Nature of a shell of young stars in the outskirts of the Small Magellanic Cloud

¹ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstr. 12–14, 69120 Heidelberg, Germany
e-mail: delgado@ari.uni-heidelberg.de
² Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory, Casilla 603, La Serena, Chile
³ Instituto de Astrofísica de Canarias (IAC), Calle Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
⁴ Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
⁵ Laboratório Interinstitucional de e-Astronomia – LIneA, Rua Gal. José Cristino 77, Rio de Janeiro 20921-400, Brazil
⁶ Observatório Nacional, Rua Gal. José Cristino 77, Rio de Janeiro 20921-400, Brazil
⁷ Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, 1442 Potsdam, Germany
⁸ Department of Astronomy, University of Virginia, 530 McCormick Road, Charlottesville, VA 22904, USA
⁹ CIERA and Department of Physics and Astronomy, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, USA
¹⁰ Instituto de Astrofisica, Pontificia Universidad Catolica de Chile, Av. Vicuna Mackenna 4860, Macul 7820436, Santiago, Chile
¹¹ Chinese Academy of Sciences South America Center for Astronomy, National Astronomical Observatories, CAS, Beijing 100101, PR China
¹² Department of Physics, University of Surrey, Guildford GU2 7XH, UK
¹³ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
¹⁴ Department of Physics, Montana State University, PO Box 173840, Bozeman, MT 59717-3840, USA
¹⁵ Astrophysics Source Code Library, University of Maryland, 4254 Stadium Drive, College Park, MD 20742, USA
¹⁶ National Center for Supercomputing Applications, 1205 West Clark St., Urbana, IL 61801, USA
¹⁷ Department of Astronomy, University of Illinois, 1002 W. Green Street, Urbana, IL 61801, USA
¹⁸ The Observatories of the Carnegie Institution for Science, 813 Santa Barbara St., Pasadena, CA 91101, USA
¹⁹ National Optical Astronomy Observatory, 950 N. Cherry Ave, Tucson, AZ 85719, USA
²⁰ Astrophoton Observatory, Edtfleck 12, 4542 Nussbach, Australia
²¹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21030, USA
²² Center for Astrophysical Sciences, Department of Physics & Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA
²³ Observatori Astronòmic, Universitat de València, Paterna (València), Spain
²⁴ Center for Astrophysics and Space Astronomy, Department of Astrophysical and Planetary Sciences, University of Colorado, 389 UCB Boulder, Colorado 80309-0389, USA
²⁵ Department of Astronomy, University of Michigan, 1085 S. University Ave., Ann Arbor, MI 48109, USA
²⁶ Research School of Astronomy and Astrophysics, Australian National University, Canberra 2611, Australia
²⁷ Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR 7550, 67000 Strasbourg, France
²⁸ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
²⁹ Institute of Astronomy and Astrophysics, Academia Sinica, PO Box 23-141, Taipei 10617, Taiwan, ROC
³⁰ Department of Astronomy, University of Illinois at Urbana-Champaign, 1002 West Green Street, Urbana, IL 61801, USA
³¹ Las Campanas Observatory, Carnegie Institution of Washington, La Serena, Chile
³² ETH Zurich, Institute of Geodesy and Photogrammetry, 8093 Zurich, Switzerland
³³ Departamento de Astronomía, Universidad de Chile, Camino del Observatorio 1515, Las Condes, Santiago, Chile

Received: 4 June 2019
Accepted: 16 September 2019

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 M_g, shell = −10.78 ± 0.02, with a surface brightness of μ_g, 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.