unWISE × ACT DR6 CMB lensing

Cross-correlations between CMB lensing and galaxy surveys are a key piece of my research. In this work we correlate the unWISE infrared galaxy catalog (~500 million galaxies, between about $z \sim 0.2$ and $z \sim 1.6$) with ACT DR6 CMB lensing, extracting information on the physics of cosmic expansion and structure growth.

For this analysis I build a measurement pipeline that carefully treats and verifies a long list of potential systematics — from foregrounds (both galactic and extragalactic) to redshift and modelling systematics. Ultimately, we delivered percent-level precision with a new level of systematic rigor for this type of analysis.

What we’ve shown

The core of this work was presented in two papers:

• The 2023 ApJ paper (arXiv:2309.05659) established the cross-correlation pipeline and measured the amplitude of structure, $S_8$, at intermediate redshift, a regime complementary to galaxy weak lensing and CMB lensing auto-spectra.
• The 2024 PRD multi-probe analysis (arXiv:2409.02109) extends this to a full joint fit of the galaxy auto-correlation, the cross-correlation, and the CMB lensing auto-spectrum — tightening constraints across multiple redshift slices.

Going beyond these two-point function analyses, we used the unWISE samples for the first detection of the three-point cross-correlation between CMB lensing and galaxies (arXiv:2311.04213), opening a new non-Gaussian probe of large-scale structure.

Foundational ACT DR6 lensing release

This work sits on top of the ACT DR6 lensing data release, which I contributed to:

• Madhavacheril et al. (arXiv:2304.05203) — DR6 gravitational lensing map and cosmological parameters
• Qu et al. (arXiv:2304.05202) — DR6 CMB lensing power spectrum
• MacCrann et al. (arXiv:2304.05196) — mitigating extragalactic foregrounds

And, more recently, the joint structure-growth analysis combining CMB lensing from ACT, SPT, and Planck:

• Qu et al. (arXiv:2504.20038, PRL) — unified structure growth measurements

Other cross-correlationas building on this work

The methodology developed here has been carried over to other galaxy samples I’ve worked on:

• DESI Legacy Survey galaxies × ACT DR6 + Planck PR4 lensing
  • Sailer et al. (arXiv:2407.04607, JCAP 2024) — cosmological constraints from the cross-correlation
  • Kim et al. (arXiv:2407.04606, JCAP 2024) — structure formation over cosmic time
  • Qu et al. (arXiv:2410.10808, Phys. Rev. D 2025) — structure growth measurements
• Quaia quasars × ACT DR6 + Planck PR4 lensing
  • Embil Villagra et al. (arXiv:2507.08798, JCAP 2025) — high-redshift structure growth