Exploring the potential for kinematically colder H i component as a tracer for star-forming gas in nearby galaxies

Atomic hydrogen (H i) dominates the mass of the cold interstellar medium, undergoing thermal condensation to form molecular gas and fuel star formation. Kinematically colder H i components, identified via kinematic decomposition of H i 21 cm data cubes, serve as a crucial transition phase between diffuse warm neutral gas and molecular hydrogen (H$_2$). We analyse these colder H i components by decomposing H i 21 cm data cubes of seven nearby galaxies – Sextans A, NGC 6822, WLM, NGC 5068, NGC 7793, NGC 1566, and NGC 5236 – spanning metallicities (0.1 $<$ Z/$Z_{\rm \odot }$$<$ 1.0) and physical scales (53–1134 pc). Using a velocity dispersion threshold of 6 km s$^{-1}$, we classify the kinematically distinct components into narrow (colder) and broad (warmer). Cross-correlation analysis between the narrow H i components and H$_2$ or star formation rate (SFR) surface density at different spatial scales reveals that dwarf galaxies exhibit the strongest correlation at $\sim$500–700 pc. The radially binned narrow H i fraction, $f_{\rm n}$ = $I_{\rm narrow {\rm{ H\,\small I}} } / I_{\rm total {\rm{ H\,\small I}} }$, in dwarf galaxies shows no clear trend with metallicity or SFR, while in spirals, $f_{\rm n}$ is lower in inner regions with higher metallicity and SFR. We find that the data set resolution significantly impacts the results, with higher physical resolution data yielding a higher median $f_{\rm n}$, $\langle$$f_{\rm n}$$\rangle$, per galaxy. With this considered, dwarf galaxies consistently exhibit a larger $f_{\rm n}$ than spiral galaxies. These findings highlight the critical role of cold H i in regulating star formation across different galactic environments and emphasize the need for high resolution H i observations to further unravel the connection between atomic-to-molecular gas conversion and galaxy evolution.