SMC west halo: a slice of the galaxy that is being tidally stripped?

Star clusters trace age and metallicity gradients^⋆,⋆⋆

¹ European Southern Observatory, Alonso de Córdova 3107, Santiago, Chile
e-mail: bdias@eso.org
² Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, Rua do Matão 1226, Cidade Universitária, 05508-900 São Paulo, Brazil
³ Laboratório de Astrofísica Teórica e Observacional, Departamento de Ciências Exatas e Tecnológicas, Universidade Estadual de Santa Cruz, Rodovia Jorge Amado km 16, 45662-000 Ilhéus, Bahia, Brazil
⁴ Universidade Federal do Rio Grande do Sul, IF, CP 15051, 91501-970 Porto Alegre, RS, Brazil
⁵ Dipartimento di Fisica e Astronomia Galileo Galilei, University of Padova, vicolo dell’Osservatorio 3, 35122 Padova, Italy
⁶ INAF–Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy

Received: 14 October 2015
Accepted: 10 April 2016

Abstract

Context. The evolution and structure of the Magellanic Clouds is currently under debate. The classical scenario in which both the Large and Small Magellanic Clouds (LMC, SMC) are orbiting the Milky Way has been challenged by an alternative in which the LMC and SMC are in their first close passage to our Galaxy. The clouds are close enough to us to allow spatially resolved observation of their stars, and detailed studies of stellar populations in the galaxies are expected to be able to constrain the proposed scenarios. In particular, the west halo (WH) of the SMC was recently characterized with radial trends in age and metallicity that indicate tidal disruption.

Aims. We intend to increase the sample of star clusters in the west halo of the SMC with homogeneous age, metallicity, and distance derivations to allow a better determination of age and metallicity gradients in this region. Positions are compared with the orbital plane of the SMC from models.

Methods. Comparisons of observed and synthetic V(B−V) colour-magnitude diagrams were used to derive age, metallicity, distance, and reddening for star clusters in the SMC west halo. Observations were carried out using the 4.1 m SOAR telescope. Photometric completeness was determined through artificial star tests, and the members were selected by statistical comparison with a control field.

Results. We derived an age of 1.23 ± 0.07 Gyr and [Fe/H] = −0.87 ± 0.07 for the reference cluster NGC 152, compatible with literature parameters. Age and metallicity gradients are confirmed in the WH: 2.6 ± 0.6 Gyr/° and −0.19 ± 0.09 dex/°, respectively. The age-metallicity relation for the WH has a low dispersion in metallicity and is compatible with a burst model of chemical enrichment. All WH clusters seem to follow the same stellar distribution predicted by dynamical models, with the exception of AM-3, which should belong to the counter-bridge. Brück 6 is the youngest cluster in our sample. It is only 130 ± 40 Myr old and may have been formed during the tidal interaction of SMC-LMC that created the WH and the Magellanic bridge.

Conclusions. We suggest that it is crucial to split the SMC cluster population into groups: main body, wing and bridge, counter-bridge, and WH. This is the way to analyse the complex star formation and dynamical history of our neighbour. In particular, we show that the WH has clear age and metallicity gradients and an age-metallicity relation that is also compatible with the dynamical model that claims a tidal influence of the LMC on the SMC.