Elsevier

Astroparticle Physics

Volume 85, December 2016, Pages 35-38
Astroparticle Physics

Active galaxies can make axionic dark energy

https://doi.org/10.1016/j.astropartphys.2016.09.008Get rights and content

Abstract

AGN jets carry helical magnetic fields, which can affect dark matter if the latter is axionic. This preliminary study shows that, in the presence of strong helical magnetic fields, the nature of the axionic condensate may change and become dark energy. Such dark energy may affect galaxy formation and galactic dynamics, so this possibility should not be ignored when considering axionic dark matter.

Introduction

As supersymmetric particles have not been observed in the LHC yet, interest in axionic dark matter is increasing. Such dark matter has a loop-suppressed interaction with the electromagnetic field, which opens up observational possibilities that aim to exploit the photon-axion conversion in astrophysical magnetic fields. Many authors have considered the electromagnetic interaction of axion particles [1]. However, the effect of this interaction to the axionic condensate itself has been largely ignored, assuming that it is negligible. In this paper we investigate the effect of an helical magnetic field on an axionic condensate. We find that, if the magnetic field is strong enough, axionic dark matter is modified to lead to the violation of the strong energy condition and behave as dark energy.1 Then we apply our findings to the helical magnetic fields in the jets of Active Galactic Nuclei (AGN). We find that the magnetic fields near the central supermassive black hole may be strong enough to make axionic dark energy and thereby affect galaxy formation and dynamics. We use natural units, for which c==1 and mP2=8πG, with mP=2.4×1018GeV being the reduced Planck mass. For the signature of the metric we take (+1,1,1,1).

Section snippets

Electromagnetically dominated axionic condensate

The axion (or an axion like particle) field ϕ, at tree level has a coupling to a fermionic field ψ of the form ϕψ¯ψ. Therefore, the axion couples to the photon via a fermionic loop as Lϕγ=14gϕγϕFμνF˜μν=gϕγϕE·B,where Fμν is the Faraday tensor, F˜μν is its dual and E and B are the electric and magnetic field respectively. In the above we have defined the dimensionful coupling gϕγ=αN/2πfa, where the fa is the Peccei-Quinn (PQ) scale, while α ≃ 1/137. We will assume N=1. The Lagrangian density for

Axionic matter in AGNs

It so happens that the conditions in Eq. (18) may be satisfied in AGN jets. Observations suggest that AGN jets feature powerful helical magnetic fields [8]. Most spiral galaxies are assumed to go through the AGN phase when their central supermassive black hole is formed. The AGN jet can be huge in length (up to Mpc scales). Its spine, however, is narrow; about d(10550)pc, which, however, is typically much larger than the axion Compton wavelength m1. Therefore, (∇Q)2 ≪ (mQ)2 is satisfied.

Conclusions

We have investigated the behaviour of axionic dark matter in the presence of a helical magnetic field and found that, when the condensate becomes electromagnetically dominated, it ceases to be dark matter and becomes dark energy instead. We have applied our findings in AGNs and showed that the helical magnetic field along the AGN jets near the AGN core can be strong enough to convert axionic dark matter into dark energy. Lacing the AGN black holes with dark energy may have profound implications

Acknowledgements

KD would like to thank A.C. Fabian, I.M. Hook, D.H. Lyth, J. McDonald and T. Raptis for discussions and comments. This work was supported (in part) by the Lancaster-Manchester-Sheffield Consortium for Fundamental Physics under STFC grant ST/L000520/1.

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