# New astrophysical bounds on ultralight axionlike particles

## TL;DR

This paper investigates ultralight axionlike particles (ULALPs) as dark matter candidates, showing they thermalize into a Bose-Einstein condensate and deriving lower mass bounds based on galactic observations.

## Contribution

It demonstrates that ULALPs thermalize via gravitational self-interactions, forming a Bose-Einstein condensate, and provides new astrophysical bounds on their mass.

## Key findings

- ULALPs thermalize and form a Bose-Einstein condensate.
- Lower mass bounds on ULALPs are around 10^{-23} to 10^{-20} eV.
- ULALPs can explain caustic rings of dark matter.

## Abstract

Motivated by tension between the predictions of ordinary cold dark matter (CDM) and observations at galactic scales, ultralight axionlike particles (ULALPs) with mass of the order $10^{-22}~{\rm eV}$ have been proposed as an alternative CDM candidate. We consider cold and collisionless ULALPs produced in the early Universe by the vacuum realignment mechanism and constituting most of CDM. The ULALP fluid is commonly described by classical field equations. However, we show that, like QCD axions, the ULALPs thermalize by gravitational self-interactions and form a Bose-Einstein condensate, a quantum phenomenon. ULALPs, like QCD axions, explain the observational evidence for caustic rings of dark matter because they thermalize and go to the lowest energy state available to them. This is one of rigid rotation on the turnaround sphere. By studying the heating effect of infalling ULALPs on galactic disk stars and the thickness of the nearby caustic ring as observed from a triangular feature in the IRAS map of our galactic disk, we obtain lower-mass bounds on the ULALP mass of order $10^{-23}$ and $10^{-20}~{\rm eV}$, respectively.