D. de la Fuente, F. Najarro, J. Borissova, S. Ramírez Alegría, M. M. Hanson, C. Trombley, D. F. Figer, B. Davies, M. Garcia, R. Kurtev, M. A. Urbaneja, L. C. Smith, P. W. Lucas, A. Herrero. 2016. Probing the Dragonfish star-forming complex: the ionizing population of the young massive cluster Mercer 30. Astronomy and Astrophysics 589, DOI: 10.1051/0004-6361/201528004
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(Mc30)(H) approximate to 6.70 x 10(50) 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) x 10(4) M-circle dot 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 (similar to 400 pc across) and located at the outer edge of the Sagittarius-Carina spiral arm (similar to 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.