A 260 pc resolution ALMA map of HCN(1-0) in the galaxy NGC 4321

TL;DR

This study uses high-resolution ALMA observations to analyze how dense gas and star formation vary within the galaxy NGC 4321, revealing environmental influences especially in the galaxy's center.

Contribution

First detailed 260 pc scale ALMA map of HCN(1-0) in NGC 4321, linking dense gas tracers with star formation across different galactic environments.

Findings

Dense gas fraction increases towards galaxy center.

Star formation efficiency of dense gas decreases in the center.

HCN/CO ratio predicts molecular gas surface density effectively.

Abstract

The star formation rate (SFR) is tightly connected to the amount of dense gas in molecular clouds. However, it is not fully understood how the relationship between dense molecular gas and star formation varies within galaxies and in different morphological environments. In this work, we study dense gas and star formation in the nearby spiral galaxy NGC 4321 to test how the amount of dense gas and its ability to form stars varies with environmental properties at 260 pc scales. We present new ALMA observations of HCN(1-0) line emission. Combined with existing CO(2-1) observations from ALMA, and H-alpha from MUSE, as well as F2100W from JWST to trace the SFR, we measure the HCN/CO line ratio, a proxy for the dense gas fraction and SFR/HCN, a proxy for the star formation efficiency of the dense gas. Towards the centre of the galaxy, HCN/CO systematically increases while SFR/HCN decreases, but these ratios stay roughly constant throughout the disc. Spiral arms, interarm regions, and bar ends show similar HCN/CO and SFR/HCN. On the bar, there is a significantly lower SFR/HCN at a similar HCN/CO. We conclude that the centres of galaxies show the strongest environmental influence on dense gas and star formation, suggesting either that clouds couple strongly to the surrounding pressure or that HCN is tracing more of the bulk molecular gas that is less efficiently converted into stars. On the contrary, across the disc of NGC 4321, where the ISM pressure is typically low, SFR/HCN does not show large variations (< 0.3 dex) in agreement with Galactic observations of molecular clouds. Despite the large variations across environments and physical conditions, HCN/CO is a good predictor of the mean molecular gas surface density at 260 pc scales.