Frühes Universum

Observations at submillimeter wavelengths are key to studying the dusty early Universe, as the emission in this regime is largely insensitive to redshift. This allows us to trace galaxy evolution equally well from when the Universe was half its current age (z~1) to only a few hundred million years after the Big Bang (z~8). Continuum surveys with single-dish telescopes such as APEX, the IRAM 30m, or the South Pole Telescope (SPT), combined with the high sensitivity and resolution of large interferometers like ALMA and NOEMA, provide access to the cold gas and dust that fuel star formation. These observations reveal the emergence of massive dust-enshrouded galaxies, the early growth of galaxy clusters, and the build-up of central galactic structures up to the epoch of reionization. Complementary optical and infrared imaging and spectroscopy are now available with JWST probing the stellar populations and ionized gas in these same systems, enabling a complete mutli-wavelength and spatially-resolved view of the first galaxies to appear in the Universe.

A complete view of stars, dust, and gas in Early Galaxies

Multi-layer view of stars, gas, and dust of an interacting galaxy in the early universe — Multi-wavelength view of an interacting galaxy at z~4.6. The near-infrared composite image (bottom) captures the emission from young (blue) and old (red) stars, and the ionized gas (green; also shown in a separate layer), while sub-millimetre wavelengths capture the neutral gas (orange) and dust (purple). The spatial offsets highlight the interplay between star formation, gas dynamics, and dust during early galaxy assembly, offering a glimpse into the baryon cycle shortly after reionization.

© Michael Romano (MPIfR) / ALPINE-CRISTAL-J

Multi-wavelength view of an interacting galaxy at z~4.6. The near-infrared composite image (bottom) captures the emission from young (blue) and old (red) stars, and the ionized gas (green; also shown in a separate layer), while sub-millimetre wavelengths capture the neutral gas (orange) and dust (purple). The spatial offsets highlight the interplay between star formation, gas dynamics, and dust during early galaxy assembly, offering a glimpse into the baryon cycle shortly after reionization.

© Michael Romano (MPIfR) / ALPINE-CRISTAL-J

High-redshift galaxies stand in stark contrast to local systems, characterized by substantially larger cold-gas reservoirs, elevated merger rates, and a more turbulent and complex interstellar medium. To understand how the earliest galaxies formed during the epoch of reionization, it is essential to capture all their baryonic components—stars, gas, and dust— at high spatial resolution. The synergy between sub-millimetre facilities such as ALMA and NOEMA, which trace the cold interstellar medium through rest-frame far-infrared continuum and key cooling lines like [C II], and the infrared capabilities of JWST, which reveal the stellar populations and ionized gas, now enables detailed, multi-wavelength studies of high-redshift dusty galaxies on sub-kiloparsec scales. Together, these observatories expose the internal structure of primordial galaxies with unprecedented clarity: pinpointing the sites of star formation, mapping the distribution of gas and dust, tracking variations in chemical enrichment, and capturing the complex dynamics of both hot ionized and cold molecular gas.
These observations pave the way to comprehensive investigations of the baryon cycle, the interplay between the interstellar and circumgalactic medium, and the early growth of supermassive black holes. Such spatially-resolved views—obtained for galaxies that existed only a few hundred million years after the Big Bang—provide direct insight into the mechanisms that regulate early galaxy growth and shape their evolution into the mature systems observed at later cosmic times.

Proto clusters in the early universe

Picture of two SPT-SMG protoclusters, SPT2349-56 and SPT0303-59, at redshift 4 — At lower angular resolution of the South Pole and APEX telescopes, dozens of dusty galaxies contribute to bright continuum and line emission—powered by prodigious star-formation—at sub-/millimetre wavelengths. While at increased spatial resolution (insets), ALMA aids the detailed study of individual massive galaxies at different cosmic epochs (starting only 1.4 billion years after the Big Bang). These extreme protocluster cores provide the environments where giant elliptical galaxies assemble, with all expected properties, during the most active phase of cluster formation.

© Nikolaus Sulzenauer, MPIfR / SPT-SMG team

At lower angular resolution of the South Pole and APEX telescopes, dozens of dusty galaxies contribute to bright continuum and line emission—powered by prodigious star-formation—at sub-/millimetre wavelengths. While at increased spatial resolution (insets), ALMA aids the detailed study of individual massive galaxies at different cosmic epochs (starting only 1.4 billion years after the Big Bang). These extreme protocluster cores provide the environments where giant elliptical galaxies assemble, with all expected properties, during the most active phase of cluster formation.

© Nikolaus Sulzenauer, MPIfR / SPT-SMG team

Galaxy evolution is influenced by environment, with nearly half of all galaxies in the local Universe residing in groups. While galaxies in the early Universe generally show elevated star-formation activity compared to their low-redshift counterparts, it remains unclear whether this extends to the densest regions. Studying the earliest cluster progenitors—protoclusters—during the epoch when both their galaxies and underlying structure are still taking shape provides a unique window into how massive galaxies assemble and how today’s clusters emerged.
The rapid evolution of these massive overdensities makes wide-area surveys essential. Protoclusters can span tens of arcminutes, and single-dish submillimetre surveys (e.g., with APEX or SPT) efficiently identify the brightest, dust-obscured galaxies within them. These include submillimetre galaxies (SMGs)—dusty, highly star-forming systems with rates often exceeding 1000 M☉ yr⁻¹—whose clustering can serve as a signpost for protocluster environments. Protoclusters also show remarkable diversity, containing both vigorously star-forming and more evolved or quiescent galaxies. Capturing this diversity requires both large, representative samples and detailed study of the physical processes shaping galaxies in dense environments. Achieving this relies on a multi-wavelength and multi-scale approach: wide-field surveys to find rare protoclusters, high-resolution interferometry (e.g., with ALMA or NOEMA) to map gas, dust, and dynamics, and near-infrared imaging (e.g., with JWST) to place limits on stellar populations, even in heavily obscured systems. Together, these observations connect the first clusters to their descendants in the local Universe, revealing how the densest regions shaped galaxy evolution from the earliest times to today.