Presence of red giant population in the foreground stellar sub-structure of the Small Magellanic Cloud

TL;DR

This study reveals a foreground stellar sub-structure in the eastern Small Magellanic Cloud, characterized by bimodal radial velocities and distances, likely formed during a recent tidal interaction approximately 307 million years ago.

Contribution

It provides the first detailed connection between bimodal radial velocity and distance distributions of RGB stars, elucidating the nature and origin of the foreground sub-structure in the SMC.

Findings

Bimodal RV distribution of RGB stars in eastern and SW regions.

Foreground RGB stars are separated by similar distances as RC stars.

Estimated formation time of the sub-structure is about 307 million years ago.

Abstract

The eastern region of the Small Magellanic Cloud (SMC) is found to have a foreground stellar sub-structure, which is identified as a distance bimodality (12 kpc apart) in the previous studies using Red Clump (RC) stars. Interestingly, studies of Red giant branch (RGB) stars in the eastern SMC indicate a bimodal radial velocity (RV) distribution. In this study, we investigate the connection between these two bimodal distributions to better understand the nature and origin of the foreground stellar sub-structure in the eastern SMC. We use the Gaia EDR3 astrometric data and archival RV data of RGB stars for this study. We found a bimodal RV distribution of RGB stars (separated by 35 - 45 km/s) in the eastern and south-western (SW) outer regions. The observed proper motion values of the lower and higher RV RGB components in the eastern regions are similar to those of the foreground and main-body RC stars respectively. This suggests that the two RGB populations in the eastern region are separated by a similar distance as those of the RC stars, and the RGB stars in the lower RV component are part of the foreground sub-structure. Based on the differences in the distance and RV of the two components, we estimated an approximate time of formation of this sub-structure as 307+/-65 Myr ago. This is comparable with the values predicted by simulations for the recent epoch of tidal interaction between the Magellanic Clouds. Comparison of the observed properties of RGB stars, in the outer SW region, with N-body simulations shows that the higher RV component in the SW region is at a farther distance than the main body, indicating the presence of a stellar Counter-Bridge in the SW region of the SMC.