Speaker
Description
The Fermi bubbles are giant bipolar structures in the Galactic halo. Their formation mechanism and the origin of their hard gamma-ray spectrum remain unclear. Most interpretations have regarded the bubbles as evidence of past Galactic center activity, with leptonic models proposed to explain the hard gamma-ray emission. More recently, some scenarios suggest that the FBs arise naturally from long-term Galactic evolution, in particular through cosmic ray (CR)-driven Galactic winds (Shimoda & Asano, 2024; Sands et al., 2025), with the gamma-rays arising from hadronic interactions. A key difficulty of this scenario, however, is that cosmic rays injected from the disk are expected to soften during transport, implying that some mechanism for spectral hardening in the halo is required. To address this issue, we focus on a multiphase Galactic halo in which hot outflows coexist with cold molecular clouds. In fact, recent observations of cold molecular gas in the Galactic nuclear outflow by Di Teodoro et al. (2020), although not directly aimed at the Fermi bubbles, suggest that the halo may be a multiphase environment rather than a uniform hot wind.
We investigate the evolution of cosmic-ray spectra in the interaction region between a Galactic wind and molecular clouds, using our two-dimensional CR-hydrodynamics simulation code that self-consistently follows cosmic-ray evolution in momentum space. Focusing on cloud size scales relevant to the molecular gas observed in the Galactic nuclear outflow, we explore how wind--cloud interactions modify the GeV-TeV hadronic gamma-ray spectrum expected in Galactic-wind scenarios. Our calculations show that radiative cooling forms transition layers at wind--cloud interfaces, where cosmic rays are locally heated and concentrated. Advection and diffusion then transport these cosmic rays into molecular clouds, enhancing the high-energy hadronic gamma-ray emissivity. These results suggest that wind--cloud interactions in a multiphase Galactic halo can provide localized sites of gamma-ray spectral hardening and may contribute to the sustained emission associated with the Fermi bubbles.
