Speaker
Description
Recent Fermi LAT observations of nearby giant molecular clouds show deficits in the gamma-ray residual map when the expected diffuse emission is modelled assuming uniformly distributed cosmic rays (CRs) [1]. The authors pointed out that the observed emission “holes” reflect the lack of penetration of <∼10 GeV CRs into denser regions, and proposed that the CR deficit is caused by slower CR diffusion in the clouds.
Several years ago, we developed a theory of CR self-modulation in dense molecular clouds [2]. We showed that the modulation occurs due to CR scattering on self-generated turbulence, excited by the CRs in the diffuse envelope. In a subsequent paper [3] we applied the theory to investigate whether the mechanism of self-modulation can explain gamma-ray features observed in the Galactic center. Assuming a simplified cloud model, with a dense core and a diffuse surrounding envelope of a constant gas density about 10 cm−3, we showed that emission is noticeably suppressed at GeV energies for the cloud column densities over 10^23 cm−2. However, the recently developed 3D dust extinction maps [4] indicate that the gas distribution in envelopes is highly inhomogeneous, decreasing monotonically from the center, and typical density values are substantially lower than that assumed in [3].
We have now generalized the theory of CR self-modulation for inhomogeneous envelopes, making it applicable to arbitrary monotonically decreasing density profiles [5]. In this case, we found that noticeable suppression of GeV gamma-rays already occurs when the gas column density exceeds a few times 10^22 cm−2. For the conditions of giant clouds [1], we obtained excellent quantitative agreement with the observed emission holes. We will present details of the developed theory, discuss comparison with the observations, and point out implications for the CR ionization in molecular clouds.
[1] Yang, R.-z., Li, G.-X., Wilhelmi, E. d. O., et al. 2023, Nature Astronomy, 7, 351
[2] Ivlev, A. V., Dogiel, V. A., Chernyshov, D. O., et al. 2018, ApJ, 855, 23
[3] Dogiel, V. A., Chernyshov, D. O., Ivlev, A. V., et al. 2018, ApJ, 868, 114
[4] Edenhofer, G., Zucker, C., Frank, P., et al. 2024, A&A, 685, A82
[5] Chernyshov, D. O., Ivlev, A. V., Kiselev A. M. 2024, PRD, 110, 043012
