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 \pm 1.7 ) kpc and a total of Q ^ { H } _ { \mathrm { Mc 30 } } \approx 6.70 \times 10 ^ { 50 } \mathrm { s } ^ { -1 } Lyman ionizing photons .
A cluster age of ( 4.0 \pm 0.8 ) Myr is found through isochrone fitting , and a total mass of ( 1.6 \pm 0.6 ) \times 10 ^ { 4 } M _ { \odot } 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 ( \sim 400 pc across ) and located at the outer edge of the Sagittarius-Carina spiral arm ( \sim 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 .