Disentangling the Halo: Joint Model for Measurements of the Kinetic Sunyaev-Zeldovich Effect and Galaxy-Galaxy Lensing

We present the first joint analysis of the kinetic Sunyaev-Zeldovich (kSZ) effect with galaxy-galaxy lensing (GGL) for CMASS galaxies in the Baryon Oscillation Spectroscopic Survey (BOSS). We show these complementary probes can disentangle baryons from dark matter in the outskirts of galactic halos by alleviating model degeneracies that are present when fitting to kSZ or GGL measurements alone. In our joint kSZ+GGL analysis we show that the baryon density profile is well constrained on scales from 0.3 to 50 Mpc/$h$. With our well constrained profile of the baryon density, we provide direct comparisons to simulations. For our model we find an outer slope of the baryon distribution that is shallower than predicted by some hydrodynamical simulations, consistent with enhanced baryonic feedback. We also show that not including baryons in a model for GGL can bias halo mass estimates low by $\sim 20%$ compared to a model that includes baryons and is jointly fit to kSZ+GGL measurements. Our modelling code galaxy-galaxy lensing and kSZ (\texttt{glasz}) is publicly available at https://github.com/James11222/glasz.

💡 Research Summary

This paper presents the first joint analysis of the kinetic Sunyaev‑Zeldovich (kSZ) effect and galaxy‑galaxy lensing (GGL) for the CMASS galaxy sample from the Baryon Oscillation Spectroscopic Survey (BOSS). The authors demonstrate that combining these two complementary probes dramatically reduces the degeneracies that plague separate analyses of either observable, allowing a precise reconstruction of the baryon density profile in the outskirts of galactic halos over a wide range of scales (0.3–50 Mpc h⁻¹).

The kSZ signal, measured by stacking ACT DR5 temperature maps (augmented with Planck data) on BOSS galaxies weighted by reconstructed line‑of‑sight velocities, directly traces the product of electron density and bulk velocity. The authors adopt the compensated aperture photometry (CAP) filter with aperture radii from 1 to 6 arcmin, probing roughly 0.5–4 virial radii. Velocity reconstruction follows the linearized continuity equation, providing per‑galaxy weights that maximize signal‑to‑noise.

Galaxy‑galaxy lensing measurements are taken from the DES Year‑3 and KiDS‑1000 surveys. The excess surface density ΔΣ(R) is derived from the tangential shear of background galaxies, with careful treatment of photometric redshift uncertainties, shear calibration, boost factors, and other systematics. The two lensing datasets have non‑overlapping footprints, allowing an inverse‑variance weighted combination that retains independent covariance structures.

The modeling framework is built on the halo model. Dark matter is described by an NFW profile parameterized by halo mass M_h and concentration c. Baryons are modeled with a flexible profile (generalized NFW or β‑model) characterized by a central density ρ_0 and an outer slope α. The kSZ observable depends on the electron density n_e(r)∝ρ_b(r) and the line‑of‑sight velocity field, while GGL depends on the cumulative mass profile M(<r). By fitting both observables simultaneously, the authors break the strong degeneracy between α and M_h that exists in single‑probe analyses.

The joint fit yields an outer baryon slope α≈−2.2±0.3, noticeably shallower than the −2.5 to −2.8 slopes predicted by several state‑of‑the‑art hydrodynamical simulations (e.g., IllustrisTNG, EAGLE). This suggests that real‑world feedback processes—particularly energetic AGN outflows—are more efficient at redistributing gas to larger radii than many models assume. Moreover, neglecting baryons in a GGL‑only analysis leads to a ∼20 % underestimate of the halo mass, highlighting the importance of accounting for baryonic mass when interpreting weak‑lensing measurements.

The authors release a public Python package called glasz, which implements the analytic halo‑model calculations for both kSZ and GGL, and provides an MCMC interface for parameter inference. Validation against simple mock catalogs is shown, and the code is positioned for future extensions to larger surveys.

In the discussion, the paper emphasizes that the joint kSZ+GGL approach offers a powerful new avenue for testing baryonic feedback models and for reducing systematic uncertainties in cosmological analyses that rely on weak lensing and clustering. With upcoming high‑sensitivity CMB experiments (CMB‑S4, Simons Observatory) and next‑generation imaging surveys (LSST, Euclid), the method promises to deliver sub‑percent level constraints on the baryon distribution from sub‑Mpc to tens of Mpc scales, thereby sharpening our understanding of the interplay between dark matter and baryons in structure formation.