Gamma-ray observations of the Orion Molecular Clouds with the Fermi Large Area Telescope

Abstract

We report on the gamma-ray observations of giant molecular clouds Orion A and B with the Large Area Telescope (LAT) on-board the Fermi Gamma-ray Space Telescope. The gamma-ray emission in the energy band between \sim100 MeV and \sim100 GeV is predicted to trace the gas mass distribution in the clouds through nuclear interactions between the Galactic cosmic rays (CRs) and interstellar gas. The gamma-ray production cross-section for the nuclear interaction is known to \sim10% precision which makes the LAT a powerful tool to measure the gas mass column density distribution of molecular clouds for a known CR intensity. We present here such distributions for Orion A and B, and correlate them with those of the velocity integrated CO intensity (WCO) at a 1° \times1° pixel level. The correlation is found to be linear over a WCO range of ~10 fold when divided in 3 regions, suggesting penetration of nuclear CRs to most of the cloud volumes. The Wco-to-mass conversion factor, Xco, is found to be \sim2.3\times10^20 cm-2(K km s-1)-1 for the high-longitude part of Orion A (l > 212°), \sim1.7 times higher than \sim1.3 \times 10^20 found for the rest of Orion A and B. We interpret the apparent high Xco in the high-longitude region of Orion A in the light of recent works proposing a non-linear relation between H2 and CO densities in the diffuse molecular gas. Wco decreases faster than the H2 column density in the region making the gas "darker" to Wco.

41 pages, 10 figures (Accepted to ApJ)

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• Gamma rays as probes of cosmic-ray propagation and interactions in galaxies
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• 3D Dust Mapping Reveals that Orion Forms Part of a Large Ring of Dust
• Runaway stars as cosmic ray injectors inside molecular clouds
• Probing the Sea of Cosmic Rays by Measuring Gamma-Ray Emission from Passive Giant Molecular Clouds with HAWC
• The Important Role of Cosmic-Ray Re-Acceleration
• The extragalactic $\gamma$-ray background: imprints from the physical properties and evolution of star-forming galaxy populations
• Probing local cosmic rays using Fermi-LAT observation of a mid-latitude region in the third Galactic quadrant
• High energy radiation from luminous and magnetized stars
• The Orion Region: Evidence of enhanced cosmic-ray density in a stellar wind forward shock interaction with a high density shell
• High Latitude, Translucent Molecular Clouds as Probes of Local Cosmic Rays
• Study of the Cosmic Rays and Interstellar Medium in Local HI Clouds using Fermi-LAT Gamma-Ray Observations
• The Neutral Atomic Hydrogen in the solar neighborhood (NeAtHood) project I. Ghost in the shell: Neutral atomic hydrogen in the extended Orion nebula