Gaia view of a stellar sub-structure in front of the Small Magellanic Cloud

Abstract

Recent observational studies identified a foreground stellar sub-structure traced by red clump (RC) stars (∼12 kpc in front of the main body) in the eastern regions of the Small Magellanic Cloud (SMC) and suggested that it formed during the formation of the Magellanic Bridge (MB), due to the tidal interaction of the Magellanic Clouds. Previous studies investigated this feature only up to 4∘.0 from the centre of the SMC due to the limited spatial coverage of the data and hence could not find a physical connection with the MB. To determine the spatial extent and properties of this foreground population, we analysed data from the Gaia Data Release 2 (DR2) of a ∼314 deg2 region centred on the SMC, which cover the entire SMC and a significant portion of the MB. We find that the foreground population is present only between 2∘.5 to ∼5°–6° from the centre of the SMC in the eastern regions, towards the MB and hence does not fully overlap with the MB in the plane of the sky. The foreground stellar population is found to be kinematically distinct from the stellar population of the main body with ∼35 km s−1 slower tangential velocity and moving to the north-west relative to the main body. Though the observed properties are not fully consistent with the simulations, a comparison indicates that the foreground stellar structure is most likely a tidally stripped counterpart of the gaseous MB and might have formed from the inner disc (dominated by stars) of the SMC. A chemical and 3D kinematic study of the RC stars along with improved simulations, including both tidal and hydro-dynamical effects, are required to understand the offset between the foreground structure and MB.