Particle acceleration in the superwinds of starburst galaxies

Abstract

Context. Starbursts are galaxies undergoing massive episodes of star formation. The combined effect of stellar winds from hot stars and supernova explosions creates a high-temperature cavity in the nuclear region of these objects. The very hot gas expands adiabatically and escapes from the galaxy creating a superwind which sweeps matter from the galactic disk. The superwind region in the halo is filled with a multi-phase gas with hot, warm, cool, and relativistic components.Aims. The shocks associated with the superwind of starbursts and the turbulent gas region of the bubble inflated by them might accelerate cosmic rays up to high energies. In this work we calculate the cosmic ray production associated with the superwind using parameters that correspond to the nearby southern starburst galaxy NGC 253, which has been suggested as a potential accelerator of ultra-high-energy cosmic rays. Methods. We evaluate the efficiency of both diffusive shock acceleration (DSA) and stochastic diffusive acceleration (SDA) in the superwind of NGC 253. We estimate the distribution of both hadrons and leptons and calculate the corresponding spectral energy distributions of photons. The electromagnetic radiation can help to discriminate between the different scenarios analyzed. Results. We find that the strong mass load of the superwind, recently determined through ALMA observations, strongly attenuates the efficiency of DSA in NGC 253, whereas SDA is constrained by the age of the starburst. Conclusions. We conclude that NGC 253 and similar starbursts can only accelerate iron nuclei beyond ∼ 10 18 eV under very special conditions. If the central region of the galaxy harbors a starved supermassive black hole of ∼ 10 6 M , as suggested by some recent observations, a contribution in the range 10 18 − 10 19 eV can be present for accretion rateṡ m ∼ 10 −3 in Eddington units. Shock energies of the order of 100 EeV might only be possible if very strong magnetic field amplification occurs close to the superwind.

The Filamentary Radio Lobes of the Seyfert–Starburst Composite Galaxy NGC 3079

“…The sound speed in the hot wind regions present in typical starbursts is about 100 km s −1 (Romero et al 2018). Hence the sonic Mach number turns out to be ∼12.…”

Cosmic rays from the nearby starburst galaxy NGC 253: the effect of a low-luminosity active galactic nucleus

“…11, the maximum energy attainable by iron nuclei entering into the electrostatic gap of the black hole magnetosphere would be ∼ 2×10 15 eV. This is even less than what Romero et al (2018) estimated for particle acceleration in the superwind. If NGC 253 and other starbursts are confirmed as sources of UHECRs of high metalicity, then the cosmic rays should probably be originated in compact , for different values of the magnetic field intensity.…”

“…4 × 1. 0, there is some room for contributions from the base of the superwind as suggested by Romero et al (2018). GeV gamma rays might also be produced in the superwind, but Fermi resolution does not allow to disentangle this radiation if present.…”

Gamma-Rays From Large-Scale Outflows In Starburst Galaxies

“…Recently, ALMA interferometer showed that the mass loading factor of NGC 253 is very high: β ∼ 12 [17]. Table 1 in [18] presents a summary of the physical properties of NGC 253.…”