Stellar disruption of axion miniclusters in the Milky Way

Abstract

Axion miniclusters are dense bound structures of dark matter axions that are predicted to form in the postinflationary Peccei-Quinn symmetry breaking scenario. Although dense, miniclusters can easily be perturbed or even become unbound by interactions with baryonic objects such as stars. Here, we characterize the spatial distribution and properties of miniclusters in the Milky Way (MW) today after undergoing these stellar interactions throughout their lifetime. We do this by performing a suite of Monte Carlo simulations which track the miniclusters' structure and, in particular, accounts for partial disruption and mass loss through successive interactions. We consider two density profiles-Navarro-Frenk-White (NFW) and power-law (PL)-for the individual miniclusters in order to bracket the uncertainties on the minicluster population today due to their uncertain formation history. For our fiducial analysis at the solar position, we find a survival probability of 99% for miniclusters with PL profiles and 46% for those with NFW profiles. Our work extends previous estimates of this local survival probability to the entire MW. We find that towards the Galactic Center, the survival probabilities drop drastically. Although we present results for a particular initial halo mass function, our simulations can be easily recast to different models using the provided data and code (github.com/bradkav/axion-miniclusters). Finally, we comment on the impact of our results on lensing, direct, and indirect detection.

Gravitational waves from dark matter isocurvature

“…In the future, these methods may restrict the amount of microhalos to be less than 1% of CDM in the range of roughly 10 −12 − 100M . We note, however, that current and future constraints depend on the density profile of the halos, their halo mass function and tidal disruption by other compact objects such as stars [108]. The study of the formation of such microhalos in our present model and their evolution until today is an interesting topic but out of the scope of this paper.…”

“…This is in contrast with PBH formation from adiabatic fluctuations in a radiation dominated universe, where any fluctuations that do not end up forming a black hole are washed away by the free streaming of radiation. The formation of microhalos from adiabatic and isocurvature fluctuations has been studied in the context of axions, with the so-called axion miniclusters [108][109][110][111], and recently by long range scalar forces [112,113] and CDM self-interactions [114,115]. Since, in general, the CDM halos are rather disperse, some constraints applicable to PBHs do not apply or apply weakly to microhalos [112,116].…”

“…When we reach matter domination density fluctuations collapse into virialized objects. However, some of the formed halos will be disrupted by interaction with, e.g., stars[108].…”

QCD Axion Kinetic Misalignment without Prejudice

“…The clumping of such heavier building blocks also leads to an increased value of the maximal mass, with observational consequences. Note, however, that these large miniclusters would likely not survive tidal stripping from other astrophysical objects such as brown dwarfs and main sequence stars, especially in high-density regions such as the Galactic center [102,103].…”

“…In all DM models, tidal interactions destroy small-scale clumps [104,105], such as axion miniclusters [102]. Recent N-body simulations have proven that the stripping mechanism is crucial for the population of miniclusters in galaxies [103].…”