Abstract
We present new spectroscopic data for 26 stars in the recently discovered Canes Venatici I (CVn I) dwarf spheroidal galaxy, obtained with the Gemini Multi-Object Spectrograph North (GMOS-N) on the Gemini North telescope. We use these data to investigate the recent claim of the presence of two dynamically inconsistent stellar populations in this system. While we find no evidence for kinematically distinct subpopulations in our sample, we also show that the available kinematic data set in CVnI is likely too small to draw robust conclusions about its subpopulations. We are, however, able to obtain a mass estimate for CVn I 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 subpopulations in small kinematic samples. We conclude that, in the absence of additional supporting observational evidence (e.g. metallicity gradients), subpopulations in small kinematic samples (typically fewer than 100 stars) should be treated with caution, as their identification depends on multiple parameters and rarely produces a signal at a high confidence level. It is therefore essential to explicitly determine the statistical significance of any suggested subpopulation.
Highlights
It is widely accepted that dwarf spheroidal galaxies are the most dark matter dominated stellar systems known in the Universe (e.g. Mateo 1998)
28 stars in the Canes Venatici I (CVn I) dwarf spheroidal (dSph) were observed on 2007 March 26 and 2007 April 7 and 8 using the Gemini Multi-Object Spectrograph North (GMOS-N) spectrograph mounted on the Gemini North telescope
We have presented a new data set of velocities and metallicities for the CVn I dSph, based on spectra taken with the GMOS-N spectrograph on the Gemini North telescope
Summary
It is widely accepted that dwarf spheroidal (dSph) galaxies are the most dark matter dominated stellar systems known in the Universe (e.g. Mateo 1998). Among the significant differences between the halo and the dSphs, the more important chemical differences are in the α-elements (Unavane, Wyse & Gilmore 1996; Venn et al 2004) It appears that the primordial dwarf satellites, which many models assume existed and were disrupted to form the Milky Way halo, must have had stellar populations distinct from those seen in the present-day dSphs (Robertson et al 2005; Font et al 2006). Ibata et al (2006) presented spectra for a sample of CVn I member stars obtained using the Deep Imaging Multi-Object Spectrograph (DEIMOS) spectrograph mounted on the Keck telescope They identified two kinematically distinct stellar populations in this data set: a spatially extended metal-poor population with high velocity dispersion and a centrally concentrated metal-rich population with a dispersion of almost zero.
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