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SWG3: Distant Universe

Key Science Case: Gas flows around and between galaxies

Additional science cases:

  • A wide-field optical IFU producing high quality spectra for all sources in the field of view at a high sensitivity to emission lines is a game-changer for the study of distant galaxies. With its blue/UV spectral coverage BlueMUSE will be a complementary and even more powerful facility compared to MUSE in the study of deep extragalactic fields. The current MUSE redshift desert (1.5 < z < 3) will be largely filled down to z = 1.88, and the large population of [OII] emitters will be probed down to z = 0. In addition, the cosmological surface brightness dimming at z ∼ 2 − 3 is a factor ∼ 4 lower in comparison to z ∼ 3 − 5 with MUSE, we therefore expect a similar gain in the ability of BlueMUSE to detect Lyman-α emission from diffuse gas around galaxies, which will allow us to detect emission from gas all the way out into the intergalactic medium.

Figure 1. Main nebular lines available to BlueMUSE as a function of redshift
  • Despite the impressive success of recent large-scale cosmological simulations at reproducing the bulk of galaxy properties across cosmic time, the fundamental questions about how galaxies acquire gas from the intergalactic medium, and how they regulate their growth through galactic winds or other pre-emptive processes, are mostly unconstrained from observations or theory. Observations with BlueMUSE in the redshift range 2 < z < 3 will be key in constraining galaxy formation because they cover the peak of cosmic star formation. Among the factors that drive this turn-over in the cosmic SFR, we may expect a change in the form of accretion flows and their interactions with galactic winds, which marks the beginning of the transition from the early Universe – where gas flows cold and collimated onto galaxies–, and the late Universe – where star formation is sustained by cooling-regulated accretion from hot coronae. Observing the evolution of the CGM through this epoch will be a key in discriminating between the various competing theoretical models.
  • The nature of the first luminous sources responsible for reionising the predominantly neutral intergalactic medium remains so far totally unknown due to the increasing opacity of the intergalactic medium with redshift which renders direct Lyman continuum (LyC) detections impossible (or extremely inefficient) at z > 4. BlueMUSE will be able to directly probe the Lyman continuum from sources at z ∼ 3 to 5 and it will allow for the first time to collect a statistical sample of LyC emitters as part of blind surveys. We expect most detections to fall in the redshift range z = 3 − 4 where Lyman-α can be used to assess the spectroscopic redshift, and where the IGM transmission remains reasonably high. BlueMUSE will be particularly efficient at detecting the faintest population of z ∼ 3 galaxies which are potentially strong leakers and may correspond to the analogues of the sources of reionisation.
  • Through the observation of massive galaxy cluster cores, we can study gravitationally magnified low luminosity galaxies that would otherwise be undetectable. Compared to current MUSE studies, BlueMUSE will fill in the redshift space between 2 < z < 4 increasing the number of background lensed sources. We expect a significant increase of giant arcs as we will be able to detect them a lower redshifts (z∼2). BlueMUSE will provide the unique opportunity to study the massive stellar content and the conditions of the ionised gas around these systems. Mapping a significant fraction of the surroundings of the critical lines with BlueMUSE has a potential of further improving the mass distribution models of the clusters.

Figure 2. Estimated LAE counts as function of redshift for a single BlueMUSE 10 hrs depth exposure (in blue) and a 5σ detection limit. In green the LAE number counts are shown when the LAE luminosity function is assumed to flatten below 1041.2 ergs−1. MUSE counts (in red) for the same depth and SNR limit, is given for comparison.

  • BlueMUSE is well tuned to discover and characterise giant Lyman-α nebulae that now we know are widespread in the early generation of galaxy groups and clusters. The presence of such nebulae demonstrates that cold gas is co-existing with hot gas inside the deep potential well of these structures, but the origin, powering mechanism and fate of the cold gas is still unclear and a matter of debate. BlueMUSE will uniquely allow us to discover Lyman-α emitting nebulae in the first generation of forming clusters including the critical redshift range z=1.8-3 where we expect cold accretion to massive haloes to peak.