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

TL;DR

This study uses ALMA CO/[CI] kinematics from the ALPAKA project and JWST NIRCam imaging to perform a robust rotation-curve decomposition for three cosmic-noon disc galaxies. The authors fit gas and stellar surface-density profiles, apply asymmetric-drift corrections, and model the rotation curves with bulge, disc, gas, and an NFW dark-matter halo, testing both fiducial (free-baryon) and fixed-baryon scenarios. They find outer rotation curves are well reproduced, but inner velocities exceed fiducial predictions, suggesting either a more concentrated central mass (older, dust-obscured bulges or overmassive black holes) or alternative halo structures; however, DM constraints remain weak due to disc-halo degeneracies. The results imply standard NFW halos can describe these z~1–3 discs within uncertainties, with central mass concentrations largely driven by baryons, and highlight the need for improved gas mass calibrations and higher-resolution data to tighten DM inferences. This work demonstrates the power of combining ALMA and JWST data to dissect mass budgets in high-redshift galaxies and guides future studies of dark matter halos during cosmic noon.

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.

Figures (20)

• Figure 1: Top row: ALPAKA subsample of galaxies analysed in this work. The galaxies are organised in ascending order of redshift, from left to right. The gas data (see Table \ref{['tab:data']}) are shown as blue contours with levels following $3 \sigma$, $6 \sigma$ and $12 \sigma$, where $\sigma$ is the root-mean-square (RMS) noise of the ALMA moment 0th map of the gas emission. We also show negative emission with dashed contours at the $-3\sigma$ level and the corresponding ALMA beam on the bottom left of each panel. In the background, we display the JWST filter used for deriving the stellar surface brightness, same as the one reported in Table \ref{['tab:data']}. The ALMA contours shown here are simply the emission levels on the unmasked 0th-moment maps, therefore they differ from the pseudo-X $\sigma$ contours shown in rizzo23. Bottom row: RGB composite JWST images. We note that the scales in the top and bottom rows are different with a larger FOV on the bottom panels that allow us to see the foreground and background galaxies surrounding ID3 and ID13.
• Figure 2: Surface brightness models of the gas (top panels) and stellar distribution (bottom panels) of ID1. Top left panel: the ALMA moment 0 map is overlayed with yellow contours for the data and orange for the best-fit model, following levels of 3$\sigma$, 6$\sigma$, 9$\sigma$, etc., where $\sigma$ is the RMS noise in the data. We also show negative contours with dashed gray lines at levels $-3\sigma$ and $-6\sigma$. Top middle panel: noise-normalised residual map defined by (Data $-$ Model) / RMS. Top right panel: 1D representation of the projected average surface brightness over inclined rings along the major axis of the galaxy. The yellow band represents the data and its uncertainties while the orange solid line represents the best-fit model. Bottom left panel: JWST image in grayscale is overlayed with teal contours for the data and blue for the best-fit model, following levels of 3$\sigma$, 9$\sigma$, 27$\sigma$, etc. above the background, where $\sigma$ is the RMS noise in the data. Bottom middle panel: noise-normalised residual map defined by (Data $-$ Model) / RMS. Bottom left panel: 1D representation of the projected average surface brightness over inclined rings along the major axis of the galaxy. The teal band represents the data and the blue solid line represents the total best-fit model which is composed by a stellar disc (dotted dark blue line) and stellar bulge (dashed red line). In both top and bottom right panels we show the circularised ALMA beam/JWST PSF and the data RMS as vertical dotted and horizontal dashed lines, respectively.
• Figure 3: Surface brightness models of the gas (top panels) and stellar distribution (bottom panels) of ID3. The panel description is the same as Figure \ref{['fig:sb_id1']}. We note that there are foreground/background galaxies seen visible in the near-infrared (bottom left panel) that are fitted simultaneously to the target galaxy.
• Figure 4: Surface brightness models of the gas (top panels) and stellar distribution (bottom panels) of ID13. The panel description is the same as Figure \ref{['fig:sb_id1']} and, similar to ID3, this galaxy is also surrounded by foreground/background galaxies that are fitted simultaneously.
• Figure 5: Fiducial mass models. The observed circular speed is shown in dark blue points and the best-fit model of the total contribution of all baryonic components and dark matter (DM) is shown as a solid black line. The individual components are shown separately: the stellar bulge and disc in red and blue dash-dotted lines, respectively; the gas disc in dashed orange lines; and the NFW dark matter halo in solid green lines. The uncertainties are shown as a shaded band around each component, data and model, indicating the corresponding 16th and 84th percentiles.