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 $\gamma$-ray emission with the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope, which is likely to be the second case of a $\gamma$-ray detection from a star-forming region (SFR) in our Galaxy. The LAT source is located in the G25 region, $1.7^{\circ }\times 2.1^{\circ }$ around $(l,b)=(25.0^{\circ },0.0^{\circ })$. The $\gamma$-ray emission is found to be composed of two extended sources and one point-like source. The extended sources have a similar sizes of about $1.4^{\circ }\times 0.6^{\circ }$. An $\sim 0.4^{\circ }$ diameter sub-region of one has a photon index of $\Gamma =1.53\pm 0.15$; 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 $\Gamma =2.1\pm 0.2$ without significant spectral curvature. Given their spatial and spectral properties, they have no clear associations with sources at other wavelengths. Their $\gamma$-ray properties are similar to those of the Cygnus cocoon SFR, the only firmly established $\gamma$-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 $\gamma$-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 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 $\gamma$-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.

31 pages, 23 figures, accepted for publication in ApJ

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