[Paper Review] Formation of Galactic Disks I: Why Did the Milky Way's Disk Form Unusually Early?

Vadim A. Semenov, Charlie Conroy+3 more|arXiv (Cornell University)|Jun 15, 2023

Stellar, planetary, and galactic studies11 citations

TL;DR

The paper analyzes MW-like galaxies in the TNG50 simulation to determine when and how disks form, finding that disk formation timing and metallicity vary widely, with about 10% forming early like the Milky Way, making the MW unusual but not outside the broader MW-mass disk population.

ABSTRACT

Recent results from spectroscopic and astrometric surveys of nearby stars suggest that the stellar disk of our Milky Way (MW) was formed quite early, within the first few billion years of its evolution. Chemokinematic signatures of disk formation in cosmological zoom-in simulations appear to be in tension with these data, implying that MW-like disk formation is delayed in simulations. We investigate the formation of galactic disks using a representative sample of MW-like galaxies from the cosmological-volume simulation TNG50. We find that on average MW-mass disks indeed form later than the local data suggest. However, their formation time and metallicity exhibit a substantial scatter, such that $\sim$10% of MW-mass galaxies form disks early, similar to the MW. Thus, although the MW is unusual, it is consistent with the overall population of MW-mass disk galaxies. The direct MW analogs assemble most of their mass early, $\gtrsim 10$ Gyr ago, and are not affected by destructive mergers after that. In addition, these galaxies form their disks during the early enrichment stage when the interstellar medium metallicity increases rapidly, with only $\sim$25% of early-forming disks being as metal-poor as the MW was at the onset of disk formation, [Fe/H] $\approx -1.0$. In contrast, most MW-mass galaxies either form disks from already enriched material or experience late destructive mergers that reset the signatures of galactic disk formation to later times and higher metallicities. Finally, we also show that earlier disk formation leads to more dominant rotationally supported stellar disks at redshift zero.

Motivation & Objective

• Assess whether the Milky Way’s early disk formation is typical within a population of MW-mass galaxies.
• Characterize the timing (spin-up) and metallicity ([Fe/H]) of disk formation across MW-like galaxies in TNG50.
• Investigate the role of mass assembly history and mergers in setting disk formation timing.
• Relate early disk formation to present-day disk dominance and kinematic structure.

Proposed method

• Use a MW-like galaxy sample (61 central galaxies) from the TNG50 simulation with criteria on halo mass, star formation, and diskiness.
• Calibrate stellar metallicities by abundance matching the observed [Fe/H] CDF from bk22 to preserve metallicity ordering.
• Analyze in situ stars within 5–11 kpc and |z|<3 kpc to compare with MW observations of chemokinematic signatures.
• Define disk spin-up by the metallicity at which the median stellar rotation velocity reaches half of its solar-metallicity value.
• Compute orbital circularity j_z/j_c to quantify diskiness.
• Investigate correlations between disk formation timing, mass assembly history, and merger activity.

Experimental results

Research questions

• RQ1 What is the distribution of disk formation times and metallicities for MW-like galaxies in TNG50?
• RQ2 Is the Milky Way’s early disk formation typical or exceptional within the MW-mass galaxy population?
• RQ3 How do mass assembly histories and mergers influence the timing and metallicity of disk spin-up?
• RQ4 How does early disk formation affect present-day disk dominance and kinematic properties?
• RQ5 What explains the observed scatter in the age–metallicity relation during disk formation?

Key findings

• On average, MW-mass disks form later in TNG50 than local data suggest, but there is substantial scatter with ~10% forming disks early like the MW.
• Early-spin-up disks form ≥10 Gyr ago and rapidly (∼1–2 Gyr), while late-spin-up disks form later with broader age ranges.
• The spin-up metallicity for MW-like disks spans ∼0.6 dex, with the MW lying at the low-metallicity tail (∼2σ below the median).
• Early-spin-up hosts assemble most mass early in their halos and show higher present-day disk dominance, while late-spin-up hosts show extended or merger-driven mass growth and more prominent bulges.
• The age–metallicity relation during disk spin-up is shaped by rapid early enrichment versus slower late enrichment, explaining the variation in spin-up ages and metallicities across the sample.