Kinematic sub-populations in dwarf spheroidal galaxies

TL;DR

This study analyzes spectroscopic data of stars in the Canes Venatici I dwarf galaxy to assess claims of multiple stellar populations, finding no evidence for such sub-populations and emphasizing caution in their detection.

Contribution

The paper provides a detailed analysis of stellar kinematics in CVnI, challenging previous claims of multiple populations and discussing the limitations of small sample sizes.

Findings

No evidence for kinematically distinct populations in CVnI

Mass estimate consistent with previous data

Detection of sub-populations in small samples is unreliable

Abstract

We present new spectroscopic data for twenty six stars in the recently-discovered Canes Venatici I (CVnI) dwarf spheroidal galaxy. We use these data to investigate the recent claim of the presence of two dynamically inconsistent stellar populations in this system (Ibata et al., 2006). We do not find evidence for kinematically distinct populations in our sample and we are able to obtain a mass estimate for CVnI that is consistent with all available data, including previously published data. We discuss possible differences between our sample and the earlier data set and study the general detectability of sub-populations in small kinematic samples. We conclude that in the absence of supporting observational evidence (for example, metallicity gradients), sub-populations in small kinematic samples (typically fewer than 100 stars) should be treated with extreme caution, as their detection depends on multiple parameters and rarely produces a signal at the 3sigma confidence level. It is therefore essential to determine explicitly the statistical significance of any suggested sub-population.