Periodic Reporting for period 2 - SPECMAP-CGM (Spectro-mapping of the circumgalactic medium across cosmic times)
Reporting period: 2024-01-01 to 2025-06-30
Galaxies form and build up gradually, through discrete merger events and through the accretion of gas from the intergalactic medium. They also lose mass, driven out by internal causes such as galactic winds or by external effects such as tidal forces or ram pressure stripping. Understanding these processes as well as their interplay is key to understanding galaxy evolution. The principal battleground between inflows, outflows, and gas recycling is located well outside of the main stellar body of a galaxy (but still bound to its gravitational potential), now generally denoted as the "circumgalactic medium" (CGM). This project is advancing a new approach to investigate the structure and the dynamics of gas flows in the CGM, by employing the technique of integral field spectroscopy, or spectro-mapping, in various emission lines, using extensive observational data from the MUSE instrument at the Very Large Telescope (VLT) of the European Southern Observatory in Chile, and from the HETDEX survey conducted with the 10m Hobby-Eberly Telescope in Texas. The principal challenge for CGM studies is the extreme faintness of the its emission, which until now made spectro-mapping unfeasible except for a few extreme objects. Due to several technological and methodical advances we have finally achieved the sensitivity required to detect the CGM directly in emission through imaging spectroscopy, routinely for large representative samples of typical galaxies over a wide range of cosmic epochs. This will close important gaps in our understanding of galaxy formation and evolution, one of the most fundamental quests in modern astrophysical research. We aim at a thorough morphological and kinematical characterisation of the CGM, including constraints on its small-scale structure, to obtain a census over different galaxy types and at different cosmic times. One specific goal is the detection and dissection of the properties of circumgalactic outflows, believed to play a crucial role in the cosmic matter cycle in and around galaxies. Another particularly challenging objective is to directly trace the emission from inflowing intergalactic gas feeding the CGM and its central galaxy. Comparisons with other observational approaches as well as dedicated physical models and computer simulations are harnessed to interpret these various lines of evidence and form a more comprehensive picture of the processes and consitions in the circumgalactic medium.
The project relies to a large extent on our own observational data. A central activity during the first two years was the completion of a Large Programme conducted at the ESO-VLT (MUSCATEL, "MUSE Cosmic Assembly survey Targetting Extragalactic Legacy fields"). After the final observing run at the telescope in Oct 2022 we worked on the complex data processing and catalogue building, including the classification of several thousand galaxies that for the first time were observed spectroscopically. Since 2024 this extraordinary dataset is available for analysis to our team and will serve as backbone of the project for the next years. In parallel to completing MUSCATEL we analysed our previously obtained MUSE data, in particular the unique "MUSE eXtremely Deep Field" (MXDF), the deepest multi-object spectroscopic observation ever taken. We systematically searched around distant galaxies for extended circumgalactic emission. We focused on specific spectral lines that can be used as tracers of various processes and components in the CGM. Of particular interest to us is the Lyman-alpha line emitted by neutral hydrogen in and around galaxies in the young universe, highlighting huge gaseous envelopes that probably originate in the superposition (and mixing) of inflowing and outflowing gas. We also obtained, for the first time, spatially resolved spectra from the outer parts of the circumgalactic regions. Measuring the precise wavelengths of the emission lines in these spectra we identified, via the line peak Doppler shifts, significant velocity differences between inner and outer regions that could be interpreted as evidence for inflowing gas as responsible for much of the emission at large radii, while outflows probably dominate in the inner regions. Another spectral line of great importance to our project is emitted by singly ionised Magnesium, one of the most frequent elements heavier than hydrogen or helium and an excellent tracer of outflowing gas enriched with fusion products from supernova explosions. By combining the signal of many galaxies in the MXDF we discovered the characteristic double-cone-shaped structures expected for circumgalactic outflows - but only for systems seen approximately edge-on. We developed a mathematical model for such circumgalactic outflows that is capable to reproduce the observed spectra and the spatial distribution of the line-emitting gas with remarkable fidelity.
Because of the unprecedented sensitivity of our MUSE deep field observations, many of our findings probe significantly into unknown territory. In particular, the velocity differences between hydrogen clouds in the inner and the outer regions that we discovered were neither expected from previous observations, nor were they predicted by any theoretical model. So far this finding relies on the average signal from many galaxies, as well as on the object-by-object analysis of a small number of relatively bright objects. An important goal for the coming years will be to substantially increase the sample size and substantiate the observed trend, as well as developing dedicated theoretical models to interpret the evidence. Another area where the project already delivered progress well beyond the state of the art is in the investigation of circumgalactic outflows, traced through the spectra of singly ionised Magnesium. Previous theoretical calculations predicted that such outflows are typically shaped as two cones extending into opposite directions from the galactic midplane. Indeed such shapes were observed in a few nearby galaxies, but only in their innermost regions. Our MXDF observations provided the first direct image showing that galactic gas is driven out far into the circumgalactic medium, reaching distances of more than 30,000 light years, and that this is actually a frequent phenomenon. With the new data from the MUSCATEL programme at hand we could then move an important step further and identify such outflow signatures in the spectra of a large number of individual galaxies. This is a breakthrough which advances the field from a few single cases towards the systematic and quantitative investigation of circumgalactic outflows on statistically significant samples. In the future we plan to further expand the sample, covering a wide range of galaxy properties and viewing angles, and to complement our simplified modelling approach with detailed computer simulations of the relevant physical processes. Finding objects where such circumgalactic outflows can be studied in several spectral lines simultaneously will greatly enhance the amount of information that can be extracted. Last not least, we also plan to make the full MUSCATEL datasets available to the worldwide scientific community in a public data release.