The equipartition magnetic field formula in starburst galaxies: accounting for pionic secondaries and strong energy losses

被引：64

作者：

Lacki, Brian C. ^{[1
]}

Beck, Rainer ^{[2
]}

机构：

[1] Inst Adv Study, Princeton, NJ 08540 USA

[2] Max Planck Inst Radioastron, D-53121 Bonn, Germany

来源：

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY | 2013年 / 430卷 / 04期

关键词：

galaxies: magnetic fields; galaxies: starburst; radio continuum: general; GAMMA-RAY EMISSION; RADIO-EMISSION; RECOMBINATION LINE; STAR-FORMATION; DISK GALAXIES; COSMIC-RAYS; LOCAL GROUP; NGC; 253; X-RAY; M82;

D O I：

10.1093/mnras/stt122

中图分类号：

P1 [天文学];

学科分类号：

0704 ;

摘要：

Equipartition arguments provide an easy way to find a characteristic scale for the magnetic field from radio emission by assuming that the energy densities in cosmic rays and magnetic fields are the same. Yet most of the cosmic ray content in star-forming galaxies is in protons, which are invisible in radio emission. Therefore, the argument needs assumptions about the proton spectrum, typically that of a constant proton/electron ratio. In some environments, particularly starburst galaxies, the reasoning behind these assumptions does not necessarily hold: secondary pionic positrons and electrons may be responsible for most of the radio emission, and strong energy losses can alter the proton/electron ratio. We derive an equipartition expression that should work in a hadronic loss-dominated environment like starburst galaxies. Surprisingly, despite the radically different assumptions from the classical equipartition formula, numerically the results for starburst magnetic fields are similar. We explain this fortuitous coincidence using the energetics of secondary production and energy loss times. We show that these processes cause the proton/electron ratio to be similar to 100 for GHz-emitting electrons in starbursts.

引用

页码：3171 / 3186

页数：16

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