Substructure, satellites & stellar streams

Beyond dwarfs and the forest, a third class of probe counts the smallest halos directly — through satellite galaxies, gravitational lensing, and perturbations to stellar streams. All measure the FDM small-halo cutoff, and all currently favour a heavy boson.

Counting the smallest halos

Fuzzy dark matter's defining prediction is a deficit of low-mass halos below the half-mode mass (Topic 7.4). Several observations count those halos, directly or by their gravitational effects, and so bound the cutoff — and the boson mass.

Satellite galaxies

The number of satellite galaxies orbiting the Milky Way counts luminous subhalos. FDM suppresses low-mass subhalos, so too strong a cutoff would leave too few satellites. The observed (and increasingly complete) satellite census sets a lower bound on $m$.

Lensing and stellar streams

Two elegant dynamical probes reach dark, starless subhalos: strong-lensing flux-ratio anomalies and gravitational imaging detect subhalo perturbations to lensed images, while cold stellar streams (tidal debris of disrupted clusters) develop gaps and wiggles when a dark subhalo passes through. Both measure the substructure abundance — the cleanest handle on the cutoff, since it needs no luminous tracer.

Worked example — the direction of the bound

Each of these — satellites, lensing, streams — finds enough low-mass structure to require $m_{22}\gtrsim$ several to tens. Like the forest, they push the boson mass up, reinforcing the tension with the dwarf cores. They probe the same cutoff, from a different observable.

Substructure probes count the smallest halos — via satellites, lensing, and stream gaps — measuring the FDM cutoff. Like the forest, they favour a heavy boson, reinforcing the tension with dwarf cores.

Complementarity

These probes are powerful precisely because they are independent: dwarfs probe halo interiors, the forest and substructure probe halo abundance. That they pull in opposite directions on the boson mass is what makes the tension serious rather than a single-probe systematic.

Subhalo counts: FDM suppresses substructure, reinforcing the heavy-boson bound.

In our research

These abundance probes measure the same low-mass cutoff our Task 1 predicts ($M_{1/2}$, the delayed collapse). They mostly reinforce the heavy-boson side of the 8.7$\sigma$ tension (Topic 10.4), against the light-boson dwarf cores of Topic 10.1.

Key references

Nadler et al. (2021), Milky Way satellite constraints on FDM, Phys. Rev. Lett. 126, 091101 (arXiv:2008.00022).
Schutz (2020), Subhalo mass function and ultralight DM, Phys. Rev. D 101, 123026 (arXiv:2001.05503).
Banik et al. (2021), Stream constraints on DM substructure, JCAP 10, 043.

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