Robust constraints of stellar-driven outflows at cosmic noon

The growth of galaxies in our Universe is regulated by their ability to accrete gas, convert this gas into stars, and expel it back to cosmological scales in large-scale galactic outflows. But how do galaxies launch these outflows? How do they propagate and break out of the galaxy? And what damage, if any, do they inflict on the gas surrounding galaxies? The fact that these questions remain unanswered highlights limitations in galaxy formation theories, posing a pressing concern for our ability to answer the key question: “How do stars and galaxies evolve?” (STFC Science Challenge A5).
This project addresses gaps in our understanding of galactic outflows by leveraging cutting-edge numerical models to guide the interpretation of new galactic outflow surveys at cosmic noon (z~2), the all-time peak of star formation in the Universe. Recent advances in spectroscopic instrumentation (e.g. Keck/KCWI, VLT/KMOS) now enable the routine detection of galactic outflows at cosmic noon and the quantification of their energetics across large samples.
However, uncertainties remain in inferring how much mass, energy, velocity, and chemical elements are flowing out of a galaxy from emission and absorption lines. Due to the multi-phase nature of outflows, different emission or absorption tracers track different spatial extents, geometries, and energetics, making it challenging to obtain a complete observational picture of outflow energetics and complicating comparisons with theoretical models.
This proposal addresses these observational challenges head-on. We will quantify potential biases and known unknowns in galactic outflow observations by leveraging a new suite of cosmological simulations from the Megatron project, in which the PL plays a leading role. These simulations are specifically designed to robustly predict the ionic structure of cosmological, stellar-driven outflows at cosmic noon. Megatron makes two critical theoretical advances: (i) solving for non-equilibrium ionic chemistry and radiative transfer together in a cosmological context to accurately predict ion densities, and (ii) using an outflow-targeted resolution strategy to accurately capture mixing between gas phases. These steps enable direct predictions of the intrinsic emission and absorption properties of galactic outflows for optically thin transitions (e.g. Ha, OIII 5007Å, SiII 1207Å, CIV 1548Å).
This project will complement this setup with Monte-Carlo radiative transfer post-processing, incorporating the spectral shapes and shifts of resonant lines (e.g. Ly-a; MgII 2803Å) and accounting for dust re-processing. This will build a complete picture of outflow tracers from the ultraviolet to the optical. Analysing these spectral diagnostics exactly as an observer would, we will quantify how outflow geometry, multi-phase structure, and kinematics bias inferences of outflow energetics at cosmic noon. Our findings will directly inform observations from new surveys on the Very Large Telescope (KMOS, MOONS, ERIS), in which this team is involved.
Our findings will also be of interest to other astronomers involved in surveys with similar science cases but covering different redshift ranges, samples, and spatial resolutions (e.g. on MUSE). To maximise impact, all synthetic emission and absorption lines will be made publicly available as part of the Megatron data release, enabling the community to test other spectral inferences unrelated to outflows (e.g. metallicity from emission lines). With project leaders combining world-leading expertise in both numerical and observational galaxy formation, all simulated data in hand by the start of the project, and a skilled RIA to deliver timely scientific analysis, this team is well poised to undertake this high-impact project.