On the consistency of rotation curves and spatially integrated H <scp>i</scp> flux profiles

ABSTRACT Resolved rotation curves (RCs) are the gold-standard measurements for inferring dark matter distributions in Lambda cold dark matter 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 $\mathrm{H}\, {\small I}$ 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 $\mathrm{H}\, {\small I}$ surface density profiles from SPARC with spatially integrated $\mathrm{H}\, {\small I}$ 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-normalized mean squared error (MSE) below 10 – whilst no galaxy deemed symmetric has a MSE above 1.2. Most cases of good agreement prefer an $\mathrm{H}\, {\small I}$ gas dispersion $\sigma _{\mathrm{H\, {\small I}}}$ of $\sim$13 km s$^{-1}$, consistent with resolved studies of gas dispersion from the literature. These results open the door for spatially integrated $\mathrm{H}\, {\small I}$ flux profiles to be used as proxies for spatially resolved dynamics, including a robust appraisal of the degree of asymmetry.