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

¹ Guizhou Provincial Key Laboratory of Radio Astronomy and Data Processing, Guizhou Normal University, Guiyang 550001, PR China
e-mail: pengfangkun@163.com
² School of Astronomy and Space Science, Nanjing University, Nanjing 210093, PR China
e-mail: xywang@nju.edu.cn
³ Guangxi Key Laboratory for Relativistic Astrophysics, Nanning 530004, PR China
⁴ Key laboratory of Modern Astronomy and Astrophysics, Nanjing University, Ministry of Education, Nanjing 210093, PR China
⁵ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, PR China
e-mail: dili@nao.cas.cn
⁶ CAS Key Laboratory of FAST, NAOC, Chinese Academy of Sciences, Beijing 100101, PR China

Received: 15 July 2018
Accepted: 6 November 2018

Abstract

Star-forming regions on different scales, such as giant molecular clouds in our Galaxy and star-forming galaxies, emit GeV gamma-rays. These are thought to originate from hadronic interactions of cosmic-ray (CR) nuclei with the interstellar medium. It has recently been shown that the gamma-ray luminosity (L_γ) of star-forming galaxies is well correlated with their star formation rates (SFR). We investigated Fermi data of eight Galactic molecular clouds in the Gould belt and found that molecular clouds do not follow the L_γ −SFR correlation of star-forming galaxies. We also compared the scaling relations of gamma-ray luminosity, SFR, and the gas mass for molecular clouds and star-forming galaxies. Using a multiple-variable regression analysis, we found different dependences of gamma-ray emission on SFR or mass for molecular clouds and star-forming galaxies. This suggests that different mechanisms may govern the production of gamma-rays in these two types of sources. Specifically, the strong dependence on mass supports that gamma-ray emission of molecular clouds primarily comes from passive interaction by diffuse Galactic CRs, whereas the strong dependence on SFR supports that gamma-ray emission of star-forming galaxies originates from CRs that are accelerated by local active sources.