One of the fundamental questions in galaxy evolution is how galaxies acquire gas from the inter and circum-galactic medium, and how they regulate their growth through galactic winds or other pre-emptive processes. Theoretical considerations on these topics still need to be constrained by observations. MUSE has demonstrated that it is possible to observe the circum-galactic medium (CGM) through its Lyman-𝛼 emission (Wisotzki et al. 2016), but it focused on the redshift range z~3-5. BlueMUSE, with the blue optimised spectral range, BlueMUSE will target the z~2-4 where the cosmological surface brightness dimming is a factor of ~4 lower in comparison to the range covered by MUSE. Furthermore, it will cover the peak of cosmic star formation epoch, which also marks a transition from the early to the late Universe. The former is characterised by cold gas flows, collimated towards galaxies, while in the later the influx of fresh gas is sustained by cooling regulated accretion from hot coronae around galaxies.
- CGM of star forming galaxies in Lyman-𝛼 emission. Ly𝛼 halos are ubiquitous around galaxies, found even surrounding low-mass galaxies at z>3, extending some other of magnitude further than the stellar counterparts, in some cases out to the virial radius (Leclercq et al. 2017, Wisotzki et al. 2018). BlueMUSE will extend MUSE results to lower redshift galaxies and provide information about motion of the gas (inflow/outflow).
- CGM flows and tomography with metal absorption lines. The streaming of the CGM around galaxies can only be mapped using bright background galaxies. Typically this is done using background quasars, which are rare, when one needs a multiple sight lines (or even more ideally background galaxies spread in arcs via gravitational lensing). BlueMUSE has an optimal wavelength range to study UV absorption lines for z<1 (e.g. MgIIλ2800, FeIIλ2600) and benefit from a large number of bright background galaxies at z>1. Furthermore, BlueMUSE has R~3500
- CGM in emission with metal lines. Ly𝛼 line traces the bulk of the gas mass, but the multi-phase characteristics of the CGM can be better traced with metal emission lines (Leclercq et al. 2022, Pessa et al. 2024). Low and moderately ionised atoms will provide maps of relatively cool (T<105K) gas, while higher ionisation states will trace the distribution of warm-to-hot gas (T>105K). Optimal redshift range for observing such systems is 0-2.5, benefiting from the increase of the surface brightness.
- CGM around AGNs. Quasars and AGNs are powerful engines able to ionise and “lit” its CGM (Borisova et al. 2016). This pumping of the energy into the CGM allows us to investigate the kinematics, ionisation and chemical enrichment properties of the multi-phase gas around galaxies, as well as AGN feedback, inflows and outflows. Crucially, the BlueMUSE wavelength range is optimised to map the peak epoch of AGN activity, and therefore observe the “sweet” spot in the AGN evolution, previously not accessible with large FoV IFS.
- Imaging the intergalactic medium (2<z<3). BlueMUSE is built with a goal to provide direct images of the cosmic web at z~1.8-3. This is achievable only through mosaicking over large areas, possible with BlueMUSE with deep integrations. Due to lower level of the cosmological surface brightness dimming, BlueMUSE will be ~16 times faster at mapping the IGM than MUSE. Furthermore, making use of bright background galaxies (still for z<3), BlueMUSE will also be able to provide a tomographic map of the cosmic web in absorption at scales of 200 kpc.