Balloon-borne telescope delivers weak-lensing catalogues for 30 merging galaxy clusters

A telescope carried into near space by a balloon has produced catalogues of galaxy shapes for 30 merging galaxy clusters. These shapes are the raw material astronomers need to measure how such clusters bend light. Writing in The Astrophysical Journal, the authors describe the data gathered by the Super-pressure Balloon-Borne Imaging Telescope, or SuperBIT, during a 45-night flight in April and May 2023. They also describe the checks they ran to show the measurements are reliable. The paper reports no new conclusions about the clusters themselves. Its contribution is the dataset and the evidence that it is clean enough to build on.

The technique at stake is weak gravitational lensing. Gravity bends light, so a large mass in the foreground slightly distorts the apparent shapes of galaxies far behind it. Galaxy clusters are vast gatherings of galaxies bound together by gravity, and merging clusters are ones caught in the act of colliding. The distortion such a cluster produces is usually too faint to see in any single background galaxy, because each galaxy has its own random shape and orientation. A coherent stretching, known as shear, appears only when the shapes of many faint galaxies are averaged together. That subtlety is what makes the method demanding. Anything else that distorts the images, from the telescope optics to the camera, can mimic the signal or hide it.

Ground-based telescopes must look through a turbulent atmosphere that blurs their images. According to the study, SuperBIT is a first-of-its-kind balloon-borne imaging telescope. It operated above 98% of Earth's atmosphere and achieved near-diffraction-limited observations, meaning images close to the sharpest its optics can physically deliver. Every telescope still spreads each point of light into a small smudge, a pattern described by its point-spread function, or PSF. That pattern has to be modelled and accounted for before galaxy shapes can be measured.

The team measured shapes with an algorithm called metacalibration. It calibrates measurements by applying small, known distortions to the images and recording how the measured shapes respond. They applied it to the target clusters and to several calibration fields. Throughout the analysis they ran diagnostics. They checked how well the PSF model matched the data and how any mismatch would affect the lensing measurements. They also tested whether galaxy shapes were correlated with properties of the galaxies or of the PSF. Such correlations would hint that something other than gravity was shaping the results.

The key quantity is the multiplicative shear bias, a systematic tendency to overestimate or underestimate the shear by a fixed fraction. To measure it, the authors built a parallel set of simulated images that reproduced the real flight conditions, including the measured SuperBIT PSFs, the observed sky backgrounds and the detector noise. They report a bias of 1.54%, with an uncertainty of 6.00%, so the result is consistent with zero.

The uncertainty is nearly four times larger than the central value, so the simulations limit the pipeline's bias only to within about six percentage points either way. The paper is the third in a series, and the catalogues are a starting point rather than an end. According to the abstract, they are designed to allow an unbiased reconstruction of the lensing signal around each cluster. What that reconstruction shows about the 30 merging systems is left to later work.