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A newly discovered young massive star cluster at the far end of the Galactic Bar

Authors

  • Ben Davies,

    Corresponding author
    1. Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA
    2. School of Physics and Astronomy, University of Leeds, Woodhouse Lane, Leeds LS2 9JT
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  • Diego de la Fuente,

    1. Centro de Astrobiologia (INTA/CSIC), Instituto Nacional de Tecnica Aeroespacial, Ctra. de Ajalvir, km 4, 28850 Torrejon de Ardoz, Madrid, Spain
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  • Francisco Najarro,

    1. Centro de Astrobiologia (INTA/CSIC), Instituto Nacional de Tecnica Aeroespacial, Ctra. de Ajalvir, km 4, 28850 Torrejon de Ardoz, Madrid, Spain
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  • Jim A. Hinton,

    1. Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH
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  • Christine Trombley,

    1. Center for Detectors, Rochester Institute of Technology, 54 Lomb Memorial Drive, Rochester, NY 14623, USA
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  • Donald F. Figer,

    1. Center for Detectors, Rochester Institute of Technology, 54 Lomb Memorial Drive, Rochester, NY 14623, USA
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  • Elena Puga

    1. Centro de Astrobiologia (INTA/CSIC), Instituto Nacional de Tecnica Aeroespacial, Ctra. de Ajalvir, km 4, 28850 Torrejon de Ardoz, Madrid, Spain
    2. Herschel Science Centre, European Space Astronomy Centre, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
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E-mail: bdavies@ast.cam.ac.uk

ABSTRACT

We present a near-infrared study of the candidate star cluster Mercer 81, located at the centre of the G338.4+0.1 H ii region and close to the TeV gamma-ray source HESS 1640–465. Using Hubble Space Telescope/NICMOS imaging and VLT/ISAAC spectroscopy, we have detected a compact and highly reddened cluster of stars, although the bright stars in the centre of the field are in fact foreground objects. The cluster contains nine stars with strong Pα emission, one of which we identify as a Wolf–Rayet (WR) star, as well as an A-type supergiant. The line-of-sight extinction is very large, AV∼ 45, illustrating the challenges of locating young star clusters in the Galactic plane. From a quantitative analysis of the WR star, we argue for a cluster age of 3.7inline image Myr, and, assuming that all emission-line stars are WR stars, a cluster mass of ≳104 M. A kinematic analysis of the cluster’s surrounding H ii region shows that the cluster is located in the Galactic disc at a distance of 11 ± 2 kpc. This places the cluster close to where the far end of the Bar intersects the Norma spiral arm. This cluster, as well as the nearby cluster [DBS2003]179, represents the first detections of active star cluster formation at this side of the Bar, in contrast to the near side which is well known to have recently undergone a ∼106 M starburst episode.

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