The Two-infall Model Revisited: Constraints on Milky Way Bulge Assembly from >30,000 Galactic Chemical Evolution Models and Machine Learning

Abstract

We constrain the formation history of the Milky Way bulge using a two-infall galactic chemical evolution (GCE) algorithm implemented in the N'OMEGA+ code. We recover a best-fit scenario in which the bulge forms through an early, rapid starburst ($t_1 \sim 0.1$ Gyr, $\tau_1 \sim 0.09$ Gyr, and star formation efficiency (SFE) $\sim 3~\mathrm{Gyr}^{-1}$), followed by a delayed, lower-mass second infall ($t_2 \sim 5.1$ Gyr, $\tau_2 \sim 1.7$ Gyr, and $\sigma_2 \sim 0.69$). Our model adopts mass- and metallicity-dependent nucleosynthetic yields from modern stellar grids and explores a wide GCE parameter space in infall timing, SFE, mass partitioning, initial mass function upper mass, and type Ia supernova normalization, optimized via a hybrid genetic algorithm with Markov Chain Monte Carlo refinement. The later infall features a reduced SFE ($\Delta\mathrm{SFE} \sim 0.72$), reproducing the metal-rich peak of the bulge metallicity distribution function (MDF) and the decline in [$\alpha$/Fe] at high [Fe/H]. Our model naturally favors the M. Joyce et al. age--metallicity relation over the ages in T. Bensby et al. Degeneracy and principal component analyses show that the infall history, SFE, and mass partitioning are strongly covariant---the bulge's observed MDF, abundance trends, and age distribution constrain only their combinations, not each parameter independently. The results support a composite bulge origin---an early, rapid collapse builds the majority of the mass, while a younger component is required to match the late-stage enrichment.