Galaxies on the Scale express
Three distant galaxies spin like giant carousels. Their stars and gas move along spiral arms, and from the speed of this motion you can calculate how much total matter the galaxy contains — visible and invisible.
Astronomers combined two views: radio telescopes revealed cold gas, while the James Webb Space Telescope showed the stars. This allowed them to precisely measure how these three galaxies rotate. The outer parts spin as expected. But right at the center, the speed turned out to be higher than calculated. As if the carousel were spinning faster near its axis than it should be for its mass.
It is not yet clear which explanation is correct. But the very fact that we can peer into the center of a galaxy billions of light-years away and notice such a 'shortfall' in our calculations is a step toward understanding how the invisible part of the Universe is structured.
Original abstract
as written by the authors arXiv:2601.03338
The rotation curves of regularly rotating disc galaxies are a unique probe of the gravitational potential and dark matter distribution. Until recently, matter decomposition of rotation curves at $$z>0.5$$ was challenging, not only due to the lack of high resolution kinematic data but also of both suitable photometry to accurately trace the stellar surface density and spatially-resolved sub-mm observations to trace the cold gas distribution. In this paper, we analyse three galaxies from the Archival Large Program to Advance Kinematic Analysis (ALPAKA) sample, combining highly resolved cold gas observations from ALMA with rest-frame near-infrared imaging from JWST to investigate their dynamical properties and constrain their dark matter halos. The galaxies, initially classified as regularly rotating discs based on ALMA observations alone, appear in JWST as extended and symmetric stellar discs with spiral arms. Our dynamical models reproduce the rotation of the discs in the outer parts well, but they systematically underpredict the inner rotation velocities, revealing a deficit of central mass relative to the data. This discrepancy indicates either an underestimation of the bulge masses due to variations in the stellar mass-to-light ratio or dust attenuation or the presence of overmassive black holes. Alternatively, it may suggest departures from standard dark-matter halo profiles, including enhanced central concentrations.
The James Webb Space Telescope sees in infrared light, so dust that hides the galactic center from ordinary telescopes does not bother it.