# PRUSSIC II -- ALMA imaging of dense-gas tracers in SDP.81: Evidence for low mechanical heating and a sub-solar metallicity in a z=3.04 dusty galaxy

**Authors:** M. Rybak, J. van Marrewijk, J. A. Hodge, P. Andreani, G. Calistro Rivera, L. Graziani, J. P. McKean, S. Viti, P. P. van der Werf  
**arXiv:** [2308.02886](https://arxiv.org/abs/2308.02886)  
**Date:** 2023-08-05  
**Summary page:** https://www.summarizepaper.com/en/paper/2308.02886v1/

## Key points

- Deep ALMA Band 3 observations of HCN, HCO+, and HNC (4-3) emission in SDP.81
- High-density gas in galaxies at redshifts greater than or equal to 1 remains unexplored
- Third detection of HCO+(4-3) emission in a high-redshift star-forming galaxy
- Unusually high HCO+/HCN intensity ratio of ≥2.2 in SDP.81
- Low mechanical heating and sub-solar metallicity (Z=0.5 Z⊙) contribute to the elevated ratio
- Impact of low mechanical heating and sub solar metallicity on gas mass estimates derived from CO observations
- Detection of CO(0-1) absorption from the foreground lensing galaxy
- Detection of CO(1-0) emission from a massive companion located approximately 50 kpc southeast of the lensing galaxy
- Launching Prussic2, a comprehensive census of dense gas tracers in high redshift star forming galaxies
- Previous work revealed low dense gas fractions and elevated dense gas star forming efficiencies in z ∼ 3 lensed DSFGs

## Summary

We present deep ALMA Band 3 observations of the HCN, HCO+, and HNC (4-3) emission in SDP.81, a well-studied z = 3.042 strongly lensed galaxy. These lines trace the high-density gas which has remained almost entirely unexplored in galaxies at redshifts greater than or equal to 1. The detection and resolution of the HCO+(4-3) emission in SDP.81 is significant as it represents only the third detection of this line in a high-redshift star-forming galaxy. Interestingly, we find an unusually high HCO+/HCN intensity ratio of ≥2.2 in SDP.81. Based on photodissociation region modeling, we propose that this elevated ratio is due to low mechanical heating - less than 10% of the total energy budget - and a sub-solar metallicity (Z=0.5 Z⊙). These conditions may not be representative of all high-redshift dusty galaxies; however, lower-than-solar metallicity could have a significant impact on gas masses inferred from CO observations. In addition to our findings on dense-gas tracers, we report the detection of CO(0-1) absorption from the foreground lensing galaxy and CO(1-0) emission from a massive companion located approximately 50 kpc southeast of the lensing galaxy. This study expands upon previous research by launching Prussic2, a comprehensive census of dense gas tracers in high redshift star forming galaxies. Our previous work with JVLA observations revealed low dense gas fractions and elevated dense gas star forming efficiencies in six z ∼ 3 lensed DSFGs. Overall, our findings provide valuable insights into the properties and characteristics of high density gas in distant galaxies and highlight the potential impact of low mechanical heating and sub solar metallicity on gas mass estimates derived from CO observations.