Cloud-9 under the line-width thermometer: implications for halo concentration and a constraint on Λm³ in superfluid dark matter
Abstract
Cloud-9 is a starless, gas-rich dark-matter halo candidate near M94 and a leading candidate for a Reionization-Limited H I Cloud (RELHIC). Its lack of stars removes stellar-feedback explanations for a shallow central potential and makes the H I distribution an unusually clean probe of halo structure. We re-analyse the published VLA column-density profile with two independent reconstruction and modelling pipelines applied to different published representations of the same VLA observation. Both use hydrostatic gas, line-of-sight projection and beam convolution, and the pipeline with the more detailed gas physics is validated by reproducing the published warm NFW solution. The central observational issue is the line width. The VLA value W50 = 12 ± 1 km s⁻¹ corresponds, if Gaussian, to σ1D = 5.1 km s⁻¹ and a purely thermal H I upper temperature of about 3.1 × 10³ K. New FAST observations, combined with the VLA data, separate Cloud-9 into a compact kinematically quiet core and a diffuse envelope; the fitted core component has W50 = 17.34 ± 0.83 km s⁻¹, corresponding to about 6.6 × 10³ K if purely thermal. Neither width is compatible with a single neutral hydrogen component at 2.09 × 10⁴ K, for which thermal broadening alone gives W50 ≈ 30.9 km s⁻¹. Under the VLA cold branch, NFW fits require c ≈ 1.3–1.5. Pipeline B gives an illustrative 6.3σ concentration displacement relative to c ≈ 13 using a conservative 0.16 dex scatter; Pipeline A gives 9.8σ because its lower best-fit mass corresponds to a mass-dependent reference concentration c ≈ 18 and the 0.11 dex scatter used in that calibration. A Pipeline-B sensitivity run using the FAST core width shifts the preferred concentration to c ≈ 3.1, reducing but not eliminating the low-concentration tension. This ordering is consistent with an independent 2026 analysis that finds Cloud-9’s best-fitting CDM halo about 7σ below the cosmological concentration–mass relation. As an alternative spatial benchmark, the n = 1/2 superfluid polytrope with P = ρ³/(12Λ²m⁶) fits the cold profile and primarily constrains the combination q = Λ(meV) [m(eV)]³ along q ∝ Msf⁻¹/³. Pipeline B gives q = 3.25 (68 per cent: 1.67–4.40) conditional on Msf = 1.6–6 × 10⁹ M⊙; the FAST-width sensitivity shifts this branch to q ≈ 5.7. We therefore present Cloud-9 not as a detection of condensate dark matter or a falsification of CDM, but as a gated test: the resolved H I line shape, environmental boundary pressure, and cube-level profile modelling determine which branch survives.
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