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The proto-Milky Way epoch forms the earliest stars in our Galaxy and sets theinitial conditions for subsequent disk formation. Recent observations fromAPOGEE and H3 surveys showed that the [$\alpha$/Fe] ratio slowly declinedbetween [Fe/H] $=-3$ and $-1.3$ until it reached the lowest value ($\sim 0.25$)among the selected in situ metal-poor stars that most likely formed during theproto-Galaxy epoch. [$\alpha$/Fe] rose to meet the traditional high valuecommonly associated with the thick disk population at [Fe/H] $=-1$. It wassuggested that the rise in [$\alpha$/Fe] could be caused by an increase in thestar formation efficiency (SFE), known as the "simmering" phase scenario.However, gas inflow also plays a vital role in shaping the star formationhistory and chemical evolution of galaxies. We investigate this unexpected[$\alpha$/Fe]-rise with a statistical experiment involving a galactic chemicalevolution (GCE). Our model has five free parameters: the mass of the initialreservoir of the cold interstellar medium (ISM) at birth, the frequency of TypeIa supernovae (SNe Ia), the cooling timescale of the warm ISM, the SFE, and theinflow rate of fresh gas. The last two free parameters were allowed to changeafter [$\alpha$/Fe] reached its lowest value, dividing the proto-Galaxy epochinto two phases. We find that the rise in [$\alpha$/Fe] is caused by a largeinflow of fresh gas and conclude that the [$\alpha$/Fe]-rise is a signature ofthe cold mode accretion whose materials formed the prototype Milky Waypreceding disk formation. Although the SFE is essential in regulating thechemical evolution, it does not necessarily increase to facilitate the[$\alpha$/Fe]-rise.

The dawn is quiet here: Rise in [$\alpha$/Fe] is a signature of massive gas accretion that fueled proto-Milky Way