Origin of the ionised-to-cold gas dispersion offset

Determine whether the systematically higher velocity dispersions measured in warm ionised gas than in colder neutral or molecular gas reflect intrinsic phase-dependent kinematics or systematic measurement effects.

Background

The paper compiles galaxies with kinematic measurements in both warm ionised and colder gas phases and finds that ionised-gas velocity dispersions are commonly higher by approximately a factor of two. Possible explanations include stronger coupling of feedback to ionised gas, diffuse extraplanar ionised gas, different spatial distributions, and biases caused by spectral resolution, masking, and modelling choices.

REBELS-25 provides an especially useful comparison because both phases are observed with ALMA at far-infrared wavelengths and comparable spectral resolution. Nevertheless, alternative modelling choices produce substantially different dispersion ratios, leaving the physical interpretation unresolved.

References

Given the heterogeneity of current datasets, it remains unclear if these differences reflect intrinsic changes in the kinematics probed, or if this could be dominated by systematics in the measurements.

— REBELS-25: multi-phase morphology and kinematics at z = 7.31  (2608.25982 - Rowland et al., 26 Aug 2026) in Section 5.2, Multi-phase gas kinematics; reiterated in Section 6, Conclusions

Taken together, the large intrinsic scatter, the susceptibility of ionised gas to non-gravitational broadening, and the still-uncertain phase-to-phase relationship motivate resolved and consistent kinematic analyses of multiple tracers within the same galaxies with careful control of resolution and modelling systematics.

— MUSE-ALMA Haloes XV: Cold molecular, ionised, and stellar kinematics of HI-rich galaxies at z $\sim$ 0.5  (2610.10113 - Barfety et al., 7 Oct 2026) in Section 1, Introduction