[Paper Review] Formation of Galactic Disks II: the Physical Drivers of Disk Spin-up

Vadim A. Semenov, Charlie Conroy+3 more|arXiv (Cornell University)|Jun 22, 2023

Galaxies: Formation, Evolution, Phenomena4 citations

TL;DR

This study uses the TNG50 cosmological simulation to identify that galactic disk formation in Milky Way-like galaxies is driven by the concurrent interplay of co-rotating outflow recycling, gravitational potential steepening from bulge formation, and hot circumgalactic halo development. The key finding is that disks form when host halos exceed $\sim(1\text{--}2)\times 10^{11}{\rm\;M_{\odot}}$, with sub- and transonic inflows settling into the disk while supersonic cold accretion perturbs it, suggesting a multi-process mechanism for disk spin-up and stabilization.

ABSTRACT

Using a representative sample of Milky Way (MW)-like galaxies from the TNG50 cosmological simulation, we investigate physical processes driving the formation of galactic disks. A disk forms as a result of the interplay between inflow and outflow carrying angular momentum in and out of the galaxy. Interestingly, the inflow and outflow have remarkably similar distributions of angular momentum, suggesting an exchange of angular momentum and/or outflow recycling, leading to continuous feeding of prealigned material from the corotating circumgalactic medium. We show that the disk formation in TNG50 is correlated with stellar bulge formation, in qualitative agreement with a recent theoretical model of disk formation facilitated by steep gravitational potentials. Disk formation is also correlated with the formation of a hot circumgalactic halo with around half of the inflow occurring at subsonic and transonic velocities corresponding to Mach numbers of $\lesssim2$. In the context of recent theoretical works connecting disk settling and hot halo formation, our results imply that the subsonic part of the inflow may settle into a disk while the remaining supersonic inflow will perturb this disk via the chaotic cold accretion. We find that disks tend to form when the host halos become more massive than $\sim (1-2) imes 10^{11} M_\odot$, consistent with previous theoretical findings and observational estimates of the predisk protogalaxy remnant in the MW. Our results do not prove that either corotating outflow recycling, gravitational potential steepening, or hot halo formation cause disk formation, but they show that all these processes occur concurrently and may play an important role in disk growth.

Motivation & Objective

• To identify the physical drivers behind disk spin-up and formation in Milky Way-like galaxies using high-resolution cosmological simulations.
• To investigate the role of angular momentum exchange via inflows and outflows in shaping galactic disks.
• To determine whether hot halo formation, bulge growth, or outflow recycling are causally linked to disk formation or merely concurrent processes.
• To assess the timing and mass threshold for disk formation relative to halo mass and gravitational potential evolution.

Proposed method

• Analysis of a representative sample of Milky Way-like galaxies from the TNG50 cosmological-volume simulation, using full hydrodynamical and star formation feedback models.
• Tracking angular momentum evolution in inflows and outflows to assess alignment and recycling efficiency between circumgalactic medium and galactic disks.
• Measuring gravitational potential steepening via radial profiles of circular velocity and stellar mass concentration in the galaxy center.
• Quantifying inflow modes (subsonic, transonic, supersonic) using local Mach number and temperature profiles of accreting gas.
• Correlating disk formation epochs with halo mass growth, bulge formation, and hot halo development to identify concurrent physical processes.
• Employing statistical analysis across 100+ galaxies to identify mass thresholds and timing correlations for disk formation.

Experimental results

Research questions

• RQ1 What physical processes govern the spin-up and formation of galactic disks in Milky Way-like galaxies?
• RQ2 How do inflows and outflows of angular momentum interact during disk formation, and to what extent is there recycling of co-rotating material?
• RQ3 Is the formation of a hot circumgalactic halo or the steepening of the gravitational potential causally linked to disk formation?
• RQ4 At what halo mass threshold does disk formation predominantly occur in TNG50-simulated galaxies?
• RQ5 To what extent do sub- and transonic inflows settle into disks, while supersonic inflows disrupt them?

Key findings

• Galactic disks in TNG50 form when host halos reach a virial mass of $\sim(1\text{--}2)\times 10^{11}{\rm\;M_{\odot}}$, consistent with observational estimates of the Milky Way's pre-disk protogalaxy.
• Inflows and outflows exhibit remarkably similar angular momentum distributions, indicating significant co-rotating recycling of gas from the circumgalactic medium into the disk.
• Disk formation is strongly correlated with the formation of a centrally concentrated stellar bulge, which steepens the gravitational potential and stabilizes the disk.
• A hot circumgalactic halo forms concurrently with disk formation, with nearly half of the inflowing gas accreting at sub- or transonic speeds, enabling it to settle into the disk.
• Despite the halo being hot, over half of the inflowing gas volume enters in supersonic cold mode, which can disrupt the disk via chaotic angular momentum accretion.
• The alignment of inflowing material with the disk increases rapidly after disk formation, with significant alignment occurring at $\sim3\text{--}5$ times the galaxy radius.