Kinematical small-scale fluctuations do not affect the measurement of the dynamical mass of galaxies

Abstract

The stellar kinematics of low-mass galaxies are usually observed to be very unsmooth with significant kinematical fluctuations in small scales, which cannot be consistent with the projected centrosymmetric stellar kinematics obtained from commonly used dynamical models. In this work, we aim to test whether the high degree of kinematical fluctuations affects the dynamical mass estimate of galaxies. We use the asymmetry parameter $\eta$ obtained from the $180^{\circ }$ rotation self-subtraction of stellar kinematics of galaxies to quantify the degree of kinematical small-scale fluctuations. We use TNG50 numerical simulation to construct a large sample of mock galaxies with known total masses, and then obtained the virial dynamical mass estimator of these mock galaxies. We find that the dynamical masses within three-dimensional $R_{\mathrm{e}}$ to the mock galaxy centres are overall averagely accurate within around 0.1 dex under the symmetric assumption, while $R_{\mathrm{e}}$ means the projected circularized half-stellar mass radius in this work. We study the local virial mass estimation bias for mock galaxies of different $\eta$. The maximum bias difference of two $\eta$ bins is around 0.16 dex, which with other local biases may help apply the observational virial mass estimators obtained from massive galaxies to other types of galaxies. We find that the Spearman's $\rho$ of $\eta$ with the intrinsic mass estimation deviations is near zero if the local bias is eliminated properly. The results indicate that even for low-mass galaxies, the existence of high degree of kinematical small-scale fluctuations does not affect the measurement of the dynamical mass of galaxies.

9 pages, 8 figures, accepted for publication in MNRAS

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