The Milky Way's dwarf satellites have drawn a curve for wave dark matter

A curve, not a number

The measurement reads as follows: at 95 per cent confidence, for models whose peak wavenumber exceeds 10,000 Mpc⁻¹, the mass of the ultralight scalar particle lies above 6 × 10⁻¹⁸ electronvolts. That number does not come from a new telescope. It comes from the dwarf satellite galaxy census compiled from Dark Energy Survey and Pan-STARRS1 observations, which we have had for years. What is new is that the census has been put, through COZMIC simulations, to a wider family of models.^[1]

The shape of the bound is its content. Instead of a single mass threshold, a relation between two quantities is given: as the peak wavenumber falls, the threshold drops quadratically. When a lower bound stops being a number and becomes a surface, the question the measurement answers changes with it; what is being ruled out is now a family, not a single particle candidate.^[1]

Where the difference between cold fuzziness and warm waves comes from

The two scenarios part company at their initial conditions. In cold fuzzy dark matter the field starts out homogeneous, and what suppresses structure is the Jeans length, which is large on astronomical scales. In warm wave dark matter the initial field is highly inhomogeneous, which produces both free-streaming suppression and Poisson-driven density fluctuations. As the authors put it, free streaming can suppress structure on still larger scales.^[1]

The abundance of small satellites can see that distinction, because both mechanisms delay the assembly of small-scale structure, and the faintest haloes around the Milky Way are the first casualties. Something else can produce the same missing satellites, however: how faint galaxies populate haloes at all. The census counts the haloes that shine, and the conversion from those to the total number of haloes is a model rather than an observation.^[1]

The assumption carrying the bound

The authors flag this themselves: in the warm ultralight models they consider, the galaxy–halo connection is assumed to be unchanged from cold dark matter, and they describe this as a natural area for follow-up work. That is where the bound draws both its strength and its fragility. If star formation in small haloes works differently in these models, the same satellite census will not yield the same curve. The place where the curve gets tested will not be a new particle, but an independent measurement of how faint galaxies come to shine.^[1]