[Paper Review] Birth of the Galactic Disk Revealed by the H3 Survey

Charlie Conroy, David H. Weinberg+13 more|arXiv (Cornell University)|Apr 6, 2022

Stellar, planetary, and galactic studies36 citations

TL;DR

The paper uses H3 survey spectroscopy and Gaia data to identify in-situ vs accreted stars, revealing a non-monotonic [alpha/Fe] vs [Fe/H] track and inferring a dramatic rise in star formation efficiency that marks the Milky Way disk birth around z~4.

ABSTRACT

We use chemistry ([alpha/Fe] and [Fe/H]), main sequence turnoff ages, and kinematics determined from H3 Survey spectroscopy and Gaia astrometry to identify the birth of the Galactic disk. We separate in-situ and accreted stars on the basis of angular momenta and eccentricities. The sequence of high-alpha in-situ stars persists down to at least [Fe/H]=-2.5 and shows unexpected non-monotonic behavior: with increasing metallicity the population first declines in [alpha/Fe], then increases over the range -1.3<[Fe/H]<-0.7, and then declines again at higher metallicities. The number of stars in the in-situ population rapidly increases above [Fe/H]=-1. The average kinematics of these stars are hot and independent of metallicity at [Fe/H]<-1 and then become increasingly cold and disk-like at higher metallicities. The ages of the in-situ, high-alpha stars are uniformly very old (13 Gyr) at [Fe/H]<-1.3, and span a wider range (8-12 Gyr) at higher metallicities. Interpreting the chemistry with a simple chemical evolution model suggests that the non-monotonic behavior is due to a significant increase in star formation efficiency, which began 13 Gyr ago. These results support a picture in which the first 1 Gyr of the Galaxy was characterized by a "simmering phase" in which the star formation efficiency was low and the kinematics had substantial disorder with some net rotation. The disk then underwent a dramatic transformation to a "boiling phase", in which the star formation efficiency increased substantially, the kinematics became disk-like, and the number of stars formed increased tenfold. We interpret this transformation as the birth of the Galactic disk at z~4. The physical origin of this transformation is unclear and does not seem to be reproduced in current galaxy formation models.

Motivation & Objective

• Identify in-situ versus accreted stellar populations in the Milky Way using chemical abundances, kinematics, and ages.
• Characterize the high-alpha in-situ population across a wide metallicity range to understand disk formation history.
• Interpret observed abundance patterns with chemical evolution modeling to infer the star formation history and disk birth epoch.

Proposed method

• Use H3 spectroscopy and Gaia astrometry to derive [Fe/H], [alpha/Fe], distances, and kinematics.
• Define in-situ and accreted samples via orbital parameters (eccentricity e and angular momentum LZ).
• Infer ages for main sequence turnoff and subgiants using the MINESweeper pipeline with flat age prior 4–14 Gyr.
• Compute kinematic trends (V_phi, sigma_Vz) and metallicity distributions for the in-situ high-alpha sequence.
• Apply a simple chemical evolution model (VICE) with fixed inflow and outflow parameters to interpret the [alpha/Fe]-[Fe/H] track in terms of star formation efficiency (SFE).
• Explore variations of model parameters to test robustness of the SFE-driven non-monotonic track.

Experimental results

Research questions

• RQ1 What are the chemical, kinematic, and age characteristics of in-situ versus accreted stars in the Milky Way?
• RQ2 Does the high-alpha in-situ population show non-monotonic chemical evolution with metallicity, and what does it imply for disk formation?
• RQ3 Can a chemical evolution model with changing star formation efficiency reproduce the observed [alpha/Fe] vs [Fe/H] track and indicate when the Galactic disk formed?
• RQ4 What is the inferred epoch and physical transformation that marks the birth of the Galactic disk?

Key findings

• The high-alpha in-situ sequence extends down to [Fe/H] ~ -2.5 with a non-monotonic [alpha/Fe] evolution: decline to [Fe/H] ~ -1.3, rise to [Fe/H] ~ -0.7, then decline again at higher metallicities.
• The in-situ high-alpha population is overwhelmingly old at [Fe/H] < -1.3 (mean ~13 Gyr) and becomes progressively younger (8–12 Gyr) at higher metallicities.
• A rapid increase in star formation efficiency about 13 Gyr ago explains the non-monotonic track, suggesting a transition from a simmering phase to a boiling phase leading to disk-like kinematics and tenfold rise in star formation.
• The transformation is interpreted as the birth of the Galactic disk at z ~ 4, with current models not fully reproducing this origin.