Extended Gamma-Ray Emission from the G25.0+0.0 Region: A Star-forming Region Powered by the Newly Found OB Association?

Abstract

We report a study of extended γ -ray emission with the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope , which is likely to be the second case of a γ -ray detection from a star-forming region (SFR) in our Galaxy. The LAT source is located in the G25 region, 1.°7 × 2.°1 around ( l , b ) = (25.°0, 0.°0). The γ -ray emission is found to be composed of two extended sources and one pointlike source. The extended sources have similar sizes of about 1.°4 × 0.°6. An ∼0.°4 diameter subregion of one has a photon index of Γ = 1.53 ± 0.15, and is spatially coincident with HESS J1837−069, likely a pulsar wind nebula. The other parts of the extended sources have a photon index of Γ = 2.1 ± 0.2 without significant spectral curvature. Given their spatial and spectral properties, they have no clear associations with sources at other wavelengths. Their γ -ray properties are similar to those of the Cygnus cocoon SFR, the only firmly established γ -ray detection of an SFR in the Galaxy. Indeed, we find bubble-like structures of atomic and molecular gas in G25, which may be created by a putative OB association/cluster. The γ -ray emitting regions appear confined in the bubble-like structure; similar properties are also found in the Cygnus cocoon. In addition, using observations with the XMM-Newton , we find a candidate young massive OB association/cluster G25.18+0.26 in the G25 region. We propose that the extended γ -ray emission in G25 is associated with an SFR driven by G25.18+0.26. Based on this scenario, we discuss possible acceleration processes in the SFR and compare them with the Cygnus cocoon.