From gas to stars along the spiral wave: CO, HCN, and star formation variations across the spiral arms in NGC 4321 and M51

TL;DR

This study investigates how molecular gas, dense gas, and star formation vary across spiral arms in two galaxies, revealing that galactic dynamics influence these variations and contribute to observed scatter in molecular ratios.

Contribution

It provides detailed observational evidence of gas and star formation variations across spiral arms at high resolution, linking galactic dynamics to molecular cloud evolution.

Findings

HCN/CO, SFR/CO, and SFR/HCN increase from upstream to downstream in NGC 4321.

Trends are less prominent in M51, indicating galaxy-specific differences.

Galactic density waves induce variations in gas density and star formation efficiency across spiral arms.

Abstract

Molecular clouds form stars from the interstellar medium via gravitational collapse, following a sequence from low-density gas to high-density cores and eventually the formation of stars. In classical density wave theory, gas clouds orbiting the galaxy experience gas compression and triggered star formation, while encountering the gravitational well of spiral arms. We aim to trace these different phases of the molecular cloud life cycle via tracers of molecular gas (CO), dense molecular gas (HCN), and star formation (H$\alpha$, 24 $\mu$m) within the spiral arms of two grand-design spiral galaxies: NGC 4321 and M51 (NGC 5194). In the spiral arms of these galaxies, we investigate the relation between molecular gas, dense gas, and star formation (CO-HCN-SFR) at matched physical resolutions of 270 pc and 125 pc in NGC 4321 and M51, respectively. We employed spiral arm masks for these galaxies and investigate trends of HCN/CO and SFR/HCN (SFR/CO), which serve as proxies for the dense gas fraction and dense (molecular) gas star formation efficiency, perpendicular to the spiral arm spines. We find that HCN/CO, SFR/CO, and SFR/HCN increase from the upstream towards the downstream side of both spiral arms of NGC 4321, while their trends are less prominent in M51. Our results indicate that large-scale galactic dynamics (e.g. density waves) can induce a sequence of gas density and star formation-to-gas density variations perpendicular to the spiral arms. This sequence contributes to the increased scatter seen among spectroscopic ratios such as HCN/CO and SFR/HCN at sub-kiloparsec scales.