MACADAM: measuring and calibrating AGN distances and masses

Abstract

High-signal-to-noise-ratio differential interferometric observations of active galactic nucleus broadline regions (AGN BLRs), combined with reverberation mapping, can provide direct angular-diameter distances through BLR parallax. Such measurements would complement standard candles and standard rulers, with applications to the Hubble tension, distance duality, sound-horizon calibration, and dark-energy evolution. Current and near-term VLTI instruments do not simultaneously provide the spectro-astrometric precision, fringe-tracking sensitivity, and target statistics required for percent-level cosmography. MACADAM (Measuring And Calibrating AGN Distances and Masses) is a compact visitorinstrument concept specifically optimized for line-differential interferometry on unresolved or marginally resolved AGNs in the 𝐽and 𝐻 bands. It combines hierarchical fringe tracking with science beam combiners that concentrate the available flux into the most informative long baselines. Broadband fringe tracking over 𝐽, 𝐻, and short-𝐾 is enabled by a wavelength-distribution and modeselection unit based on dispersed pupils projected onto linear MEMS micromirror arrays. This unit routes selected spectral intervals to single-band integrated-optics chips while correcting transverse chromatic dispersion and residual tip–tilt. Population simulations indicate that a fringe-tracking limit near 𝐾 = 16 could provide statistically significant BLR-parallax samples in several redshift bins up to 𝑧 ∼ 1. A preliminary optical design shows that the required functions can be implemented within the VLTI visitor-instrument volume using largely demonstrated technologies. The ultimate cosmographic accuracy will depend critically on improved BLR modelling and systematic-error control.