Neutral hydrogen and spiral structure in M33

Observations of neutral hydrogen (H I) in the galaxy M33 are presented which have sufficient angular resolution ( 47 × 93 arcsec) to distinguish detailed HI spiral structure for the first time. HI spiral features extend over the entire disc; the pattern is broken and multi-armed with the best defined arms lying at radii outside the brightest optical ‘features’. Several very narrow spiral ‘filaments’ are unresolved by the beam, implying true widths ≲ 160 pc. The H I is well correlated with other Population I material. The only direct evidence for the presence of a density-wave in M33 is derived from the morphology of the inner southern arm. The radial velocity field, with resolution 16 km s–1 , is perturbed near the inner spiral arms. These perturbations agree with the predictions of density-wave theory but may simply arise from the self-gravity of massive arms whether or not they are a quasi-stationary wave phenomenon. If the outer spiral features form a rigidly rotating density-wave pattern, the absence of large radial streaming motions along the features implies a small pattern speed |<\,25\,km\,s⁻¹kpc⁻¹|⁠, with corotation in the outer parts of the disc.