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

TL;DR

This study investigates colder atomic hydrogen components in nearby galaxies and their potential as tracers for star-forming gas, revealing their significance varies with galaxy type and observational resolution.

Contribution

It introduces a kinematic decomposition method to analyze cold HI components across diverse galaxy types and scales, emphasizing the importance of resolution in understanding gas-phase transitions.

Findings

Dwarf galaxies show the strongest correlation between cold HI and star formation at 500-700 pc scales.

The cold HI fraction is higher in dwarf galaxies compared to spirals, regardless of metallicity.

Higher resolution data increases the median cold HI fraction, affecting interpretation of gas dynamics.

Abstract

Atomic hydrogen (HI) dominates the mass of the cold interstellar medium, undergoing thermal condensation to form molecular gas and fuel star formation. Kinematically colder HI components, identified via kinematic decomposition of HI 21 cm data cubes, serve as a crucial transition phase between diffuse warm neutral gas and molecular hydrogen (H$_{2}$). We analyse these colder HI components by decomposing HI 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 < $Z/Z_{\odot}$ < 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 HI components and H$_{2}$ or star formation rate (SFR) surface density at different spatial scales reveals that dwarf galaxies exhibit the strongest correlation at ~500-700 pc. The radially binned narrow HI fraction, $f_{\rm n} = I_{\rm narrowHI}/I_{\rm totalHI}$, 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 dataset 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 HI in regulating star formation across different galactic environments and emphasise the need for high-resolution HI observations to further unravel the connection between atomic-to-molecular gas conversion and galaxy evolution.