2018MNRAS.479.2505K

We present new L-band (1-2 GHz) observations of the nearby spiral galaxy M33 with 80 pc resolution obtained with the Karl G. Jansky Very Large Array. The H I observations, combined with H I measurements from the Green Bank Telescope, improve the spectral resolution and sensitivity (2.8 K rms noise in a 0.2 km s^–1 channel) compared to previous observations. We find individual profiles are usually non-Gaussian, harbouring line wings, multiple components, and asymmetries. Given this spectral complexity, we quantify the motions in the atomic interstellar medium through moment analysis of the spectra and fits to aligned, stacked profiles. The measured value of the H I line width depends strongly on the method used, with the velocity stacked profiles aligned to the peak velocity giving the minimum value of σ = 7 km s^–1 and all other methods giving higher values (σ ∼ 10 km s^–1). All measurements of the line width show a shallow radial trend, with σ decreasing by ∼2 km s^–1 from R_gal = 0 to R_gal = 8 kpc. We consider a number of energy sources that might maintain the line width against turbulent dissipation, but no single source is adequate. We find excess emission relative to a Gaussian in the stacked profile line wings, ranging from 9 per cent to 26 per cent depending on how the spectra are aligned. By splitting the line wings into symmetric and asymmetric components, we find that the lagging rotational disc accounts for one-third of the line wing flux. We also find emission far from the rotation-axis of the galaxy in multiple discrete H I clouds, including a filament with a projected length of ∼8 kpc.