Tracking galaxy evolution with precise and accurate metal abundances in the interstellar medium

Project Information

ISM-METALS

Grant agreement ID: 101077573

DOI

10.3030/101077573

EC signature date 28 November 2022

Start date 1 October 2023

End date 30 September 2028

Funded under

• European Research Council (ERC)

Total cost

€ 1 500 000,00

EU contribution

€ 1 500 000,00

Coordinated by

RUPRECHT-KARLS-UNIVERSITAET HEIDELBERG
Germany

Periodic Reporting for period 1 - ISM-METALS (Tracking galaxy evolution with precise and accurate metal abundances in the interstellar medium)

Reporting period: 2023-10-01 to 2026-03-31

Summary of the context and overall objectives of the project

Throughout their life, stars produce new elements in their interiors that, in their death, enrich the interstellar medium with atoms such as carbon, oxygen, nitrogen and sulfur. The introduction of these heavy elements (metals) into the interstellar medium (ISM) sets the heating and cooling balance in star-forming regions, regulating the ability of galaxies to grow their stellar mass and evolve. The gas-phase metal abundance (metallicity) is the most accessible measure of the build-up of chemical enrichment over cosmic time. Typically measured via bright emission lines, which are detectable even in galaxies in the early universe, highly ionized star-forming HII regions provide essential probes of the chemical composition of galaxies. However, the emissivity of the lines is determined by not just the abundance of a given element, but also the temperature of the ionized gas, along with a mix of other effects, including the intensity of the ionizing source, the overall surrounding radiation field, and the intrinsic density structure of the nebula – all of which evolve over time. Research has been plagued by these systematic uncertainties impacting our metallicity measurements, to the extent that we do not know if most galaxies are metal-rich or metal-poor compared to the Milky Way.

ISM-METALS tackles this problem by applying a data-driven approach to emission line measurements in our own Milky Way, Local Group galaxies and more distant Local Volume galaxies. It will compile and study vast new homogeneous data sets resolving tens of thousands of individual HII regions across these galaxy samples, and aims to resolve long-standing discrepancies in absolute metallicity calibrations and develop new and robust prescriptions that directly address electron temperature uncertainties. The key open questions ISM-METALS will address are:
- How to optimally determine robust absolute metallicities from optical emission lines?
- What is the temperature structure of the ionized ISM?
- How effectively and quickly do metals mix within galaxies?

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The ISM-METALS group has played a leading role in two collaborations, bringing expertise on understanding the metallicities measured within new optical spectroscopic datasets. With a focus on the Milky Way and Large and Small Magellanic Clouds (LMC, SMC), the Sloan Digital Sky Survey (SDSS) Local Volume Mapper (LVM) project is carrying out an optical integral field unit (IFU) survey that resolves the internal structures of HII regions at 1-10 pc resolution. With a focus on more distant galaxies, the PHANGS (Physics at High Angular resolution in Nearby GalaxieS) collaboration has surveyed 42 nearby galaxies with the optical IFU instrument MUSE on the Very Large Telescope, isolating tens of thousands of individual HII regions at 100 pc scales. The ISM-METALS project overcomes limitation of past projects by bridging the high resolution HII region studies in LVM with large, statistical integrated HII region studies in PHANGS. In support of our scientific analysis, we are also contributing to the refinement of the data calibration and reduction pipelines associated with these two projects.

Using the more distant sample of HII regions, we have characterized the temperatures of different ions, finding that the N+ emission lines provide remarkably robust constraints on the temperature within the full volume of ionized gas. Leveraging this insight, we have determined precise radial metallicity gradients for PHANGS galaxies, finding very low scatter that is indicative of a systematically well-mixed ISM. Within one particular, peculiar galaxy located in a low-density void environment, we have used metallicity constraints for the N/O abundances relative to O/H to present chemodynamic evidence of ongoing pristine gas accretion. This insight links internal and external processes that balance to regulate galaxy growth.

We have also published the first scientific analysis from the LVM dataset. We constructed the largest IFU map made (to date) of the Milky Way and providing insights into the physical conditions of well-known nebulae within the Orion constellation, including the Horsehead Nebula and Flame Nebula. In addition, we supported projects analyzing the ionization structure of the Rosette Nebula, characterizing collisionless shocks in a Galactic supernova remnant, and identifying ionized helium emission associated with extreme Wolf-Rayet stars in the SMC. A total area of 3000 square degrees have now been observed by the LVM, an unprecedented dataset.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The scientific publications resulting from this project, combined with the contributions to the LVM and PHANGS data products, has already had a major impact on multiple projects related to ionized gas studies of HII regions. The progress towards survey completion in LVM has been substantial. By obtaining spectroscopy everywhere across broad areas of the sky, we are enabling a very large discovery potential for future researchers. This data will be made public, with the first public release of LVM data already available in SDSS DR19.