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We investigate the relationship between CN N = 1 − 0 and HCN J = 1 − 0 emission on scales from 30 to 400 pc using ALMA archival data, for which CN is often observed simultaneously with the CO J = 1 − 0 line. In a sample of nine nearby galaxies ranging from ultra-luminous infrared galaxies to normal spiral galaxies, we measure a remarkably constant CN/HCN line intensity ratio of 0.86 ± 0.07 (standard deviation of 0.20). This relatively constant CN/HCN line ratio is rather unexpected, as models of photon dominated regions have suggested that HCN emission traces shielded regions with high column densities while CN should trace dense gas exposed to high ultraviolet radiation fields. We find that the CN/HCN line ratio shows no significant correlation with molecular gas surface density but shows a mild trend (increase of ∼1.3 per dex) with both star formation rate surface density and star formation efficiency (the inverse of the molecular gas depletion time). Some starburst and active galactic nuclei show small enhancements in their CN/HCN ratio, while other nuclei show no significant difference from their surrounding discs. The nearly constant CN/HCN line ratio implies that CN, like HCN, can be used as a tracer of dense gas mass and dense gas fraction in nearby galaxies.

 

Abstract The properties of young massive clusters (YMCs) are key to understanding the star formation mechanism in starburst systems, especially mergers. We present Atacama Large Millimeter/submillimeter Array high-resolution (∼10 pc) continuum (100 and 345 GHz) data of YMCs in the overlap region of the Antennae galaxy. We identify six sources in the overlap region, including two sources that lie in the same giant molecular cloud (GMC). These YMCs correspond well with radio sources in lower-resolution continuum (100 and 220 GHz) images at GMC scales (∼60 pc). We find most of these YMCs are bound clusters through virial analysis. We estimate their ages to be ∼1 Myr and that they are either embedded or just beginning to emerge from their parent cloud. We also compare each radio source with a Pa β source, and find they have consistent total ionizing photon numbers, which indicates they are tracing the same physical source. By comparing the free–free emission at ∼10 pc scale and ∼60 pc scale, we find that ∼50% of the free–free emission in GMCs actually comes from these YMCs. This indicates that roughly half of the stars in massive GMCs are formed in bound clusters. We further explore the mass correlation between YMCs and GMCs in the Antennae and find it generally agrees with the predictions of the star cluster simulations. The most massive YMC has a stellar mass that is 1%–5% of its host GMC mass.