Galaxy assembly

Galaxy assembly across physical scale

Galaxy evolution unfolds across an immense range of physical scales, from star formation in parsec-sized molecular clouds to kiloparsec-scale secular evolution and galactic flows, to megaparsec-scale cosmic environments. Understanding the interplay between galaxies, their dark matter haloes, and the large-scale structures has long been limited by observational gaps across these regimes. The WST will bridge these scales with unprecedented accuracy and statistical power, offering a unified view of galaxy evolution.

Galaxy assembly in the Cosmic Web

Key star formation physics occurs below ~100 pc, where the Schmidt–Kennicutt law breaks down, and the cycle between clouds, HII regions, and star clusters can be resolved. The large field-of-view of the WST IFS, coupled with cloud scale resolution, will be essential for linking nebulae to their ionising sources for direct measurements of feedback pressures, escape fractions, and enrichment processes.

The WST will be able to perform a SDSS-like survey of several millions of galaxies across a wide range of redshifts (z~0.2-1.5) over ~150 sq. degrees, thereby providing a comprehensive map of the matter and galaxy distributions, enabling us to link the photometric and spectral properties (and inferred physical properties) of galaxies to the cosmic web structures on scales of 10 s to 100 Mpc (from filaments to groups and rich clusters).

Cartoon of the circum-galactic medium and gas recycling in a galaxy: blue filaments flow into the galaxy, while pink gas flows outward and surrounds it. — Cartoon view of the CGM. The central galaxy is fed by filamentary accretion from the cosmic web (blue). Outflows emerge from the galaxy, enriching the CGM (pink): some of this gas escapes the halo, some is recycled back into the central galaxy. The CGM is a dynamical and multiphase cosmic interface where a large diversity of physical mechanisms occurs at different spatial and temporal scales. Credits: Tsinghua University.

The WST will be able to map the gas in filaments in cosmologically representative portions of the Cosmic Web, which will allow us to understand how diffuse matter is distributed in the intergalactic medium, measure the morphology of those filaments, study their kinematics, and topological properties. At the same time, it will be possible to directly image the Circum-galactic Medium in a large sample of galaxies, therefore allowing us to understand how gas flows in and out of galaxies and how these processes are connected to the distribution of matter on the large scales sampled by the Cosmic Web.

The Galaxy as a Rosetta Stone for galaxy assembly

The Milky Way (MW) and its satellite galaxies are the environment in which the cosmological paradigms of galaxy formation can be put to the most stringent and detailed tests. The unique power of the WST for resolved stellar studies will be in large scale surveys of targets of all types, including also pulsating stars, throughout the Milky Way and in resolved galaxies providing stellar radial velocities and chemical abundances at the faint end of the Gaia and Rubin LSST catalogues. The combination of the WST LR and HR data, along with the IFS in crowded regions, will greatly expand our view of the Milky Way and provide a powerful perspective on galaxy assembly that is complementary to high-redshift studies. Some of the science questions that will be addressed include: how do the diverse in situ and accreted stellar populations in the Milky Way and its surroundings compare to galaxy formation and evolution models? Can we uniquely determine the complete accretion history over time? How did the bulge form? How old is the stellar disc? Just to make a few specific examples, the WST will provide a full mapping of the Milky Way’s halo phase space, disentangling the numerous stellar substructures, remnants of past dwarf galaxies and globular clusters, that it harbors. At the same time, identification of ancient stars in the inner Galaxy can place constraints on early nucleosynthesis and the formation of the first generations of stars in environments similar to those being observed at high redshift. The WST will also investigate the origin and evolution of in-situ stellar populations across the Galactic disc, bar, and bulge. It will deliver low- and high-resolution spectroscopy over >30,000 sq. deg., yielding homogeneous abundances, ages (also using the so-called “cosmic-clocks”), and 6D kinematics for tens of millions of stars out to 20 kpc. This will reconstruct the disc’s star-formation history, constrain inside-out growth and gas accretion, trace echoes of past interactions, probe the primitive disc, and link the bulge to globular clusters, the nuclear disc, and the nuclear star cluster.