Wave–Tide Locking in Thin Stellar Streams as an Analytic Mass Spectrometer for Ultralight Axion Dark Matter

Thin stellar streams provide sensitive probes of subgalactic gravitational structure. We
point out a simple analytic relation linking three length scales in a fuzzy-dark-matter stream
progenitor: the tidal radius of a bound dark core, its gravitational Bohr radius, and the axion
de Broglie wavelength evaluated at the stripped-star velocity scale. If the stream width is
w = Cwrt, with Cw = O(1), then
rt
rB
= 4π2
C2
w
w
λdB
2
. (1)
Thus the condition λdB ∼ w automatically implies rt/rB ∼ 25–40: the dark wave core survives
well inside the tidal boundary while the extended stellar envelope is stripped into a thin stream.
This “wave–tide locking” gives a no-simulation axion-mass estimator,
ma ≃ 2.69 × 10−19 eV
√2
qκ
! R
10 kpc
38 pc
w
2 w
ℓripple
220 km s−1
vc

, (2)
where R is Galactocentric radius, qκ = κ/Ω, and ℓripple is an observed small-scale ripple or
coherence length identified with λdB. In the simplest locked case ℓripple ∼ w, representative clean
thin streams point to ma ∼ 2–5×10−19 eV. We emphasize that this is not a proof that axions are
the dark matter; it is a falsifiable analytic mass-spectrometer test for an intermediate ultralight
axion component, to be followed by homogeneous stream-catalog analyses and Schr¨odinger–
Poisson simulations