[Paper Review] Molecular line emission in NGC1068 imaged with ALMA: II. The chemistry of the dense molecular gas

S. Viti, S. García‐Burillo+20 more|arXiv (Cornell University)|Jul 18, 2014

Astrophysics and Star Formation Studies65 references50 citations

TL;DR

This study uses ALMA and PdBI observations of molecular lines in NGC 1068’s circumnuclear disk to analyze the dense gas chemistry. It finds that multiple gas-phase components—differing in density, temperature, and chemistry—are required to explain observed line ratios, with the AGN region showing distinct molecular depletion and chemical differentiation across the disk.

ABSTRACT

We present a detailed analysis of ALMA Bands 7 and 9 data of CO, HCO+, HCN and CS, augmented with Plateau de Bure Interferometer (PdBI) data of the ~ 200 pc circumnuclear disk (CND) and the ~ 1.3 kpc starburst ring (SB ring) of NGC~1068, a nearby (D = 14 Mpc) Seyfert 2 barred galaxy. We aim at determining the physical characteristics of the dense gas present in the CND and whether the different line intensity ratios we find within the CND as well as between the CND and the SB ring are due to excitation effects (gas density and temperature differences) or to a different chemistry. We estimate the column densities of each species in Local Thermodynamic Equilibrium (LTE). We then compute large one-dimensional non-LTE radiative transfer grids (using RADEX) by using first only the CO transitions, and then all the available molecules in order to constrain the densities, temperatures and column densities within the CND. We finally present a preliminary set of chemical models to determine the origin of the gas. We find that in general the gas in the CND is very dense (> 10^5 cm^-3) and hot (T> 150K), with differences especially in the temperature across the CND. The AGN position has the lowest CO/HCO+, CO/HCN and CO/CS column density ratios. RADEX analyses seem to indicate that there is chemical differentiation across the CND. We also find differences between the chemistry of the SB ring and some regions of the CND; the SB ring is also much colder and less dense than the CND. Chemical modelling does not succeed in reproducing all the molecular ratios with one model per region, suggesting the presence of multi-gas phase components. The LTE, RADEX and chemical analyses all indicate that more than one gas-phase component is necessary to uniquely fit all the available molecular ratios within the CND.

Motivation & Objective

• To determine the physical and chemical conditions of dense molecular gas in the circumnuclear disk (CND) and starburst ring of NGC 1068.
• To disentangle whether observed line ratio variations are due to excitation differences (density/temperature) or chemical differentiation.
• To assess the role of AGN and starburst feedback in shaping molecular gas chemistry in galactic nuclei.
• To evaluate the limitations of LTE and RADEX models in reproducing observed molecular line ratios in extragalactic environments.

Proposed method

• Used ALMA Bands 7 and 9 data for CO, HCO⁺, HCN, and CS transitions in the CND and starburst ring of NGC 1068.
• Applied Local Thermodynamic Equilibrium (LTE) models to estimate column densities of molecular species.
• Conducted large one-dimensional non-LTE radiative transfer modeling using RADEX to constrain gas densities, temperatures, and column densities.
• Combined RADEX results with a time-dependent gas-grain chemical model (UCL_CHEM) to explore chemical drivers in different regions.
• Compared RADEX solutions with a grid of chemical models to assess consistency with observed line ratios.
• Used PdBI data to supplement ALMA resolution for the ~200 pc CND and ~1.3 kpc starburst ring.

Experimental results

Research questions

• RQ1 What are the physical conditions (density, temperature, column density) of dense molecular gas in the CND of NGC 1068?
• RQ2 Are the observed variations in molecular line ratios across the CND due to excitation differences or chemical differentiation?
• RQ3 How do the chemical properties of the CND compare to those of the starburst ring?
• RQ4 Can a single chemical model reproduce the observed molecular line ratios across the CND, or is a multi-phase component model required?
• RQ5 What role do X-ray and cosmic-ray ionization play in shaping the molecular chemistry in the AGN-dominated region?

Key findings

• The CND hosts extremely dense (n > 10⁵ cm⁻³) and hot (T > 150 K) gas, with the AGN position showing the lowest CO/HCO⁺, CO/HCN, and CO/CS column density ratios.
• RADEX modeling indicates significant temperature variations across the CND, with the AGN region being the hottest and the E Knot the coolest.
• The starburst ring is substantially cooler and less dense than the CND, with chemistry possibly resembling the W Knot but at lower excitation conditions.
• Chemical modeling with UCL_CHEM suggests that each subregion of the CND likely contains a three-phase ISM: two dense gas phases with high ionization and one shocked component where CS and possibly HCN originate.
• The inability of a single chemical model to reproduce all observed ratios across the CND implies that multiple gas-phase components are necessary to explain the data.
• A higher number of molecular transitions—especially vibrationally excited lines—at ALMA resolution is required to fully resolve the physical and chemical complexity of the CND.