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

Technical Summary: Disentangling the Halo via Joint kSZ and GGL Modeling

Problem Statement
Modern weak lensing (WL) and clustering analyses face a significant challenge in modeling baryonic feedback, which suppresses the matter power spectrum on non-linear scales. Current hydrodynamical simulations exhibit a wide spread in predictions regarding the amplitude and scale of this suppression, leading to substantial uncertainties in cosmological inference. While the kinetic Sunyaev-Zeldovich (kSZ) effect offers a direct probe of electron density (and thus baryons) in the outskirts of galactic halos, and galaxy-galaxy lensing (GGL) probes the total matter distribution, fitting these probes individually suffers from model degeneracies. Specifically, kSZ-only analyses often rely on uncertain stellar-to-halo mass relations (SHMR) to estimate halo mass, while GGL-only analyses struggle to disentangle the baryon density profile from the dark matter distribution without external constraints.

Methodology
The authors present the first joint analysis of kSZ effect measurements and GGL measurements for CMASS galaxies in the Baryon Oscillation Spectroscopic Survey (BOSS). The study utilizes:

• Data: Stacked kSZ temperature profiles from the Atacama Cosmology Telescope (ACT) DR5 and Planck maps, weighted by reconstructed peculiar velocities. GGL excess surface mass density ($\Delta\Sigma$) profiles are derived from cross-correlations with background source galaxies from the Dark Energy Survey Year 3 (DES Y3) and KiDS-1000.
• Modeling Framework: A fast, analytic halo model based on the assumption of axisymmetric density profiles. The total matter density is separated into baryonic ($\rho_b$) and dark matter ($\rho_{dm}$) components, each comprising a 1-halo term (central halo) and a 2-halo term (surrounding neighbors).
  • Dark Matter: Modeled as a Navarro-Frank-White (NFW) profile.
  • Baryons: Modeled as a Generalized NFW (GNFW) profile, parameterized by an amplitude, core scale fraction, and inner/intermediate/outer power-law slopes. The model assumes baryons are fully ionized gas and normalizes the profile such that the mass fraction of baryons reaches the cosmic baryon fraction at a large truncation radius ($r_b \approx 10 \times r_{200}$).
  • Observables: The model predicts the kSZ temperature fluctuation (via line-of-sight integration of gas density convolved with the beam) and the GGL $\Delta\Sigma$ (via the projected surface mass density).
• Statistical Approach: A joint likelihood analysis is performed using an affine-invariant ensemble Markov Chain Monte Carlo (MCMC) algorithm. The analysis fits for halo mass ($M_{halo}$), baryon profile parameters (specifically the outer slope $\beta$), and a 2-halo amplitude nuisance parameter ($A_{2h}$), while fixing other parameters (e.g., inner slope $\gamma$, intermediate slope $\alpha$) to alleviate degeneracies.

Key Contributions

1. Joint Modeling Framework: The paper introduces glasz, a public code that consistently models both kSZ and GGL signals within a single halo model framework, allowing for the simultaneous inference of halo mass and baryon density parameters.
2. Disentanglement of Components: The study demonstrates that combining kSZ and GGL data effectively breaks the degeneracies present when fitting either dataset alone. The kSZ signal constrains the baryon distribution in the halo outskirts, while GGL tightly constrains the total halo mass and the 2-halo term amplitude.
3. Direct Simulation Comparison: By obtaining a well-constrained baryon density profile, the authors enable direct comparisons with hydrodynamical simulations without relying on intermediate SHMR estimates.

Results

• Parameter Constraints: The joint fit yields tight constraints on the halo mass ($\log_{10}(M_{halo}/M_\odot) \approx 13.44 \pm 0.05$) and the outer power-law slope of the baryon distribution ($\beta \approx 2.45^{+0.30}_{-0.23}$). The baryon density profile is constrained across scales from 0.3 to 50 $h^{-1}$ Mpc.
• Impact on Halo Mass Estimates: The authors find that neglecting baryons in a GGL-only model shifts the estimated halo mass by approximately 20% compared to the joint kSZ+GGL model. This highlights the necessity of including baryonic physics to avoid bias in mass estimation.
• Baryonic Feedback Strength: The inferred outer slope $\beta$ is shallower than predicted by the AGN feedback model in the Battaglia (2016) hydrodynamical simulations. A shallower slope implies baryons are more extended, suggesting that the observed feedback is stronger than what these specific simulations predict.
• Comparison to Simulations: The observed suppression of $\Delta\Sigma$ at small scales aligns more closely with the Illustris simulation (which features more extreme feedback) than with the IllustrisTNG-300 simulation.

Significance and Claims
The paper claims that this joint analysis provides a robust method to "disentangle" baryons from dark matter in galactic halos. By alleviating model degeneracies, the approach allows for:

• More accurate halo mass estimates that account for baryonic content.
• Direct empirical constraints on baryonic feedback strength, revealing a preference for stronger feedback than some standard hydrodynamical simulations predict.
• A pathway to utilize small-scale weak lensing measurements in future cosmological analyses, which are currently often discarded due to baryonic modeling uncertainties.

The authors note that their model currently neglects satellite galaxies and miscentering effects, which could introduce systematic biases (estimated at the $\sim10-20\%$ level for mass and $\beta$). They posit that future work incorporating a Halo Occupation Distribution (HOD) framework will be necessary for precision cosmology, but the current analytic framework successfully demonstrates the complementarity of kSZ and GGL probes.