Tracing the circumgalactic medium with optical emission lines

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Galaxies are surrounded by large reservoirs of gas that are fed by inflows from the intergalactic medium and by outflows from galactic winds. Absorption-line measurements along the lines of sight to bright and rare background quasars have shown that this “circumgalactic medium” (CGM) extends far beyond the stellar bodies of galaxies. We know that the CGM has a complex multiphase structure and is, globally speaking, probably neither in thermal nor in dynamic equilibrium. However, the spatial distribution and kinematics of the various CGM phases remain poorly constrained, due to the scarcity of useful absorption lines of sight and the difficulties of detecting the extremely low surface brightness CGM gas directly in emission. Here we propose to perform a novel investigation of the cool-warm ionized component of the CGM in galaxies by detecting and characterising the CGM in optical emission lines. We will use the Multi-Unit Spectroscopic Explorer (MUSE) instrument at the ESO Very Large Telescope, in particular taking advantage of the ample Guaranteed Time Observations (GTO) allocation to our team in return for constructing and delivering MUSE. The high throughput and the large multiplexing factor afforded by the large field of view, combined with our innovative “MUSE Deep Fields” observing strategy of exposing for several tens of hours per single pointing, will provide at least an order of magnitude improvement in sensitivity compared to existing measurements. We target hydrogen recombination lines (H-alpha and H-beta) as well as collisionally excited metal lines, so that also gas-diagnostic line ratios can be measured. Such data will constrain the geometry, layering, and clumping of the warm ionized gas, elucidate the principal excitation mechanisms, and help in the characterisation of the gas flow patterns by tracing spatially resolved gas kinematics. It will also open a new dimension for the comparison with numerical simulations. We will pursue this goal by two avenues: We search for spatially extended line emission around individual galaxies that stretches significantly beyond their stellar bodies; we also measure mean CGM properties by stacking representative (sub)samples of the galaxy population. This experiment builds on our recent breakthrough results on extended Lyman-alpha haloes around high-redshift (z>3) galaxies which we demonstrated to be ubiquitous, even around faint and low-mass systems. Our Lyman-alpha work also provides us with a good understanding of how to push towards extremely faint surface brightness limits while controling the systematics, as well as with the necessary analysis tools. Upon outcome this investigation will substantially advance the observational state-of-the-art in this field and open up a new window for studying the circumgalactic medium of normal galaxies.