Fermi Gamma-ray Space Telescope

Study of the ISM and CRs in Chamaeleon region using the Planck thermal dust optical depth

Katsuhiro Hayashi
(T. Mizuno, Y. Fukui, A. Okumura, H. Tajima, on behalf of the Fermi-LAT collaboration)


Diffuse gamma-ray emission from the interstellar medium (ISM) is a powerful probe of the interstellar matter and cosmic rays (CRs). Many studies of local molecular cloud regions using Fermi-LAT data have revealed gas properties and CR density in the solar neighborhood through comparisons with gas distribution measured by other wavelengths. Most of these studies assume that the gamma-ray emitting interstellar gas is decomposed into atomic hydrogen, molecular hydrogen and some excess gas not traced by standard HI and CO surveys. In this study, we examine the Chamaeleon molecular cloud complex using a total gas column density (NH) model based on the dust optical depth at 353 GHz (τ353) obtained from the Planck thermal dust emission model. In fitting gamma-ray data with several NH models as a function of τ353, we found that a non-linear relation of τ353 proportional to the ~1.2-1.3-th power of NH gives the best fit, which may indicate the effect of dust evolution. Based on the best-fit NH model, we will discuss gas properties and CR density of the Chamaeleon region in comparison with other studies of the local ISM.