Periodic Reporting for period 3 - MOPPEX (MOlecules as Probes of the Physics of EXternal galaxies)
Reporting period: 2023-06-01 to 2024-11-30
The MOPPEX project exploits the chemical richness of the dense molecular gas in galaxies to disentangle, in space and time, the many processes that determine the physical appearance of a galaxy, and to understand the role of each of these processes in the cosmic history of a galaxy.
Molecules pervade the cooler, denser parts of our Universe, in particular the reservoirs of the matter that forms stars and planets, and the gas in the centres of galaxies. In the Milky Way we routinely use molecules to discover and explore these regions and the more complex the chemistry, the more details of the gas the molecules reveal. There are one hundred billion galaxies in the observable Universe. About 200 or so are our neighbours. However, due to their distance, we are still not able to zoom in and observe individual clouds of dense gas. Nevertheless with the advent of ever more sensitive telescopes such as ALMA, we are discovering that chemistry in external galaxies is as complex as in our own Milky Way. Molecules, it seems, are universal and widespread.
In MOPPEX I am using molecules to shed light on the physical and chemical structure of our local galaxies, namely (i) what the energetic processes that determine their appearance are and (ii) where the matter that will form stars or fuels black holes is, with the ultimate goal to understand how galaxies form, evolve and interact with each other.
The ultimate objective is to fundamentally change the way molecular observations are interpreted for external galaxies and thus to cause a paradigm shift in the use of molecules as tools to determine the chemistry and physics of galaxies.
Molecules pervade the cooler, denser parts of our Universe, in particular the reservoirs of the matter that forms stars and planets, and the gas in the centres of galaxies. In the Milky Way we routinely use molecules to discover and explore these regions and the more complex the chemistry, the more details of the gas the molecules reveal. There are one hundred billion galaxies in the observable Universe. About 200 or so are our neighbours. However, due to their distance, we are still not able to zoom in and observe individual clouds of dense gas. Nevertheless with the advent of ever more sensitive telescopes such as ALMA, we are discovering that chemistry in external galaxies is as complex as in our own Milky Way. Molecules, it seems, are universal and widespread.
In MOPPEX I am using molecules to shed light on the physical and chemical structure of our local galaxies, namely (i) what the energetic processes that determine their appearance are and (ii) where the matter that will form stars or fuels black holes is, with the ultimate goal to understand how galaxies form, evolve and interact with each other.
The ultimate objective is to fundamentally change the way molecular observations are interpreted for external galaxies and thus to cause a paradigm shift in the use of molecules as tools to determine the chemistry and physics of galaxies.
To date we have performed work on two fronts, each covering a Theme of the MOPPEX project:
THEME 1: An important part of the project is the enhancements of our in-house codes (Theme 1 WP1) which are all publicly available on https://uclchem.github.io(opens in new window). To date we have
(i) augmented our chemical networks with carbon fractionation reactions and studied their effects on the gas of galaxies; (ii) devised methods for exploiting the topology of reaction networks; (iii) devised an open-source publicly available emulator (CHEMULATOR) for dynamical models; (iv) we employed Bayesian inference and neural network statistical emulation to estimate binding energies and probabilities of reactions for surface reactions; (vi) we devised an application of the MOPED (Massive Optimised Parameter Estimation and DATA) compression algorithm to the determination of which molecules on dust grains should be prioritised by future observations with the JWST.
As the new tools became available, we applied them to generate databases of physically and statistically informed quantities that will allow the community to ultimately use molecules observed in galaxies as rigorous tools to derive the necessary information to determine the formation and the evolution of a galaxy.
In particular we (i) used the latest UCLCHEM chemical code coupled with radiative transfer modelling and statistical approaches, to produce a list of molecular species for which the gas history in a galaxy or in a cloud can be ignored, removing a major modelling complexity. We then determine the best of these species to observe when attempting to constrain various physical parameters and created a public web-based tool (https://hits.strw.leidenuniv.nl/(opens in new window)) whereby a user can search the database we created and determine the best observational lines for their purpose; (ii) created a publicly available, open source first version of the code (UCLCHEMCMC) that allows the user, via a web interface (https://uclchemcmc.strw.leidenuniv.nl/(opens in new window)) to estimate the posterior probability distribution of the gas parameters.
