars and thus produces a more stable signal. The signal behaves as expected with respect to quasar magnitude, transitioning from a positive to a negative angular correlation when going from brighter to fainter quasars and extending up to 100-200 arcmin (tens Mpc). Additionally, in some specific cases, a large-scale excess correlation appears, which was already observed in a previous work using background submillimeter galaxies and its relationship to the specifics of the lens sample is studied. All in all, the measurements presented in this work exemplify the importance of galaxy-quasar correlations for cosmology, since the observable is shown to be very sensitive to both the cosmological matter density and clustering amplitude parameters.
