Consequences of radially correlated rotation curves for galaxy mass models

TL;DR

This study demonstrates that considering radially correlated rotation curve data improves galaxy mass model fits and alters the inferred preference between dark matter halo profiles, emphasizing the importance of accounting for data correlations.

Contribution

It introduces a data-driven method to incorporate rotation curve correlations into galaxy mass modeling, improving fit quality and changing model preference conclusions.

Findings

Including correlations yields better fit quality ($\,\chi^2_\mathrm{r}\approx 1$).

Correlation parameters are physically plausible, around 20 km/s and 5 kpc.

Ignoring correlations biases the preference towards pseudo-isothermal models.

Abstract

Consecutive points in rotation curve measurements are correlated with each other, but this is usually ignored when constructing galaxy mass models. We apply the data-driven approach proposed by Posti (2022) to include the characteristic amplitude and scale length of such correlations as `nuisance parameters'. We construct mass models for $134$ galaxies from the SPARC rotation curve compilation with Navarro-Frenk-White (NFW) and pseudo-isothermal sphere (pISO) models for the dark halo. Allowing for correlations in the rotation curves generally improves the goodness of fit for both halo models, often yielding a formally good fit ($\chi^2_\mathrm{r}\approx 1$) and model uncertainties that seem more representative of the constraining power of the data. For both halo models the inference on the typical correlation amplitude and scale length are very similar and physically plausible, $\sim 20\,\mathrm{km}\,\mathrm{s}^{-1}$ and $\sim 5\,\mathrm{kpc}$, respectively. The parametric form that we use to describe the correlations is intentionally simple, and our fitting approach makes the parameters describing possible correlations prone to `absorbing' other systematic errors, so we regard these estimates as upper limits. Without allowing for correlations we find a statistical preference for the pISO over the NFW model for $88$/$134$ galaxies; this preference essentially disappears when correlations are allowed for. Accounting for correlations in rotation curves when constructing mass models fundamentally affects how they are interpreted, highlighting an important systematic uncertainty that affects evidence for cusps or cores in dark matter haloes.