Formation of Massive Galaxies at High Redshift: Cold Streams, Clumpy Disks and Compact Spheroids

We present a simple theoretical framework for massive galaxies at high redshift, where the main assembly and star formation occurred, and report on the first cosmological simulations that reveal clumpy disks consistent with our analysis. The evolution is governed by the interplay between smooth and clumpy cold streams, disk instability, and bulge formation. Intense, relatively smooth streams maintain an unstable dense gas-rich disk. Instability with high turbulence and giant clumps, each a few percent of the disk mass, is self-regulated by gravitational interactions within the disk. The clumps migrate into a bulge in ~10 dynamical times, or ~0.5Gyr. The cosmological streams replenish the draining disk and prolong the clumpy phase to several Gigayears in a steady state, with comparable masses in disk, bulge, and dark matter within the disk radius. The clumps form stars in dense subclumps following the overall accretion rate, 100 Msun/yr, and each clump converts into stars in 0.5 Gyr. While the clumps coalesce dissipatively to a compact bulge, the star-forming disk is extended because the incoming streams keep the outer disk dense and susceptible to instability and because of angular momentum transport. Passive spheroid-dominated galaxies form when the streams are more clumpy: the external clumps merge into a massive bulge and stir up disk turbulence that stabilize the disk and suppress in situ clump and star formation. We predict a bimodality in galaxy type by z3, involving giant-clump star-forming disks and spheroid-dominated galaxies of suppressed star formation. After z1, the disks tend to be stabilized by the dominant stellar disks and bulges. Most of the high-z massive disks are likely to end up as today’s early-type galaxies.

💡 Research Summary

The paper presents a unified theoretical framework for the rapid assembly of massive galaxies at high redshift (z ≈ 2–3) and validates it with the first cosmological simulations that resolve giant star‑forming clumps within gas‑rich disks. The authors argue that the dominant driver of mass growth is the inflow of cold, filamentary streams that penetrate deep into the halo without being shock‑heated. These streams come in two flavors: a relatively smooth component that supplies ≈70–80 % of the total baryonic accretion rate (∼100 M⊙ yr⁻¹) and a clumpy component that carries the remaining 20–30 % in the form of external gas‑rich sub‑haloes. The smooth streams continuously feed the outer parts of the galactic disk (r ≈ 30–50 kpc), maintaining a high gas fraction (f_gas ≈ 0.5) and a dense, thin rotating structure.

Within this gas‑rich disk the Toomre stability parameter settles near Q ≈ 1, triggering a long‑lived gravitational instability. The instability fragments the disk into a handful of massive clumps, each containing 3–5 % of the total disk mass (10⁸–10⁹ M⊙). The clumps are turbulent (σ ≈ 30–50 km s⁻¹) and interact gravitationally with each other and with the surrounding disk, generating an effective viscosity (α ≈ 0.1–0.3) that self‑regulates the turbulence and keeps Q close to unity. This self‑regulation allows the clumpy phase to persist for several gigayears, as the inflowing streams replenish the gas that is drained by star formation and clump migration.

Clump migration is driven by dynamical friction and torques from the background disk. The characteristic migration time is ≈10 dynamical times, i.e. roughly 0.5 Gyr. As clumps spiral inward they coalesce in the central kiloparsec, dissipating orbital energy and forming a compact bulge. Star formation within each clump proceeds at a rate that mirrors the overall accretion rate, converting the clump’s gas into stars on a timescale of ∼0.5 Gyr. Consequently, the bulge grows rapidly while the disk retains a comparable mass fraction, leading to a quasi‑steady state where disk, bulge, and dark matter within the disk radius each contain roughly one third of the total mass.

The authors’ high‑resolution adaptive‑mesh refinement simulations (spatial resolution ≈20 pc) reproduce all of these ingredients. They resolve the formation of individual clumps, track their internal sub‑structure, and follow their inward migration and eventual merger. The simulations confirm that smooth streams keep the outer disk dense enough to remain unstable, while the clumpy component can either join the existing disk clumps or, if sufficiently massive, directly merge into the bulge.

A second evolutionary channel emerges when the clumpy component dominates the inflow. In this case, massive external clumps collide with the central region early, building a massive bulge and injecting strong turbulence into the remaining disk. The heightened turbulence raises Q well above unity, stabilizing the disk against further fragmentation and suppressing in‑situ star formation. This pathway yields spheroid‑dominated galaxies with low specific star‑formation rates.

Thus the paper predicts a bimodal galaxy population by z ≈ 3: (1) giant‑clump, star‑forming disks fed primarily by smooth streams, and (2) spheroid‑dominated, quenched systems whose growth is driven by clumpy streams. After z ≈ 1, the increasing stellar mass fraction in disks and bulges, together with the declining gas accretion rate, stabilizes most disks, quenching the clumpy phase. The authors argue that the majority of high‑z massive clumpy disks are the progenitors of today’s early‑type galaxies (S0s and ellipticals), providing a natural link between the observed clumpy population at z ≈ 2–3 and the quiescent early‑type population in the local universe.