Abstract Ultra compact HII (UCHII) G25.4NW is a bright IR source in the region of the inner Galaxy. New HI images from the Very Large Array Galactic Plane Survey show clear absorption features associated with the UCHII region up to 95 km s−1, and there are no other absorptions up to the tangential velocity. This reveals that G25.4NW has a near-side distance of 5.7 kpc, and it is located in the region of the inner Galactic molecular ring. Using the new distance, the bolometric luminosity of G25.4NW is estimated as 105.6L⊙, which corresponds to an O6 star. It contains 460 M⊙ of ionized gas. A high-resolution 13CO image from the Galactic Ring Survey reveals that G25.4NW is part of a more extended star-forming complex with about 104M⊙ of molecular gas.
Keywords radiation mechanisms: thermal — ISM: atoms — ISM: HII regions — ISM: molecules
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