Hubble Space Telescope proper motions of Large Magellanic Cloud star clusters -- II. Kinematic structure of young and intermediate-age clusters

TL;DR

This paper uses high-precision HST proper motions and homogeneous isochrone fitting to derive 3D positions and velocities for 19 young and intermediate-age LMC clusters. By transforming to the LMC frame and combining with literature LOS velocities, it maps the full kinematic structure, revealing that most young clusters orbit near the disc while NGC 1850 shows a peculiar, distance-sensitive inclination. The analysis finds no compelling evidence for accretion from the SMC, with clusters aligning along the LMC age–metallicity relation. A set of simple dynamical models suggests that the observed kinematic patterns can be qualitatively explained by multiple recent LMC–SMC interactions, including two disc-crossings in the past 0.3–0.9 Gyr and a recent close passage, though the models are simplified and inner-disc dynamics require more detailed treatment.

Abstract

In this paper, we explore the kinematic properties of a sample of 19 young (<1 Gyr) and intermediate-age (1-2.5 Gyr) massive star clusters within the Large Magellanic Cloud (LMC). We analyse the proper motions of the clusters, which have been measured based on multi-epoch Hubble Space Telescope (HST) observations. Additionally, we infer from the HST data homogeneous and robust estimates for the distances, ages and metallicities of the clusters. This collection of information, in combination with literature line-of-sight velocities, allows us to investigate the full 3D dynamics of our sample of clusters within the frame of the LMC in a self-consistent way. While most young clusters orbit the LMC close to the stellar disc plane, NGC 1850 (~100 Myr old) depicts a peculiar case. Depending on the exact distance from the disc, it follows either a highly inclined, retrograde orbit or an eccentric orbit along the bar structure. The orbits of young clusters that formed North of the LMC centre show signs that might be connected to the resettling motion of the LMC bar structure. Based on the dynamic properties in combination with the positions of the clusters in the age-metallicity space, we find no clear-cut evidence for clusters in our sample that could have been stripped from the Small Magellanic Cloud (SMC) onto the LMC. We finally compare the kinematics of the intermediate-age clusters with a suite of simple numerical simulations of the Magellanic system to interpret the cluster motions. A possible interaction history of the LMC with the SMC, where the SMC had two past crossings of the LMC disc plane (about 300 and 900 Myr ago), in combination with the recent SMC pericentre passage, can qualitatively explain the observed kinematic structure of the clusters analysed in this work.

Figures (10)

• Figure 1: Velocities and positions of the young and intermediate-age clusters within the LMC as a function of the radial distance from the galaxy's centre, R. In each panel, the clusters with less robust isochrone fits are denoted with diamond symbols and grey error bars. Panel (a) presents the tangential velocity V$_{\phi}$. Theoretical circular velocities resulting from a pure NFW profile (orange solid line) and a model composed of a dark matter halo and a stellar disc and bar (green dashed line) are also shown. The size of the stellar bar is indicated by the vertical dashed line. Panel (b) shows the radial velocity V$_{R}$; the out-of-plane velocity V$_{Z}$ is presented in panel (c); and panel (d) shows the vertical distance from the plane Z. In all panels, the clusters are colour-coded by the logarithm of their ages (in years).
• Figure 2: Reconstructed orbits of the six young ($<$1 Gyr) clusters within the LMC. The orbits have been integrated backwards for the inferred lifetime of the clusters. The coloured circles represent their current positions within the galaxy and the black crosses indicate the birth places of the clusters. For orientation, the shown contours follow the density of the LMC field star population. The plot is oriented such that North is approximately to the top and East approximately to the left of the image.
• Figure 3: Age-metallicity relation of the young and intermediate-age clusters studied in this work. The clusters are colour-coded according to their cylindrical galactocentric distance R. Clusters with less robust isochrone fits are illustrated with diamond symbols and grey error bars. Also shown are analytical models of the chemical evolution of the LMC (orange solid line) and SMC (green dashed line), assuming a bursting star formation rate Pagel98.
• Figure 4: Predictions for three individual model realizations with different LMC--SMC orbital histories. The left column shows for each model the vertical out-of-plane distance (Z) while the right column presents the vertical out-of-plane velocity (V$_{Z}$). The rows, from top to bottom, show the following model realisations for which the LMC experiences different interaction events with the SMC: only an SMC pericentre passage about 150 Myr ago with an impact parameter Z$_{\mathrm{peri}}=-$6.6 kpc; only an SMC pericentre passage, but now more recently, about 110 Myr ago with a smaller impact parameter of only Z$_{\mathrm{peri}}=-$2 kpc; two events of the SMC crossing the disc plane of the LMC at $\sim$340 Myr and $\sim$910 Myr ago, in addition to the SMC pericentre passage of the SMC $\sim$150 Myr ago. In the bottom-left panel, the locations of the disc crossings are indicated with a blue and orange cross. The intermediate-age clusters in our sample are indicated by coloured circles, where the colour represents the measured out-of-plane distance Z (left panels), and the out-of-plane velocity V$_{Z}$ (right panels), respectively. Clusters with less robust isochrone fits are illustrated with diamond symbols. The black contours shown in all panels follow the stellar density of the LMC bar structure.
• Figure 5: Selection of cluster member stars using the example of NGC 1856. (a) The m$_{\rm F814W}$ vs m$_{\rm F438W}$--m$_{\rm F814W}$ CMD of well-measured stars in the field of the cluster for which PMs have been determined (b) 1-D relative PMs as a function of the m$_{\rm F814W}$ magnitude. The blue line (drawn by hand) follows our selection of likely cluster members (black dots) based on their motions. The dependence on the magnitude is to account for the varying PM uncertainties as a function of magnitude. (c) CMD of PM-selected cluster members. (d) CMD of PM-selected cluster stars, including only stars within one effective radius from the cluster centre.