Overview

• Researchers measured a mass of about 1.13 × 10^6 solar masses at an assumed redshift of 0.881, roughly 100 times lower than previous cosmological lensing detections.
• No light has been detected from the object at optical, infrared, or radio wavelengths, leaving open whether it is a dark-matter clump or a compact inactive dwarf galaxy.
• The signal was identified as a narrowing in a high-resolution radio arc in the lens system JVAS B1938+666 and was confirmed with both non-parametric and parametric models.
• The result is highly precise, with a 26σ detection significance, a 3.3% fractional mass uncertainty, and an astrometric uncertainty of 194 microarcseconds.
• Observations combined the GBT, VLBA, and EVN into an Earth-spanning array correlated at JIVE, and companion papers in Nature Astronomy and MNRAS outline plans to find more such objects to test cold dark matter predictions.