# The Herschel Comprehensive (U)LIRG Emission Survey (HerCULES): CO Ladders, fine structure lines, and neutral gas cooling

**Authors:** M. J. F. Rosenberg, P. P. van der Werf, S. Aalto, L. Armus, V. Charmandaris, T. Díaz-Santos, A. S. Evans, J. Fischer, Y. Gao, E. González-Alfonso, T. R. Greve, A. I. Harris, C. Henkel, F. P. Israel, K. G. Isaak, C. Kramer, R. Meijerink, D. A. Naylor, D. B. Sanders, H. A. Smith  
**arXiv:** [1501.02985](https://arxiv.org/abs/1501.02985)  
**Date:** 2015-01-13  
**Summary page:** https://www.summarizepaper.com/en/paper/1501.02985v1/

## Key points

- (U)LIRGs are known for extreme infrared luminosities
- The HerCULES survey analyzed 29 (U)LIRGs using data from the Herschel Space Observatory
- Key emission lines observed include [CII] 157 μm, [OI] 63 μm, and [OI] 145 μm with PACS, CO J=4-3 through J=13-12, [CI] 370 μm, and [CI] 609 μm with SPIRE
- CO ladders classified based on excitation levels reveal insights into molecular gas properties
- Self-absorption observed in [OI] 63 μm emission line in 13 galaxies
- CO excitation correlates more strongly with far-infrared colors than IRAS ratios
- Cooling budgets highlight fine-structure line flux deficits in certain objects with high far-infrared fluxes
- Galaxies with large CO linewidths tend to have high-excitation CO ladders and lower AGN contributions, suggesting mechanical heating as an important factor

## Summary

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(Ultra) Luminous Infrared Galaxies ((U)LIRGs) are objects known for their extreme infrared luminosities. The Herschel Comprehensive ULIRG Emission Survey (HerCULES) provides a comprehensive analysis of 29 (U)LIRGs across a wide range of luminosities, utilizing data from the Herschel Space Observatory. The survey observes key emission lines such as [CII] 157 μm, [OI] 63 μm, and [OI] 145 μm with PACS, CO J=4-3 through J=13-12, [CI] 370 μm, and [CI] 609 μm with SPIRE, along with low-J CO transitions from ground-based telescopes. The CO ladders in the sample are classified based on their excitation levels, revealing interesting insights into the molecular gas properties of these galaxies. Notably, self-absorption is observed in the [OI] 63 μm emission line in 13 of the galaxies. By comparing CO excitation to IRAS ratios and far-infrared luminosity, it is found that CO excitation correlates more strongly with far-infrared colors. Furthermore, cooling budgets for the galaxies are presented, highlighting fine-structure line flux deficits in certain objects with high far-infrared fluxes. Interestingly, this deficit is not observed in CO transitions within a specific range. To investigate molecular gas heating mechanisms, a combination of diagnostic quantities including CO excitation levels and AGN contributions are analyzed. The results suggest that galaxies with large CO linewidths tend to exhibit high-excitation CO ladders and lower AGN contributions, indicating mechanical heating as an important factor in shaping the molecular gas properties of these (U)LIRGs. Overall, the HerCULES survey provides valuable insights into the complex interplay between different emission lines and heating mechanisms in (U)LIRGs across varying luminosity ranges.