The velocity dispersion of LMC Carbon stars: possible detection of a kinematically distinct population

Abstract

We analyze the velocity residuals of 551 carbon stars relative to a rotating-disk model of the inner $\sim 70 \rm deg^2$ of the Large Magellanic Cloud (LMC). We find that the great majority of the stars in this sample are best fit as being due to two different populations, a young disk population containing 20% of the stars with a velocity dispersion of $8 \kms$, and an old disk containing the remaining stars with a velocity dispersion of $22 \kms$. The young disk population has a metallicity $\sim 0.25$ dex higher than the old disk. With less certainty, the data also suggest at the $2σ$ level that there may be a third kinematically distinct population that is moving towards us at 30 km/sec relative to the LMC, consistent with measurements of 21 cm velocities. If real, this population contains about 7% of the carbon stars in the sample. It could be a feature in the disk of the LMC or it could be tidal debris in the foreground or background. If it is tidal debris, this population could account for some or all of the microlensing events observed towards the LMC.