Parity-violating Dark Photon Halos

TL;DR

The paper introduces a parity-violating mechanism for structure formation in the late universe by coupling an ultralight axion to a dark U(1) gauge field through a Chern-Simons term. Through a trapped misalignment scenario, resonant, Floquet-type instabilities amplify transverse dark-photon modes during the matter-dominated era, generating helically polarized, gravitating halos with intrinsic vorticity. Predicted halos span $M_{ m halo} \sim 10^{5}-10^{11} \; M_\odot$ and sizes $R \sim 1-10^{6}$ pc, with virial velocities around tens of km/s, and may act as primordial seeds for SMBHs while imprinting parity-odd signatures in lensing and CMB observables. The work motivates detailed simulations and observational searches to test this parity-violating channel for early structure formation and black-hole seeding in the JWST era.

Abstract

We propose a mechanism for the generation of gravitationally bound dark photon halos during the matter-dominated era. Coupled to an ultralight axion field through a parity-violating Chern-Simons term, dark photons can be produced by the tachyonic instability of axion coherent oscillation. The dark photons with a net helicity lead to a metric vorticity and can generate chiral substructures. For axion masses in the range $10^{-28} \, \mathrm{eV} \lesssim m_a \lesssim 10^{-22} \, \mathrm{eV}$, the resulting inhomogeneities collapse to form halos with masses spanning $M_{\rm halo} \sim 10^5 \, M_{\odot}$ to $10^{11} \, M_{\odot}$, with halo sizes ranging from $O(1)$ to $O(10^{6}) \, \mathrm{pc}$. During halo collapse, the induced vorticity could mediate efficient angular-momentum transport, which enables monolithic collapse and provides primordial seeds for the early formation of supermassive black holes.

Figures (2)

• Figure 1: Stability diagram of the Mathieu equation. The color scale represents the Floquet exponent, representing the growth rate of unstable modes. White trajectories show the evolution of individual Fourier modes, with arrows indicating the direction of time flow.
• Figure 2: Left: Power spectrum of produced dark photons from axion background oscillations. The blue and red solid lines represent the transverse polarizations for the heavy ($m_{\gamma'} = 0.5 m_a$) and light ($m_{\gamma'} = 0.2 m_a$) dark photon cases, respectively. Right: the energy density of dark photon polarization modes as a function of axion-dark-photon mass ratio $m_{\gamma'}/m_a$. The longitudinal mode is scaled by $10^{30}$ for visibility. For both plots, we assume $m_a = 10^{-29}~\text{eV}$, $\lambda = 5$, $f_a = 10^{17}~\text{GeV}$, $z_* = 20$, and the initial axion angle $\phi_*/f_a = 1$.