The Next Generation Virgo Cluster Survey XVI. The Angular Momentum of Dwarf Early-Type Galaxies from Globular Cluster Satellites

TL;DR

This study investigates the rotation and velocity dispersion of globular cluster systems in six dwarf early-type galaxies in the Virgo cluster, revealing that rotation detection is challenging and often biased by observational limitations.

Contribution

It provides a detailed analysis of the kinematics of GC systems in dwarf galaxies, highlighting the importance of data quality and the potential for overestimating rotation.

Findings

Some galaxies show signs of rotation, but statistical significance is often marginal.

Measurement biases depend on the number of GCs and velocity uncertainties.

High-quality data with small uncertainties are crucial for reliable kinematic measurements.

Abstract

We analyze the kinematics of six Virgo cluster dwarf early-type galaxies (dEs) from their globular cluster (GC) systems. We present new Keck/DEIMOS spectroscopy for three of them and reanalyze the data found in the literature for the remaining three. We use two independent methods to estimate the rotation amplitude (Vmax) and velocity dispersion (sigma_GC) of the GC systems and evaluate their statistical significance by simulating non-rotating GC systems with the same number of GC satellites and velocity uncertainties. Our measured kinematics agree with the published values for the three galaxies from the literature and, in all cases, some rotation is measured. However, our simulations show that the null hypothesis of being non-rotating GC systems cannot be ruled out. In the case of VCC1861, the measured Vmax and the simulations indicate that it is not rotating. In the case of VCC1528, the null hypothesis can be marginally ruled out, thus, it might be rotating although further confirmation is needed. In our analysis, we find that, in general, the measured Vmax tends to be overestimated and the measured sigma_GC tends to be underestimated by amounts that depend on the intrinsic Vmax/sigma_GC, the number of observed GCs (N_GC), and the velocity uncertainties. The bias is negligible when N_GC>~20. In those cases where a large N_GC is not available, it is imperative to obtain data with small velocity uncertainties. For instance, errors of <2km/s lead to Vmax<10km/s for a system that is intrinsically not rotating.