No evidence for Keplerian taper of far-out galactic rotation

TL;DR

This study analyzes 175 galactic rotation curves to test if the observed data supports a Keplerian taper at large radii, finding no evidence for such tapering and supporting flat rotation curves instead.

Contribution

It provides a model-independent statistical test of the rotation curve slope, challenging the expectation of Keplerian decline in galactic halos.

Findings

Data is compatible with flat rotation curves, showing no Keplerian taper.

Milky Way data indicates a clear fall-off in rotation velocity at large radii.

Supports the hypothesis of non-convergent mass distribution in galactic halos.

Abstract

We present a statistical analysis of the 175 SPARC galactic rotation curves to test the hypothesis of whether the Keplerian velocity tapering at large radii ($V(r)\propto 1/\sqrt{r}$) germane to a convergent mass distribution in typical spherical halo models agrees with observational data. The null hypothesis is Rubin's flat-rotation curve, $V(r)=\text{constant}$ -such as can be obtained from a spherical, isothermal-like density profile, or alternatively with a very prolate halo-. To decide whether we adopt the null (Rubin behaviour) or alternative (Keplerian behaviour) hypothesis, we evaluate the derivative in each galaxy of $V(r)$ with its last data points. The test is model independent inasmuch we are testing for the \emph{slope} of the dark matter rotation curve, whether it is or not compatible with zero. We conclude that the data is presently compatible with the null hypothesis -no taper off, no decline of $V(r)$ is seen. Separately, beyond SPARC, our own Milky Way galaxy, for which recent data sets have been reported, does show clear $V(r)$ fall-off at the level of 20\%.