Weak Lensing by Photometric Density Ridges

Abstract

Ridges in galaxy density fields measured by photometric surveys are 2D projections of filaments in the cosmic web, and so should lens light from background galaxies. We report on a detection of this effect in Dark Energy Survey Year 3 data at high significance, though not independently of galaxy-galaxy lensing. We describe improvements to the existing subspace-constrained mean shift algorithm to locate these ridges efficiently at scale, and examine the dependence of the signal in simulations on cosmological and algorithmic parameters. We find that it depends primarily on $S_8=σ_8 \left( Ω_m / 0.3 \right)^{1/2}$, and discuss improvements to our methodology that would be needed to allow precision parameter estimation.

Figures (12)

• Figure 1: A schematic of the simulation and analysis procedure used in this paper.
• Figure 2: Schematic overview of the ridge-to-filament identification pipeline in Radians.
• Figure 3: Top: a schematic of the identification of nearest points on a ridge to background galaxies. Bottom: A zoom-in on one pair showing the separation angle $\theta$ and the rotation angle $\phi$ to convert to tangential shears.
• Figure 4: The input redshift distributions for the simulation foreground lenses (on whose density the ridges are defined) and background sources (whose orientation around the ridges we measure). The vertical line shows $z=0.4$: we cut the lenses to $z<0.4$ and the sources to $z>0.4$, defined on the true redshifts in the simulations, to maximize the signal-to-noise by ensuring that the sources are behind the lenses.
• Figure 5: The tangential shear around ridges in a fiducial shape noise-free simulation as a function of separation, compared with the galaxy-galaxy lensing signal from the same data.