# Comparative study of gamma-ray emission from molecular clouds and star-forming galaxies

**Authors:** Fang-Kun Peng, Shao-Qiang Xi, Xiang-Yu Wang, Qi-Jun Zhi, Di Li  
**arXiv:** [1811.07117](https://arxiv.org/abs/1811.07117)  
**Date:** 2018-11-17  
**Summary page:** https://www.summarizepaper.com/en/paper/1811.07117v1/

## Key points

- Study title: "Comparative study of gamma-ray emission from molecular clouds and star-forming galaxies"
- Investigate GeV gamma-ray emission from star-forming regions on different scales
- Focus on giant molecular clouds in our Galaxy and star-forming galaxies
- Recent discovery of correlation between gamma-ray luminosity ($L_\gamma$) and star formation rates (SFR) in star-forming galaxies
- Analysis of \textsl{Fermi} data from eight Galactic molecular clouds in the Gould belt reveals lack of adherence to $L_\gamma-SFR$ correlation observed in star-forming galaxies
- Multiple-variable regression analysis shows distinct dependencies of gamma-ray emission on SFR or mass for molecular clouds versus star-forming galaxies
- Different mechanisms likely govern the production of gamma-rays in these two types of sources:
- Gamma-ray emission from molecular clouds primarily passive interaction by diffuse Galactic cosmic rays (CRs)
- Gamma-ray emission in star-forming galaxies originates from CRs accelerated by local active sources
- Comparative study sheds light on complex interplay between cosmic rays and interstellar environments within different astronomical structures, providing insights into diverse mechanisms driving gamma-ray emissions.

## Summary

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In their study titled "Comparative study of gamma-ray emission from molecular clouds and star-forming galaxies," Fang-Kun Peng, Shao-Qiang Xi, Xiang-Yu Wang, Qi-Jun Zhi, and Di Li investigate the emission of GeV gamma-rays from star-forming regions on different scales. Specifically, they focus on giant molecular clouds in our Galaxy and star-forming galaxies, which are believed to emit these gamma-rays due to hadronic interactions of cosmic-ray (CR) nuclei with the interstellar medium. The authors note a recent discovery that the gamma-ray luminosity ($L_\gamma$) of star-forming galaxies is closely correlated with their star formation rates (SFR). To further explore this phenomenon, the researchers analyze \textsl{Fermi} data from eight Galactic molecular clouds located in the Gould belt. Surprisingly, they find that these molecular clouds do not adhere to the $L_\gamma-{\rm SFR}$ correlation observed in star-forming galaxies. By comparing the scaling relations of gamma-ray luminosity, SFR, and gas mass for both types of sources, the team conducts a multiple-variable regression analysis to uncover distinct dependencies of gamma-ray emission on SFR or mass for molecular clouds versus star-forming galaxies. The results suggest that different mechanisms likely govern the production of gamma-rays in these two types of sources. Specifically, while the strong dependence on mass supports the notion that gamma-ray emission from molecular clouds primarily stems from {\em \textup{passive}} interaction by diffuse Galactic CRs, the strong dependence on SFR implies that gamma-ray emission in star-forming galaxies originates from CRs accelerated by local {\em \textup{active}} sources. This comparative study sheds light on the complex interplay between cosmic rays and interstellar environments within different astronomical structures, offering valuable insights into the diverse mechanisms driving gamma-ray emissions in varying astrophysical contexts. The findings contribute to our understanding of how star formation processes and cosmic ray interactions shape the observed gamma-ray signatures in both molecular clouds within our own Galaxy and distant star-forming galaxies across cosmic distances.