A Candidate for the Most Luminous OB Association in the Galaxy

Abstract

The Milky Way harbors giant H II regions which may be powered by star complexes more luminous than any Galactic OB association known. Being across the disk of the Galaxy, however, these brightest associations are severely extinguished and confused. We present a search for one such association toward the most luminous H II region in the recent catalog by Murray and Rahman, which, at ~9.7 kpc, has recombination rate of ~7x10^{51} sec${}^{-1}$. Prior searches have identified only small scale clustering around the rim of this shell-like region, but the primary association has not previously been identified. We apply a near-infrared color selection and find an overdensity of point sources toward its southern central part. The colors and magnitudes of these excess sources are consistent with O- and early B-type stars at extinctions $0.96<A_K<1.2$, and they are sufficiently numerous (406 +/- 102 after subtraction of field sources) to ionize the surrounding H II region, making this a candidate for the most luminous OB association in the Galaxy. We reject an alternate theory, in which the apparent excess is caused by localized extinction, as inconsistent with source demographics.

To appear in ApJ Letters

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• Young massive star clusters
• UBVJHK synthetic photometry of Galactic O stars
• Stellar Feedback in Molecular Clouds and its Influence on the Mass Function of Young Star Clusters
• Star Formation in Massive Clusters via the Wilkinson Microwave Anisotropy Probe and the Spitzer Glimpse Survey
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Cited by in corpus (10)

• The 6-GHz methanol multibeam maser catalogue IV: Galactic longitudes 186 to 330 including the Orion-Monoceros region
• OB associations and their origins
• The Milky Way Project: Leveraging Citizen Science and Machine Learning to Detect Interstellar Bubbles
• Probing embedded star clusters in the HII complex NGC 6357 with VVV
• Spectroscopic Confirmation of the Dragonfish Association: The Galaxy's Most Luminosity OB Association
• On the Diagnostic Power of FIR/Sub-mm SED Fitting in Massive Galactic Molecular Clumps
• The Galactic HII Region Luminosity Function at Radio and Infrared Wavelengths
• Probing the Dragonfish star-forming complex: the ionizing population of the young massive cluster Mercer 30
• OB Associations at the Upper End of the Milky Way Luminosity Function
• Cloud structure and young star distribution in the Dragonfish complex