Abstract

Context. Understanding the evolutionary history of the Magellanic Clouds requires an in-depth exploration and characterization of the stellar content in their outer regions, which ultimately are key to tracing the epochs and nature of past interactions. Aims. We present new deep images of a shell-like overdensity of stars in the outskirts of the Small Magellanic Cloud (SMC). The shell, also detected in photographic plates dating back to the fifties, is located at ∼1.9° from the center of the SMC in the north-east direction. Methods. The structure and stellar content of this feature were studied with multiband, optical data from the Survey of the MAgellanic Stellar History (SMASH) carried out with the Dark Energy Camera on the Blanco Telescope at Cerro Tololo Inter-American Observatory. We also investigate the kinematic of the stars in the shell using the Gaia Data Release 2. Results. The shell is composed of a young population with an age ∼150 Myr, with no contribution from an old population. Thus, it is hard to explain its origin as the remnant of a tidally disrupted stellar system. The spatial distribution of the young main-sequence stars shows a rich sub-structure, with a spiral arm-like feature emanating from the main shell and a separated small arc of young stars close to the globular cluster NGC 362. We find that the absolute g-band magnitude of the shell is Mg, shell = −10.78 ± 0.02, with a surface brightness of μg, shell = 25.81 ± 0.01 mag arcsec−2. Conclusion. We have not found any evidence that this feature is of tidal origin or a bright part of a spiral arm-like structure. Instead, we suggest that the shell formed in a recent star formation event, likely triggered by an interaction with the Large Magellanic Cloud and or the Milky Way, ∼150 Myr ago.

Highlights

  • The Magellanic Clouds are the largest satellites of the Milky Way (MW), and the only irregular galaxies in its immediate surroundings

  • We study the structure, stellar and gas content, kinematics and possible formation scenarios of this overdensity by means of its resolved stellar populations as traced by the imaging data obtained in the Survey of the MAgellanic Stellar History (SMASH, Nidever et al 2017) and the proper motions available from the Gaia Data Release 2 (DR2; Gaia Collaboration et al 2018a)

  • Our analysis of the resolved stellar populations of the elongated overdensity detected in our Canon 200 images reveals that it is the brighter optical part of a more extended structure mainly traced by blue, young stars distributed in an intricate and complex structure

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Summary

Introduction

The Magellanic Clouds are the largest satellites of the Milky Way (MW), and the only irregular galaxies in its immediate surroundings. The density distribution of these inter-cloud stars suggests that they belong to the extended outer regions of the two Clouds (e.g., Skowron et al 2014; Jacyszyn-Dobrzeniecka et al 2017; Wagner-Kaiser & Sarajedini 2017). This would support a scenario where the Bridge formed in a close encounter between the Clouds some 250 Myr ago (D’Onghia & Fox 2016 and references therein). We study the structure, stellar and gas content, kinematics and possible formation scenarios of this overdensity by means of its resolved stellar populations as traced by the imaging data obtained in the Survey of the MAgellanic Stellar History (SMASH, Nidever et al 2017) and the proper motions available from the Gaia Data Release 2 (DR2; Gaia Collaboration et al 2018a)

Canon 200 lens data
SMASH data
A shell in the outer region of the SMC
Spatial distribution as traced with different stellar populations
Stellar content
Young star clusters
Cepheids i
Stellar kinematics
H I and Hα emission
Findings
Discussion
Full Text
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