2025ApJ...980...85M

Spectral siren measurements of the Hubble constant (H₀) rely on correlations between observed detector-frame masses and luminosity distances. Features in the source-frame mass distribution can induce these correlations. It is crucial, then, to understand (i) which features in the source-frame mass distribution are robust against model (re)parameterization, (ii) which features carry the most information about H₀, and (iii) whether distinct features independently correlate with cosmological parameters. We study these questions using real gravitational-wave observations from the LIGO-Virgo-KAGRA Collaborations' third observing run. Although constraints on H₀ are weak, we find that current data reveals several prominent features in the mass distribution, including peaks in the binary black hole source-frame mass distribution near ∼9M_⊙ and ∼32M_⊙ and a roll-off at masses above ∼46M⊙. For the first time using real data, we show that all of these features carry cosmological information and that the peak near ∼32M_⊙ consistently correlates with H₀ most strongly. Introducing model-independent summary statistics, we show that these statistics independently correlate with H₀, exactly what is required to limit systematics within future spectral siren measurements from the (expected) astrophysical evolution of the mass distribution.