Molecular transitions as probes of the physical conditions of extragalactic environments

Abstract

Aims. We present a method to interpret molecular observations and molecular line ratios in nearby extragalactic regions. Methods. Ab initio grids of time dependent chemical models, varying in gas density, temperature, cosmic ray ionization rate, and radiation field, are used as inputs into RADEX calculations. Tables of abundances, column densities, theoretical line intensities, and line ratios for some of the most used dense gas tracers are provided. The degree of correlation as well as degeneracy inherent in molecular ratios is discussed. Comparisons of the theoretical intensities with example observations are also provided. Results. We find that, within the parameters space explored, chemical abundances can be constrained by a well-defined set of gas density, gas temperature, and cosmic ray ionization rates for the species we investigate here. However, line intensities, and more importantly line ratios, from different chemical models can be very similar, thereby leading to a clear degeneracy. We also find that the gas subjected to a galactic cosmic ray ionization rate will not necessarily have reached steady state in 1 million years. The species most affected by time dependency effects are HCN and CS, which are both high density tracers. We use our ab initio method to fit an example set of data from two galaxies, i.e. M82 and NGC 253. We find that (i) molecular line ratios can be easily matched even with erroneous individual line intensities, (ii) no set of species can be matched by a one-component interstellar medium (ISM), and (iii) a species may be a good tracer of an energetic process but only under specific density and temperature conditions. Conclusions. We provide tables of chemical abundances and line intensities ratios for some of the most commonly observed extragalactic tracers of dense gas for a grid of models. We show that by taking the chemistry behind each species and the individual line intensities into consideration, many degeneracies that arise by just using molecular line ratios can be avoided. Finally we show that using a species or a ratio as a tracer of an individual energetic process, such as cosmic rays and UV, ought to be done with caution.

A Hard X-Ray Test of HCN Enhancements As a Tracer of Embedded Black Hole Growth

“…This suggests two possibilities: 1) the HCN/HCO + enhancements trace an alternate physical process than AGN activity, or 2) these MIR classified starbursts host AGN that are behind obscuring screens sufficiently thick to render MIR diagnostics ineffective. The former is supported by theoretical work showing that differences in the HCN/HCO+ line intensity ratios can arise due to many different densities, temperatures, radiation fields, and evolutionary states of activity (e.g., Viti 2017). The latter scenario is plausible as the necessary optical depths are seen in the "compact obscured nuclei" (CONs; Sakamoto et al 2010;Aalto et al 2015;Falstad et al 2015;Scoville et al 2015) which can have obscuring columns in excess of N H > 10 25 cm −2 and show emission from vibrationally excited HCN molecules (HCN-VIB).…”

“…During Phase III (1 − 2 Myr) the cosmic ray flux is reduced back to the Galactic value and the chemical evolution is followed; this phase is intended to approximate the molecular gas chemistry after rapid cessation of SMBH accretion. We note that these models are general and it is not appropriate to compare these values with observed intensity ratios; the models do not include any observational considerations, including resolution effects (see e.g., Viti 2017). ing, the chemical evolution may be much slower. Thus, HCN enhancements can persist long past the time in which an AGN has turned off, consistent with our finding that HCN enhancements are poor indicators of current SMBH growth.…”

A chemical study of carbon fractionation in external galaxies

“…We have in fact compared the abundances of the main nitrogen-bearing species, their isotopologues and the fractionation ratios and found that in small cases the behavioural trends remain unchanged. We ran UCLCHEM in two phases in a very similar manner as in Viti (2017) where theoretical abundances for extragalactic studies were derived. In Phase I, the gas is allowed to collapse from a diffuse atomic state (∼ 10 cm −3 ) up to a high density (chosen by the user) by means of a free-fall collapse.…”

Astrochemical Kinetic Grid Models of Groups of Observed Molecular Abundances: Taurus Molecular Cloud 1 (TMC-1)

“…Cl + 1.0(-9) F 6.68(-9) * Increasing oxygen abundance corresponds to decreasing oxygen depletion for dark cloud conditions (Hincelin et al 2011). have also been used to determine time-dependent column densities, benchmark chemical networks and models, and predict molecular emission line intensities in starburst galaxies from statistical equilibrium calculations in the LVG approximation (Viti 2017). We automate the execution of the Nautilus code (Ruaud, Wakelam, and Hersant 2016) over a 7-D parameter space (gas kinetic temperature T gas , density n, visual extinction A V , cosmic ray ionization rate ζ, dust temperature T dust , reactive desorption efficiency a, and diffusion to binding energy ratio b), as shown in Table 1, and for each molecule i we construct a 9-D data structure {X i , t, T gas , n, A V , ζ, T dust , a, b} containing the abundance X i at every time t and every combination p of free parameters {T gas , n, A V , ζ, T dust , a, b}.…”