Based on high-resolution spectra obtained during gravitational microlensing events we present a detailed elemental abundance analysis of 32 dwarf and subgiant stars in the Galactic bulge .
Combined with the sample of 26 stars from the previous papers in this series , we now have 58 microlensed bulge dwarfs and subgiants that have been homogeneously analysed .
The main characteristics of the sample and the findings that can be drawn are : ( i ) The metallicity distribution ( MDF ) is wide and spans all metallicities between [ Fe / H ] = -1.9 to +0.6 ; ( ii ) The dip in the MDF around solar metallicity that was apparent in our previous analysis of a smaller sample ( 26 microlensed stars ) is no longer evident ; instead it has a complex structure and indications of multiple components are starting to emerge .
A tentative interpretation is that there could be different stellar populations at interplay , each with a different scale height : the thin disk , the thick disk , and a bar population ; ( iii ) The stars with [ Fe / H ] \lesssim - 0.1 are old with ages between 10 and 12 Gyr ; ( iv ) The metal-rich stars with [ Fe / H ] \gtrsim - 0.1 show a wide variety of ages , ranging from 2 to 12 Gyr with a distribution that has a dominant peak around 4-5 Gyr and a tail towards higher ages ; ( v ) There are indications in the [ \alpha / Fe ] - [ Fe / H ] that the “ knee ” occurs around [ Fe / H ] = -0.3 to -0.2 , which is a slightly higher metallicity as compared to the “ knee ” for the local thick disk .
This suggests that the chemical enrichment of the metal-poor bulge has been somewhat faster than what is observed for the local thick disk .
The results from the microlensed bulge dwarf stars in combination with other findings in the literature , in particular the evidence that the bulge has cylindrical rotation , indicate that the Milky Way could be an almost pure disk galaxy .
The bulge would then just be a conglomerate of the other Galactic stellar populations ( thin disk , thick disk , halo , and … ?
) , residing together in the central parts of the Galaxy , influenced by the Galactic bar .