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

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

Problem Statement
Rotation curves of regularly rotating disc galaxies serve as a primary probe for gravitational potentials and dark matter distributions. While mass decomposition is well-established in the local Universe ($z=0$) using neutral hydrogen (HI) and high-resolution photometry, extending these analyses to $z > 0.5$ ("cosmic noon") has been historically challenging. Previous studies at these redshifts relied on warm ionized gas tracers (e.g., H$\alpha$, [OII]), which may be influenced by non-circular motions driven by feedback or AGN, and lacked spatially resolved rest-frame near-infrared (NIR) imaging to accurately constrain stellar surface densities. Furthermore, the bulk of the gas mass (cold atomic and molecular) was often estimated via scaling relations rather than direct observation, introducing significant uncertainties in baryonic mass decomposition.

Methodology
This study analyzes three regularly rotating disc galaxies from the ALMA Archival Large Program to Advance Kinematic Analysis (ALPAKA) sample, selected for their high-quality cold gas kinematics and available JWST/NIRCam data. The redshifts of the targets are $z=0.561$ (ID1), $z=1.445$ (ID3), and $z=2.103$ (ID13).

The methodology combines:

1. Cold Gas Kinematics: ALMA observations of CO or [CI] emission lines provide spatially resolved moment 0 maps and rotation velocity profiles, corrected for asymmetric drift to derive circular speeds ($V_c$).
2. Stellar Photometry: JWST/NIRCam imaging in rest-frame NIR filters (F277W and F444W) traces the bulk of the stellar populations with minimal dust attenuation.
3. Surface Brightness Modeling:
   • Gas: Modeled as a single exponential disc using a Bayesian nested-sampling approach (DYNESTY) on ALMA moment 0 maps.
   • Stars: Modeled using GALFIT with a multi-component approach (exponential disc + de Vaucouleurs bulge + sky background), accounting for foreground/background sources where necessary.
4. Mass Decomposition: The authors construct dynamical models assuming axisymmetric potentials. The total circular velocity is the quadrature sum of contributions from a stellar bulge, stellar disc, gas disc, and a spherical Navarro–Frenk–White (NFW) dark matter halo.
   • Fiducial Models: The total stellar mass ($M_*$), total gas mass ($M_{gas}$), and dark matter virial mass ($M_{200}$) are treated as free parameters, constrained by priors on the mass-to-light ratio ($\Upsilon$) difference between bulge and disc (fixed at 1.4) and the concentration-mass relation.
   • Fixed Baryonic Models: A secondary analysis fixes baryonic parameters to literature values (SED-derived masses, standard $\alpha_{CO/[CI]}$ conversion factors) to attempt constraining the dark matter halo properties directly.

Key Results

1. Rotation Curve Discrepancy: The fiducial dynamical models successfully reproduce the rotation curves in the outer regions of all three galaxies. However, they systematically underpredict the observed circular velocities in the inner regions (within the effective radius).
2. Mass Constraints:
   • In the fiducial models, stellar masses are well-constrained ($\log(M_*/M_\odot) \approx 10.6 - 10.9$), while gas and dark matter masses remain degenerate due to the limited radial extent of the data relative to the halo scale.
   • The derived dark matter fractions within the stellar effective radius are low ($f_{DM} < R_{eff} \approx 0.2$), consistent with values found in local massive discs and other high-$z$ samples.
3. Failure of Fixed Baryonic Models: When baryonic parameters are fixed to literature values, the models struggle to reproduce the inner rotation velocities. For ID1 and ID3, the models require unphysically high dark matter halo concentrations ($c_{200} > 50$) to fit the inner data. For ID13, the fixed baryonic mass leads to a physically unfeasible dark matter halo mass ($\log(M_{200}/M_\odot) \approx 10.4$) that is lower than the baryonic mass.
4. Investigation of Central Mass Deficit: The authors explore several physical mechanisms to explain the inner velocity deficit:
   • Massive Bulges: Increasing the bulge-to-disc mass-to-light ratio ($\Upsilon_b/\Upsilon_d$) from 1.4 to 1.6 improves the fit, potentially due to older stellar populations or dust attenuation in the bulge.
   • Supermassive Black Holes (SMBHs): Including SMBHs following local mass relations does not resolve the discrepancy. However, SMBHs with masses $\sim 1.5\% - 4.4\%$ of the total stellar mass (consistent with recent high-$z$ JWST findings) could account for the inner velocities, though they may not be sufficient alone.
   • Metallicity Gradients: A positive metallicity gradient (implying a negative $\alpha_{CO}$ gradient) was tested but found to have a negligible effect on the rotation curve shape.

Significance and Claims
The paper claims to present the first mass decomposition of galaxies at $z > 0.5$ using robust, spatially resolved constraints on both cold gas kinematics (ALMA) and stellar distributions (JWST). The primary significance lies in the finding that standard NFW dark matter halos, combined with plausible baryonic configurations (e.g., massive bulges or overmassive black holes), can reproduce the observed rotation curves without invoking dark matter cores.

The authors caution that previous claims of "cored" dark matter profiles at $z > 1$, based on warm gas tracers and scaling relations, should be treated with extreme caution. Their analysis suggests that the low dark matter fractions observed are consistent with standard NFW profiles, and the inner velocity discrepancies likely arise from underestimations of central baryonic mass (due to dust or stellar population variations) or the presence of massive central black holes, rather than a fundamental departure from CDM halo structures. The study highlights the necessity of high-resolution cold gas and NIR data to break the degeneracy between baryonic and dark matter components in high-redshift galaxies.