EXtra-galactic science cases

An outstanding issue in current extra-galactic studies is a better physical understanding of the interplay between dark, stellar and gaseous material including how primordial and metal-enriched gas flows in and out of galaxies on various scales. Such questions can only be addressed with a large multi-object spectroscopic survey at redshifts 1 < z < 5, corresponding to a formative period in cosmic history. The unique combination of large field of view, primary diameter, high-multiplex MOS and monolitic IFS makes WST the ideal facility to address this issue.

Left: the large-scale structure of the Universe as predicted by a cosmological simulation (Springel et al. 2005). WST with its MOS capabilities (high multiplex and wide field-of-view) will be able to reconstruct (shed light) on the filamentary structure of the Universe. Right: WST with its monolithic IFS will be able to study the IGM in emission at z~3 as traced with Lya on Mpc scales. The figure displays an example of what can be currently done with a mosaic of 9 (3x3 arcmin2) MUSE very long exposures (Bacon et al. 2021).

Tracking galaxy assembly and the formation of the cosmic web: to understand the interplay between dark, stellar and gaseous material, inside and outside galaxies.

The most fundamental feature of structure formation is the complex distribution of matter referred to as the cosmic web. Galaxies form and assemble within this evolving network of dark matter haloes, filaments and voids. WST ’s unique combination of a panoramic MOS and an on-axis IFS is ideally suited for understanding the baryonic process that occur in the cosmic web, both on small scales where gaseous flows regulate star formation and chemical enrichment, and in larger cosmic volumes where environmental trends can be charted. WST ’s large aperture ensures that such synergies between galaxy-scale processes and larger cosmic structures can be studied at high redshifts (z> 2) where star formation and AGN activities are at their peak, as well as at low redshifts (z< 1.5) where finer details and improved S/N is possible.  By the late 2020s, deep multi-band imaging over very large fields from ground and space-based facilities will ensure reliable target selection with minimal contamination from interlopers. In addition to undertaking a traditional photometrically-selected spectroscopic survey defining the 3-D galaxy distribution over 0 <z< 7, within a restricted redshift range 2 <z< 3 a more powerful technique has emerged based on studies of the 3D topology of the Lyman alpha forest seen in absorption along the line of sight to background sources. These clouds of intergalactic hydrogen trace the linear regime of density fluctuations and hence act as a valuable proxy for the dark matter distribution.

The small-scale matter cycle.

Galaxy evolution is driven by feedback processes occurring on a variety of scales. On the scales of giant molecular clouds, the star formation efficiency and multi-phase interstellar medium (ISM) is shaped by small-scale processes. On galaxy-wide scales, outflows and secular evolution drive the interplay between galaxies, their dark matter halos, and the circum- and intergalactic media. Understanding the distribution of baryonic matter and energy across these scales remains a key challenge for both theoretical and observational astrophysics. Observationally, large galaxy surveys provide the statistics needed to study the assembly of galaxy subcomponents, and the link between galaxies and their large-scale environments. Addressing the physics of star formation, however, requires a statistical perspective on the individual components of the matter cycle (HII regions, star clusters) at a spatial resolution comparable with the size and separation length characteristic of star-forming regions (~100 pc).
The current generation of IFS (e.g., MUSE at the VLT) has enabled mapping of nearby galaxies, demonstrating the power of this approach. However, such samples are relatively modest and span a limited range of environments within galaxies (typically the inner few kpc), impeding our ability to match any insights drawn with results obtained from much larger galaxy surveys probing kpc scales (e.g., MaNGA and SAMI). To make progress requires ‘cloud-scale’ (<100 pc) mapping of a sufficiently large sample of galaxies (>103) across a range of morphological features (e.g., spiral arms, bars, bulges) and integrated properties (e.g., Mstar, SFR, local environmental density). With its large IFS, WST is uniquely placed to perform an ambitious spectroscopic survey in the Local Volume (D < 25 Mpc) at better than 100 pc resolution to address the key question of the small-scale matter cycle. Resolving individual nebulae will allow statistical studies of the metallicity of HII regions, in combination with the stellar population, uncovering the small-scale production and flow of metals in galaxies. WST's large aperture will ensure the detection of weak auroral lines yielding direct metallicity estimates beyond the inner gas-rich regions in poorly explored environments for star formation, where conversion from neutral to molecular hydrogen becomes highly inefficient. A sample of ~104 regions would adequately span approximately 2 dex in the metallicity, N/O ratio, and ionization parameter. Such a sample would be two orders of magnitude larger than the current state-of-the-art (e.g., CHAOS). Gas phase metallicity measurements will be combined with those from integrated-light spectroscopy and, (where available) resolved stars to chart the local history of chemical enrichment,of resolved stars to chart the local history of chemical enrichment, the impact of metal diffusion, and the role of morphological features in mixing.

An illustration of the level of detail achievable in surveys that resolve the average distance between HII regions ('cloud-scale') and those that observe galaxies at kpc resolution. Right: comparison of a WST survey of the Local Volume with existing cloud-scale and kpc-scale surveys. The grey band indicates the typical distance between clouds. Only WST will be able to provide a statistical sample of galaxies at cloud-scale resolution. Adapted from Emsellem et al. (2022).