Probing the Dragonfish star-forming complex: the ionizing population of the young massive cluster Mercer 30^⋆

¹ Centro de Astrobiología (CSIC/INTA), ctra. de Ajalvir km. 4, 28850 Torrejón de Ardoz, Madrid, Spain
e-mail: delafuente@cab.inta-csic.es
² Departamento de Física y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Av. Gran Bretaña 1111, Playa Ancha, Casilla 5030, Valparaíso, Chile
³ Millennium Institute of Astrophysics (MAS), 7820436 Macul, Santiago, Chile
⁴ Department of Physics, University of Cincinnati, Cincinnati, OH 45221-0011, USA
⁵ Center for Detectors, Rochester Institute of Technology, 74 Lomb Memorial Drive, Rochester, NY 14623, USA
⁶ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
⁷ Institute for Astro- and Particle Physics, University of Innsbruck, Technikerstr. 25/8, 6020 Innsbruck, Austria
⁸ Centre for Astrophysics Research, Science and Technology Research Institute, University of Hertfordshire, Hatfield AL10 9AB, UK
⁹ Instituto de Astrofísica de Canarias, 38200, La Laguna, Tenerife, Spain
¹⁰ Departamento de Astrofísica, Universidad de La Laguna, 38205, La Laguna, Tenerife, Spain

Received: 18 December 2015
Accepted: 7 February 2016

Abstract

It has recently been claimed that the nebula, Dragonfish, is powered by a superluminous but elusive OB association. However, systematic searches in near-infrared photometric surveys have found many other cluster candidates in this region of the sky. Among these, the first confirmed young massive cluster was Mercer 30, where Wolf-Rayet stars were found.We perform a new characterization of Mercer 30 with unprecedented accuracy, combining NICMOS/HST and VVV photometric data with multi-epoch ISAAC/VLT H- and K-band spectra. Stellar parameters for most of spectroscopically observed cluster members are found through precise non-LTE atmosphere modeling with the CMFGEN code. Our spectrophotometric study for this cluster yields a new, revised distance of d = (12.4 ± 1.7) kpc and a total of Q^H_Mc30 ≈ 6.70 × 10⁵⁰ s^-1 Lyman ionizing photons. A cluster age of (4.0 ± 0.8) Myr is found through isochrone fitting, and a total mass of (1.6 ± 0.6) × 10⁴M_⊙ is estimated, thanks to our extensive knowledge of the post-main-sequence population. As a consequence, membership of Mercer 30 to the Dragonfish star-forming complex is confirmed, allowing us to use this cluster as a probe for the whole complex, which turns out to be extremely large (~400 pc across) and located at the outer edge of the Sagittarius-Carina spiral arm (~11 kpc from the Galactic center). The Dragonfish complex hosts 19 young clusters or cluster candidates (including Mercer 30 and a new candidate presented in this work) and an estimated minimum of nine field Wolf-Rayet stars. All these contributions account for, at least 73% of the ionization of the Dragonfish nebula and leaves little or no room for the alleged superluminous OB association; alternative explanations are discussed.