On the consistency of rotation curves and spatially integrated HI flux profiles

TL;DR

This study demonstrates that spatially integrated HI flux profiles can reliably approximate resolved rotation curves in galaxies, enabling dynamical analysis when detailed data is unavailable, and introduces a new asymmetry metric for flux profiles.

Contribution

It develops a forward model linking rotation curves and HI flux profiles, validating the use of flux profiles as proxies for galaxy dynamics and introducing a new asymmetry metric.

Findings

Most galaxies show good agreement between RCs and flux profiles with low MSE.

Symmetric galaxies have MSE below 1.2, indicating high consistency.

Optimal HI gas dispersion is around 13 km/s, aligning with previous studies.

Abstract

Resolved rotation curves (RCs) are the gold-standard measurements for inferring dark matter distributions in $\Lambda$CDM and testing alternative theories of dynamics in galaxies. However they are expensive to obtain, making them prohibitive for large galaxy samples and at higher redshift. Spatially integrated HI flux profiles are more accessible and present the information in a different form, but -- except in a highly compressed form, as linewidths -- have not so far been compared in detail with RCs or employed for dynamical inferences. Here we study the consistency of RCs and HI surface density profiles from SPARC with spatially integrated HI flux profiles from ALFALFA, by combining the resolved properties in a forward model for the flux profile. We define a new metric for asymmetry in the flux profiles, enabling us to cleanly identify those unsuitable for our axisymmetric method. Among all SPARC galaxies the agreement between RCs and flux profiles is satisfactory within the limitations of the data -- with most galaxies having an uncertainty normalised mean squared error (MSE) below 10 -- while no galaxy deemed symmetric has a MSE above 1.2. Most cases of good agreement prefer a HI gas dispersion $\sigma_{\mathrm{HI}}$ of ~13 km/s, consistent with resolved studies of gas dispersion from the literature. These results open the door for spatially integrated HI flux profiles to be used as proxies for spatially resolved dynamics, including a robust appraisal of the degree of asymmetry.