Lensing the darkness: The matter density profile in cosmic voids from UNIONS

TL;DR

This paper measures the matter density profile of cosmic voids using weak lensing around spectroscopically identified voids from BOSS, overlapping with UNIONS imaging. It introduces a novel analytic Gaussian covariance for void lensing, constructs multiple void catalogues, and fits a five-parameter HSW density profile while correlating with the void-galaxy cross-correlation to constrain $b_{Vg}$. The stacked lensing signal is detected at $6.2\sigma$, with size-dependent profiles showing deeper central underdensities in smaller voids; the inferred galaxy bias in underdense environments is $b_{Vg}/b_g\approx1.36\pm0.27$ and broadly consistent with linear-bias expectations. These results demonstrate the viability of void-lensing as a cosmological probe and set the stage for future tomographic analyses with Euclid, Roman, and other Stage-IV surveys.

Abstract

We measure the distribution of matter contained within the emptiest regions of the Universe: cosmic voids. We use the large overlap between the Ultraviolet Near-Infrared Optical Northern Survey (UNIONS) and voids identified in the LOWZ and CMASS catalogues of the Baryon Oscillation Spectroscopic Survey (BOSS) to constrain the excess surface mass density of voids using weak lensing. We present and validate a novel method for computing the Gaussian component of the conventional weak lensing covariance, adapted for use with void studies. We detect the stacked weak lensing void density profile at the $6.2σ$ level, the most significant detection of void lensing from spectroscopically-identified voids to date. We find that large and small voids have different matter density profiles, as expected from numerical studies of void profiles. This difference is significant at the $2.3σ$ level. Comparing the void profile to a measurement of the void-galaxy cross-correlation to test the linearity of the relationship between mass and light, we find good visual agreement between the two, and a galaxy bias factor of $2.45\pm0.36$, consistent with other works. This work represents a promising detection of the lensing effect from underdensities, with the goal of promoting its development into a competitive cosmological probe.

Figures (13)

• Figure 1: Footprints of the two surveys used in this analysis and their overlapping region. The UNIONS footprint at the time of writing is in purple, and the CMASS footprint is shown in light blue. The overlapping region of the two surveys is shown in yellow.
• Figure 2: Void size distribution for the BOSS LOWZ+CMASS void catalogue from Woodfinden2022.
• Figure 3: Void redshift distribution for the BOSS LOWZ and CMASS void catalogues from Woodfinden2022 with the UNIONS source redshift distribution. The UNIONS source distribution plotted in this figure is one of the blinded distributions. This is done in order to preserve the blinding of the cosmological analysis. It is close, but not identical, to the unblinded redshift distribution which is used for the analysis in this paper.
• Figure 4: Results from measuring the $\Delta\Sigma$ profile around 30 sets of random points from mocks. Each data point is an average of the 30 random catalogues for each radial bin, and the error bars are the square root of the diagonal of the covariance matrix found from the randoms divided by the 30 catalogues that went into the measurement.
• Figure 5: Comparison between the diagonals of the analytic covariance calculation and the empirical estimate of the covariance around mock-random lenses for the Full void catalogue and the four additional subsets used in this analysis.