Stellar Disruption of Axion Minihalos and Consequences for Direct Axion Detection

TL;DR

This work analyzes how axion minihalos formed from isocurvature fluctuations and PQ scenarios with post-inflation symmetry breaking are disrupted by stellar encounters. It advances a sequential stripping framework that tracks halo profile changes after each disruption and incorporates relaxation to a Hernquist-like state, then switches between relaxed and unperturbed branches to fit simulations across concentrations. Extending the analysis to the Milky Way with Monte Carlo orbital populations and a Galactic disk model, the authors show that cumulative stellar perturbations are more destructive than previously thought, reducing minihalo mass retention at the Solar radius to about 30% for certain axion masses, and increasing inter-minihalo axion density which can enhance haloscope detection prospects. The results have direct implications for the local axion density, the shape of the surviving minihalo mass function, and the interpretation of direct detection signals, motivating more detailed modeling of tidal disruption and its consequences for axion searches.

Abstract

Scenarios such as the QCD axion with the Peccei-Quinn symmetry broken after inflation predict an enhanced matter power spectrum on sub-parsec scales. These theories lead to the formation of dense dark matter structures known as minihalos, which provide insights into early Universe dynamics and have implications for direct detection experiments. We examine the mass loss of minihalos during stellar encounters, building on previous studies that derived formulas for mass loss and performed N-body simulations. We propose a new formula for the mass loss that accounts for changes in the minihalo profile after disruption by a passing star. We also investigate the mass loss for multiple stellar encounters. We demonstrate that accurately assessing the mass loss in minihalos due to multiple stellar encounters necessitates considering the alterations in the minihalo's binding energy after each encounter, as overlooking this aspect results in a substantial underestimation of the mass loss. We further extend our analysis to the Galactic environment by more accurately incorporating multiple stellar encounters and dynamical relaxation timescales, simulating minihalo orbits in the Galactic potential. Our results show stellar interactions are more destructive than previously estimated, reducing minihalo mass retention at the solar system to ~30%, compared to earlier estimates of ~60%. This enhanced loss arises from cumulative energy injections when relaxation periods between stellar encounters are accounted for. The altered minihalo mass function implies a larger fraction of axion dark matter occupies inter-minihalo space, potentially increasing the local axion density and improving haloscope detection prospects. This thesis highlights the significance of detailed modeling of stellar disruptions in shaping the axion dark matter distribution.

Figures (19)

• Figure 1: Survival fraction (SF) as a function of the normalized total injected energy $E_{\rm frac}$ into the minihalo as a result of stellar interaction, for concentration parameters $c=10,30,100,500$. The solid curves are the output of our implementation of K2021's analytical approach. The dots are numerical simulation data from S2023. The dashed curves are the empirical fitting functions used by S2023.
• Figure 2: The normalized injected energy per unit mass $\vert\Delta\epsilon\vert$ in a minihalo due to a stellar interaction is plotted as a function of the normalized radius $x$. In doing so, the total injected energy $E_{\rm frac}$ is set to 0.1, 1, 10. The normalized relative potential $\psi$ is also plotted as a function of $x$. The $\psi(x)$ curve does not vary with $E_{\rm frac}$. In all cases for this figure, the concentration parameter $c=10$. The value of $x$ at which $\vert\Delta\epsilon(x)\vert$ and $\psi(x)$ curves intersect is called the normalized crossover radius $x^*$.
• Figure 3: The same as Fig. \ref{['fig:SF vs Efrac for four values of concentration']} except that the solid curves are the output of our analytical approach using the sequential stripping model of mass loss in the minihalo.
• Figure 4: The top and bottom panels show the density profile of an NFW minihalo (black curve) which has a stellar encounter with impact parameters $b = 2\times10^{-5}\rm kpc$ and $b = 5\times10^{-5}\rm kpc$ respectively. The NFW minihalo has an initial concentration $c_{\rm s}=100$ and virial mass $M_{\rm vir} = 10^{-10}\rm M_\odot$. The orange curve shows the case where the remnant minihalo has relaxed to a Hernquist density profile, which is a broken power law (BPL) profile with $k=3$. The blue dots are numerical data points of the resulting density profile stabilized at $t = 2.5$ Gyr post-stellar interaction taken from S2023. We performed a curve fit of these data points to a broken power law profile and obtained its three parameters. The broken power law profile is shown as the red curves.
• Figure 5: The same as Fig. \ref{['fig:SF vs Efrac - sequential stripping model']} except that the solid curves are the output of our analytical approach using the sequential stripping model of mass loss in the minihalo and taking into account relaxation of the remnant minihalo to a Hernquist profile.