# Heating of Milky Way disc Stars by Dark Matter Fluctuations in Cold Dark Matter and Fuzzy Dark Matter Paradigms

**Authors:** Benjamin V. Church, Jeremiah P. Ostriker, Philip Mocz  
**arXiv:** [1809.04744](https://arxiv.org/abs/1809.04744)  
**Date:** 2018-09-13  
**Summary page:** https://www.summarizepaper.com/en/paper/1809.04744v1/

## Key points

- Cold Dark Matter (CDM) models have discrepancies at galactic scales, predicting unobserved over-dense "cusps" in dwarf galaxies and overestimating their formation rate.
- Fuzzy Dark Matter (FDM), an ultra-light axion-like scalar boson, offers a solution by avoiding cusps and suppressing small-scale power while delaying galaxy formation through macroscopic quantum pressure.
- FDM introduces non-virialized over-dense wavelets formed by quantum interference patterns, serving as a more efficient source of heating for galactic discs compared to subhalos.
- Wavelet heating can lead to the oldest disc stars acquiring a velocity dispersion exceeding $30 \: \mathrm{km} \: \mathrm{s}^{-1}$ within a Hubble time in the solar neighborhood.
- Dynamical substructure heating in both CDM and FDM could cause a flaring that terminates the Milky Way disc at $15 - 20 \: \mathrm{kpc$}.
- The study sets a lower limit on the FDM particle mass at $m_a > 0.57 \times 10^{-22} \mathrm{eV}$ and suggests that FDM wavelets may play a crucial role in producing observed disc thickening over time.
- Previous research provides insights into various distribution functions of Milky Way stars and data on thin and thick disc velocity dispersion, supporting the idea that Fuzzy Dark Matter could offer new perspectives on galactic dynamics.

## Summary

In their study, Benjamin V. Church, Jeremiah P. Ostriker, and Philip Mocz delve into the discrepancies between Cold Dark Matter (CDM) models and observations at galactic scales. CDM models have been successful on cosmological scales but predict unobserved over-dense "cusps" in dwarf galaxies and overestimate their formation rate. To address these issues, the researchers consider Fuzzy Dark Matter (FDM), an ultra-light axion-like scalar boson. The FDM model offers a promising solution by avoiding cusps and suppressing small-scale power while delaying galaxy formation through macroscopic quantum pressure. By comparing galactic dark matter halos comprised of ultra-light axions with conventional CDM results, the study reveals intriguing findings. In addition to self-gravitating subhalos, FDM introduces non-virialized over-dense wavelets formed by quantum interference patterns. These wavelets serve as a more efficient source of heating for galactic discs compared to subhalos. The researchers demonstrate that in the solar neighborhood, wavelet heating can lead to the oldest disc stars acquiring a velocity dispersion exceeding $30 \: \mathrm{km} \: \mathrm{s}^{-1}$ within a Hubble time if all energy delivered to the disc is converted into velocity dispersion. Furthermore, calculations of radius-dependent disc velocity dispersion and corresponding scale height resulting from dynamical substructure heating in both CDM and FDM indicate that these effects could cause a flaring that terminates the Milky Way disc at $15 - 20 \: \mathrm{kpc}$. While the exact source of thickened discs remains uncertain, it is established that perturbations caused by dark substructure cannot surpass the total disc velocity dispersion. As a result, this work sets a lower limit on the FDM particle mass at $m_a > 0.57 \times 10^{-22} \mathrm{eV}$ and suggests that FDM wavelets may play a crucial role in producing observed disc thickening over time. Moreover, focusing on the Milky Way as one of the most extensively studied galactic discs, previous research by Binney (2010) provides valuable insights into various distribution functions of Milky Way stars and data on thin and thick disc velocity dispersion as a function of vertical distance. This comprehensive study sheds light on how Fuzzy Dark Matter could offer new perspectives on galactic dynamics and provides compelling evidence for considering FDM wavelets as an essential mechanism contributing to observed phenomena in galactic structures like our own Milky Way.