Fast rotations in galaxies at cosmic noon indicate central concentration of stars, dark matter or massive black holes

Fernanda Roman-Oliveira, Francesca Rizzo, Filippo Fraternali

First listed 2026-01-06 | Last updated 2026-02-05

Abstract

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.

Short digest

Three ALPAKA discs at z=0.56, 1.45, and 2.10 are modeled by combining ALMA CO/[CI] kinematics (CO(2-1), CO(5-4), [CI] 3P2–3P1) with JWST/NIRCam rest‑NIR imaging (F277W/F444W) to decompose their rotation curves. The models fit the outer discs but systematically underpredict inner speeds, revealing a central mass excess beyond stars+cold gas. The authors attribute this to underestimated bulge masses from M/L variations or dust, or to overmassive black holes; alternatively, dark‑matter halos may be more centrally concentrated than standard profiles. This is among the first robust baryon‑anchored decompositions at z>0.5, pointing to early central mass build‑up at cosmic noon.

Key figures to inspect

• Rotation‑curve mass decompositions for the three ALPAKA galaxies: inspect the inner few kpc where model curves fall below the data to gauge the size of the velocity excess per galaxy.
• JWST/NIRCam morphology (F277W for the z=0.56 disc; F444W for the z=1.45 and z=2.10 discs): check spiral structure, central light concentration, and dust lanes that could bias bulge M/L.
• ALMA moment‑0 maps and velocity fields with beam ellipses: verify disc regularity, radial extent, and the number of independent resolution elements along the major axis (≈3.8/4.2/4.2) feeding the kinematic fits (3D BAROLO).
• Halo‑model comparison panels (e.g., NFW vs enhanced‑concentration variants): assess how inner density slope/concentration shifts reduce residuals in the central rotation.
• Tests adding a central point mass or radial M/L gradients: see how much BH mass or M/L increase is required to match the observed inner velocities.