[Paper Review] The coupling of dynamics and molecular chemistry in galaxies

F. Combes|ArXiv.org|Sep 24, 2007

Astronomy and Astrophysical Research3 citations

TL;DR

This paper reviews how molecular line ratios—particularly HCN, HCO⁺, and CO—serve as diagnostics of dense gas and physical conditions in galaxies, distinguishing between starburst (PDR) and AGN (XDR) environments. It shows that HCN/CO ratios >0.3 in ULIRGs likely stem from XDR chemistry rather than dense gas alone, creating a degeneracy in using HCN as a star formation tracer.

ABSTRACT

While the best tracer of the molecular component and its dynamics in galaxies is the CO molecule, which excitation is revealed by its isotopic and (2-1)/(1-0) ratios, the denser gas is revealed by molecules such as HCN, HNC, HCO+ or CN, which are now widely used to probe star formation regions, or to quantify the impact of the nuclear activity on the interstellar medium. This paper reviews recent observations in nearby galaxies, where these molecular line ratios serve as diagnostic tools of the physical conditions of the gas and also of its chemical properties. Those differ significantly according to the proximity of an AGN or of a starburst. The origin of the differences is not yet well known and could be due to different densities, temperatures, chemical abundances or non-collisional excitation of the gas (e.g. Aalto et al 2007, Krips et al 2007). HCN or HNC line enhancements can be caused not only by higher gas densities/temperatures, but also UV/X-ray radiation, and global IR pumping. The chemistry can be dominated by PDR regions near a starburst, or X-ray dominated in a molecular torus surrounding an AGN (XDR regions). The molecular line ratios expected in those regions vary according to the different models (Meijerink et al. 2007).

Motivation & Objective

• To understand the coupling between galactic dynamics (shocks, bars, outflows) and molecular chemistry in external galaxies.
• To resolve the degeneracy in interpreting enhanced HCN emission in ULIRGs as either dense gas tracers or signatures of X-ray dominated regions (XDR).
• To compare PDR (UV-irradiated) and XDR (X-ray-irradiated) chemistry models in starburst and AGN environments.
• To assess the reliability of molecular line ratios (e.g., HCN/CO, HCO⁺/CO) as tracers of star formation and nuclear activity.

Proposed method

• Analysis of high-resolution molecular line observations from IRAM, ALMA, and other telescopes in nearby galaxies (NGC 253, M82, NGC 1068).
• Use of multi-line molecular surveys to detect 35 new extragalactic species and 13 isotopic substitutes, including HCN, HNC, HCO⁺, CN, SiO, and complex organics.
• Comparison of observed line ratios (e.g., HCN/CO, HCO⁺/CO, HCN/HCO⁺) with predictions from PDR and XDR models.
• Modeling of X-ray and UV irradiation effects on molecular abundances, including non-collisional excitation and dust grain chemistry (e.g., PAH destruction).
• Use of CO(1-0), CO(2-1), and HCN(1-0) luminosity ratios to trace gas density, temperature, and excitation conditions.
• Spatially resolved mapping of SiO, HCN, and HCO⁺ emission to identify shock regions, outflows, and nuclear disks.

Experimental results

Research questions

• RQ1 What causes the enhanced HCN and HCO⁺ emission in ULIRGs—dense gas formation or X-ray-driven chemistry in XDR regions?
• RQ2 How do PDR and XDR models differ in predicting molecular line ratios such as HCN/CO and HCO⁺/CO?
• RQ3 To what extent do shocks from bars, outflows, or supernovae enhance SiO and other shock tracers in star-forming galaxies?
• RQ4 Why do PDR models fail to reproduce the observed high abundances of HCO⁺ and CO⁺ in M82’s starburst disk?
• RQ5 Can HCN/CO ratios alone distinguish between star formation and AGN activity in galactic nuclei?

Key findings

• HCN/CO ratios exceed 0.3 in ULIRGs, indicating XDR chemistry dominance rather than dense gas alone, challenging HCN as a pure star formation tracer.
• HCO⁺/CO ratios are low in ULIRGs (as per Gracia-Carpio et al. 2006), suggesting that HCN enhancement is not due to high gas density but likely X-ray-driven chemistry.
• SiO emission in NGC 253 is linked to bar-induced shocks and cloud-cloud collisions, with abundance ~10⁻⁹ in the circumnuclear disk, much higher than in PDRs.
• The HCN/CO(1-0) ratio increases with IR luminosity, correlating with LIR, but this correlation may reflect XDR chemistry, not dense gas or star formation rate.
• XDR models predict higher CO excitation and warmer CO molecules than PDRs, explaining the observed low CO(1-0)/H₂ ratio in XDR regions.
• PDR models fail to reproduce observed HCO⁺ and CO⁺ abundances in M82, suggesting missing physics such as PAH destruction or non-thermal excitation.