A SkyMapper view of the Large Magellanic Cloud: the dynamics of stellar populations

Abstract

We present the first SkyMapper stellar population analysis of the Large Magellanic Cloud (hereafter LMC),including the identification of 3578 candidate Carbon Stars through their extremely red g − r colours. Coupled with Gaia astrometry, we analyse the distribution and kinematics of this Carbon Star population, finding the LMC to be centred at (R.A., Dec.) = (80.90 • ±0.29, −68.74 • ±0.12), with a bulk proper motion of (µ α , µ δ ) = (1.878 ± 0.007, 0.293 ± 0.018) mas yr −1 and a disk inclination of i = 25.6 • ± 1.1 at position angle θ = 135.6 • ± 3.3 • . We complement this study with the identification and analysis of additional stellar populations, finding that the dynamical centre for Red Giant Branch (RGB) stars is similar to that seen for the Carbon Stars, whereas for young stars the dynamical centre is significantly offset from the older populations. This potentially indicates that the young stars were formed as a consequence of a strong tidal interaction, probably with the Small Magellanic Cloud (SMC). In terms of internal dynamics, the tangential velocity profile increases linearly within ∼ 3 kpc, after which it maintains an approximately constant value of V rot = 83.6 ± 1.7 km s −1 until ∼ 7 kpc. With an asymmetric drift correction, we estimate the mass within 7 kpc to be M LMC (< 7 kpc) = (2.5±0.1)×10 10 M and within the tidal radius (∼ 30 kpc) to be M LMC (< 30 kpc) = (1.06 ± 0.32) × 10 11 M , consistent with other recent measurements.

The Magellanic Edges Survey I: Description and first results

“…The radial velocity dispersion (σ R ) in field 18 (24.4 ± 2.7km s −1 ) is, within uncertainty, equal to the azimuthal velocity dispersion. This is consistent with the behaviour reported in Vasiliev (2018) and Wan et al (2020). The magnitude of the radial dispersion measured here is again somewhat smaller than that reported in Wan et al (2020); the difference can be attributed to the same reasons outlined above for the azimuthal velocity dispersion.…”

“…However, various studies of the Clouds have reported COM positions which differ by up to a degree on the sky, depending on the chosen tracer (see e.g. Wan et al 2020). Given that our sample is primarily red clump stars, for consistency we adopt the COM position reported by van der Marel & Kallivayalil (2014), for their 'PMs+Old v LOS Sample': i.e.…”

“…Vasiliev 2018;Guiglion et al 2015;Noordermeer et al 2008). As the MagES fields are situated at substantially larger galactocentric radii (∼10.5 • from the LMC COM) than the Wan et al (2020) data, it is reasonable that the azimuthal velocity dispersion in the MagES fields is correspondingly smaller.…”

“…van der Marel & Kallivayalil 2014; Vasiliev 2018); though some more recent studies suggest ∼25 • (e.g. Wan et al 2020;Choi et al 2018). However, all such measurements have been derived using stars at much smaller radial distances from the LMC COM than even the innermost of our fields.…”

“…This has allowed substructures to be kinematically traced out to 25 • from the centre of the Clouds (e.g. Belokurov et al 2017), and detailed analyses of internal LMC dynamics based on proper motions (Gaia Collaboration et al 2018c;Vasiliev 2018;Wan et al 2020) to be performed. However, the Clouds are sufficiently distant that the Gaia spectrograph does not reach the faint magnitudes required to provide line-of-sight velocities for the old stellar populations in the Clouds that comprise the peripheral substructures.…”