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

TL;DR

This study uses Fermi LAT gamma-ray data to map gas distribution in Orion Molecular Clouds, revealing insights into cosmic ray penetration and the CO-to-H2 conversion factor across different cloud regions.

Contribution

First gamma-ray based gas mass distribution maps of Orion A and B, showing cosmic ray penetration and regional variations in the CO-to-H2 conversion factor.

Findings

Gamma-ray emission correlates linearly with CO intensity over a tenfold range.

The Xco factor varies between regions, being higher in the high-longitude part of Orion A.

Wco decreases faster than H2 density, indicating 'dark' molecular gas regions.

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{\deg} \times1{\deg} 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{\deg}), \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.