First detection of the GI-type of intrinsic alignments of galaxies using the self-calibration method in a photometric galaxy survey

TL;DR

This paper reports the first detection of intrinsic galaxy alignments using a self-calibration method in a photometric survey, aiding in understanding galaxy formation and improving lensing measurements.

Contribution

It introduces the first direct measurement of GI and Ig intrinsic alignments using self-calibration in a photometric survey, without relying on specific IA models.

Findings

Detection of GI correlation at 3.51σ significance

Detection of Ig correlation at 3.65σ significance

Validation of self-calibration as a tool for IA analysis

Abstract

Weak gravitational lensing is one of the most promising cosmological probes to constrain dark matter, dark energy, and the nature of gravity at cosmic scales. Intrinsic alignments (IAs) of galaxies have been recognized as one of the most serious systematic effects facing gravitational lensing. Such alignments must be isolated and removed to obtain a pure lensing signal. Furthermore, the alignments are related to the processes of galaxy formation, so their extracted signal can help in understanding such formation processes and improving their theoretical modeling. We report in this Letter the first detection of the gravitational shear--intrinsic shape (GI) correlation and the intrinsic shape--galaxy density (Ig) correlation using the self-calibration method in a photometric redshift survey. These direct measurements are made from the KiDS-450 photometric galaxy survey with a significance of 3.65$\sigma$ in the third bin for the Ig correlation, and 3.51$\sigma$ for the GI cross-correlation between the third and fourth bins. The self-calibration method uses the information available from photometric surveys without needing to specify an IA model and will play an important role in validating IA models and IA mitigation in future surveys such as the Rubin Observatory Legacy Survey of Space and Time, Euclid, and WFIRST.