THEME 2: the overarching aim of this Theme is to make use of state of the art observational data of two galaxies (used as astrochemical laboratories and bench-marking cases) as bed-tests for the application of the methodologies developed in Theme 1. As part of our effort to establish the physical and chemical characteristics of parsec-scale molecular gas in nearby galaxies we have been heavily involved in the ALMA large program ALCHEMI and we led or significantly contributed to (i) the study of the emission of the C2H molecule in the central zone of NGC 253 and used it to give initial constraints on the cosmic ray ionization in the centre of the galaxy NGC 253; (ii) the study of the emission of the H3O+ and SO emission in NGC 253 which allowed us to firmly constrain the cosmic ray ionization rate; (iii) the study of the emission of HCN and HNC in NGC 253 to trace the sources of heating in this galaxy. The ALCHEMI work above dealt with the starburst galaxy NGC 253. Our second test-bed was the AGN dominated galaxy NGC 1068; for this galaxy (iv) we - for the first time - performed a systematic analysis of the usefulness of specific molecular ratios to "isolate" AGN-dominated galaxies.
These galaxies are very energetic and the molecular gas in them is highgly affected by shocks. In order to locate and characterize the shock components in these two different types of galaxies: (i) we have determined for the first time the shock structure and location of the shocks in the central nucleus of NGC 1068
and (ii) the nucleus of NGC 253.
All the work performed so far and briefly described above has been published in peer-reviewed articles.
THEME 1: An important part of the project is the enhancements of our in-house codes (Theme 1 WP1) which are all publicly available on https://uclchem.github.io(opens in new window). To date we have
(i) augmented our chemical networks with carbon fractionation reactions and studied their effects on the gas of galaxies; (ii) devised methods for exploiting the topology of reaction networks; (iii) devised an open-source publicly available emulator (CHEMULATOR) for dynamical models; (iv) we employed Bayesian inference and neural network statistical emulation to estimate binding energies and probabilities of reactions for surface reactions; (vi) we devised an application of the MOPED (Massive Optimised Parameter Estimation and DATA) compression algorithm to the determination of which molecules on dust grains should be prioritised by future observations with the JWST.
As the new tools became available, we applied them to generate databases of physically and statistically informed quantities that will allow the community to ultimately use molecules observed in galaxies as rigorous tools to derive the necessary information to determine the formation and the evolution of a galaxy.
In particular we (i) used the latest UCLCHEM chemical code coupled with radiative transfer modelling and statistical approaches, to produce a list of molecular species for which the gas history in a galaxy or in a cloud can be ignored, removing a major modelling complexity. We then determine the best of these species to observe when attempting to constrain various physical parameters and created a public web-based tool (https://hits.strw.leidenuniv.nl/(opens in new window)) whereby a user can search the database we created and determine the best observational lines for their purpose; (ii) created a publicly available, open source first version of the code (UCLCHEMCMC) that allows the user, via a web interface (https://uclchemcmc.strw.leidenuniv.nl/(opens in new window)) to estimate the posterior probability distribution of the gas parameters.
THEME 2: the overarching aim of this Theme is to make use of state of the art observational data of two galaxies (used as astrochemical laboratories and bench-marking cases) as bed-tests for the application of the methodologies developed in Theme 1. As part of our effort to establish the physical and chemical characteristics of parsec-scale molecular gas in nearby galaxies we have been heavily involved in the ALMA large program ALCHEMI and we led or significantly contributed to (i) the study of the emission of the C2H molecule in the central zone of NGC 253 and used it to give initial constraints on the cosmic ray ionization in the centre of the galaxy NGC 253; (ii) the study of the emission of the H3O+ and SO emission in NGC 253 which allowed us to firmly constrain the cosmic ray ionization rate; (iii) the study of the emission of HCN and HNC in NGC 253 to trace the sources of heating in this galaxy. The ALCHEMI work above dealt with the starburst galaxy NGC 253. Our second test-bed was the AGN dominated galaxy NGC 1068; for this galaxy (iv) we - for the first time - performed a systematic analysis of the usefulness of specific molecular ratios to "isolate" AGN-dominated galaxies.
These galaxies are very energetic and the molecular gas in them is highgly affected by shocks. In order to locate and characterize the shock components in these two different types of galaxies: (i) we have determined for the first time the shock structure and location of the shocks in the central nucleus of NGC 1068
and (ii) the nucleus of NGC 253.
All the work performed so far and briefly described above has been published in peer-reviewed articles.
Our progress beyond the state of the art can be summarized in three main results: (1) our chemical coeds have reached a complexity and a completness that has already gone beyond the state of the art, especially as they are fully modular and open source (https://uclchem.github.io/(opens in new window)). These codes are now routinely used by the community at large. (2) We are the main developers of a whole new subfield in astrochemistry: statistical astrochemistry. Our statistical tools, including an emulator for chemical modelling, is very novel. (3) We are interpreting molecular observations for nearby galaxies in a novel way: by making sure we always understand and take into consideration the chemistry that leads to the physical appearance of the galaxies we study.
We expect - by the end of MOPPEX - to establish a set of unique molecular tracers characterizing different phases of the neutral gas in nearby galaxies.
We expect - by the end of MOPPEX - to establish a set of unique molecular tracers characterizing different phases of the neutral gas in nearby galaxies.