Kinematics of young star clusters in the outer north-eastern region of the Small Magellanic Cloud

TL;DR

This study probes whether the outer north-eastern region of the SMC exhibits kinematic disturbances induced by tidal interactions with the LMC by measuring 3D velocities of three young star clusters formed in situ. Using Gemini GMOS-S spectroscopy and Gaia DR3 proper motions, the author computes residual velocities $\Delta V$ relative to extrapolated rotating-disk models, revealing a gradient when adopting a young-disk reference but not for an old-disk reference. The anisotropy parameter $\beta$ indicates that the clusters’ kinematics resemble an older, more ordered rotation in some models, underscoring that multiple rotation disks can describe the same region depending on tracer population. The results corroborate that LMC tides affect the outer SMC irrespective of stellar age, and they caution against using outer-region kinematics alone to infer the epoch of interaction.

Abstract

It has been suggested since recent time that the magnitude of the interaction between galaxies could be measured from the level of kinematic disturbance of their outer regions with respect to the innermost ones. Here, I proved that the outer north-eastern region of the Small Magellanic Cloud (SMC), a relatively recent stellar structure with a tidal origin from the interaction with the Large Magellanic Cloud, is imprinted by a residual velocity pattern. I obtained from GEMINI GMOS spectra mean radial velocities of star clusters formed in situ, which added to derived mean proper motions and heliocentric distances, allowed to compute their 3D space velocity components. These space velocities differentiate from those that the clusters would have if they instead orderly rotated with the galaxy, i.e., their residual velocities are larger than the upper limit for an object pertaining to the SMC main body rotation disk. The level of kinematic disturbance depends on the SMC rotation disk adopted; galaxy rotation disks traced using relatively old objects are discouraged.The resulting kinematic disturbance arises in younger and older stellar populations, so that the epoch of close interaction between both Magellanic Clouds cannot be uncovered on the basis of the kinematics behavior of stellar populations populating the outer SMC

Figures (6)

• Figure 1: Left: Sky map with stars observed by SMASH (gray points) and those observed in this work (large magenta circles) in the field of HW 64; symbol size being proportional to the $g$ brightness of the star. Filled magenta circles represent cluster members (see details in Section 3). Right: SMASH color-magnitude diagram with the isochrone betal12 corresponding to the distance, reddening, metallicity and age of the cluster piatti2022d, superimposed.
• Figure 2: Same as Figure \ref{['fig1']} for IC 1655.
• Figure 3: Same as Figure \ref{['fig1']} for IC 1660.
• Figure 4: RV versus scaled CMD distance of stars in HW 64 (left panel), IC 1655 (middle panel), and IC 1660 (right panel), respectively. The magenta circles represent the selected clustered stars.
• Figure 5: $\Delta$$V$ versus heliocentric distance of young SMC clusters located in the north-eastern shell region. The solid and dashed vertical lines represent the galaxy center and its depth according to the cluster rotation disk piatti2021b, while the horizontal line represents a boundary between bound and kinematically perturbed clusters. Magenta and red filled circles represent the present studied clusters and those taken from the literature, respectively. A kinematic gradient is present when compared to a young stellar model (middle panel), but absent in comparison to the old stellar and cluster models (right and left panels), respectively (see text for details).