Constraints on Cosmic Rays in the Milky Way Circumgalactic Medium from O viii Observations

Abstract

We constrain the cosmic-ray (CR) population in the circumgalactic medium (CGM) of the Milky Way by comparing the observations of absorption lines of O viii ions with predictions from analytical models of the CGM: the precipitation (PP) and isothermal (IT) models. For a CGM in hydrostatic equilibrium, the introduction of CR suppresses thermal pressure and affects the O viii ion abundance. We explore the allowances given to the ratio of CR pressure to thermal pressure (P CR/P th = η), with varying boundary conditions, CGM mass content, photoionization by extragalactic ultraviolet background, and temperature fluctuations. We find that the allowed maximum values of η are η ≲ 10 in the PP model and η ≲ 6 in the IT model. We also explore the spatial variation of η: rising (η = Ax) or declining (η = A/x) with radius, where A is the normalization of the profiles. In particular, the models with a declining ratio of CR to thermal pressure fare better than those with a rising ratio with suitable temperature fluctuation (higher σ lnT for PP and lower for IT). The declining profiles allow A ≲ 8 and A ≲ 10 in the case of the IT and PP models, respectively, thereby accommodating a large value of η (≃200) in the central region but not in the outer regions. These limits, combined with the limits derived from the γ-ray and radio background, can be useful for building models of the Milky Way CGM including the CR population. However, the larger amount of CRs can be packed in the cold phase, which may be one way to circumvent these constraints.