The cosmic ray ionization and γ-ray budgets of star-forming galaxies.

Cosmic rays in star-forming galaxies are a dominant source of both diffuse γ-ray emission and ionization in gas too deeply shielded for photons to penetrate. Though the cosmic rays responsible for γ-rays and ionization are of different energies, they are produced by the same star formation-driven sources, and thus galaxies' star formation rates, γ-ray luminosities, and ionization rates should all be linked. In this paper, we use up-to-date cross-section data to determine this relationship, finding that cosmic rays in a galaxy of star formation rate [Formula: see text] and gas depletion time produce a maximum primary ionization rate ζ ≈ 1 × 10( /Gyr) s and a maximum γ-ray luminosity [Formula: see text] erg s in the 0.

The diffuse γ-ray background is dominated by star-forming galaxies.

The Fermi Gamma-ray Space Telescope has revealed a diffuse γ-ray background at energies from 0.1 gigaelectronvolt to 1 teraelectronvolt, which can be separated into emission from our Galaxy and an isotropic, extragalactic component. Previous efforts to understand the latter have been hampered by the lack of physical models capable of predicting the γ-ray emission produced by the many candidate sources, primarily active galactic nuclei and star-forming galaxies, leaving their contributions poorly constrained.

Giant magnetized outflows from the centre of the Milky Way.

The nucleus of the Milky Way is known to harbour regions of intense star formation activity as well as a supermassive black hole. Recent observations have revealed regions of γ-ray emission reaching far above and below the Galactic Centre (relative to the Galactic plane), the so-called 'Fermi bubbles'. It is uncertain whether these were generated by nuclear star formation or by quasar-like outbursts of the central black hole and no information on the structures' magnetic field has been reported.

Fermi bubbles: giant, multibillion-year-old reservoirs of Galactic center cosmic rays.

Recently evidence has emerged for enormous features in the γ-ray sky observed by the Fermi-LAT instrument: bilateral "bubbles" of emission centered on the core of the Galaxy and extending to around ± 10 kpc from the Galactic plane. These structures are coincident with a nonthermal microwave "haze" and an extended region of x-ray emission. The bubbles' γ-ray emission is characterized by a hard and relatively uniform spectrum, relatively uniform intensity, and an overall luminosity 4×10(37)   erg/s, around 1 order of magnitude larger than their microwave luminosity while more than order of magnitude less than their x-ray luminosity.

A lower limit of 50 microgauss for the magnetic field near the Galactic Centre.

The amplitude of the magnetic field near the Galactic Centre has been uncertain by two orders of magnitude for several decades. On a scale of approximately 100 parsecs (pc), fields of approximately 1,000 microgauss (microG; refs 1-3) have been reported, implying a magnetic energy density more than 10,000 times stronger than typical for the Galaxy. Alternatively, the assumption of pressure equilibrium between the various phases of the Galactic Centre interstellar medium (including turbulent molecular gas, the contested 'very hot' plasma, and the magnetic field) suggests fields of approximately 100 microG over approximately 400 pc size scales.