Chemical characterization of the inner Galactic bulge:North–South symmetry

Abstract

While the number of stars in the Galactic bulge with detailed chemical abundance measurements is increasingly rapidly, the inner Galactic bulge ( |b| < 2$^\circ$) remains poorly studied, due to heavy interstellar absorption and photometric crowding. We have carried out a high-resolution IR spectroscopic study of 72 M giants in the inner bulge using the CRIRES (ESO/VLT) facility. Our spectra cover the wavelength range of 2.0818 - 2.1444 $\mu$m with the resolution of R$\sim$50,000 and have signal-to-noise ratio of 50 - 100. Our stars are located along the bulge minor axis at l = 0$^\circ$, b = $\pm$0$^\circ$, $\pm$1$^\circ$, $\pm$2$^\circ$ and +3$^\circ$, selected with the aim of investigating any North-South asymmetries in the metallicity distribution function and composition, and comparing them to the outer bulge fields. Our sample was analysed in a homogeneous way using the most current K-band line list. We clearly detect a bimodal MDF with a metal-rich peak at $\rm \sim +0.3\,dex$ and a metal-poor peak at $\rm \sim -0.5\,dex$. Only a single star is found to exceed $\rm [Fe/H]=+0.5\,dex$. They show a symmetric behaviour along the $\pm$1$^\circ$, $\pm$2$^\circ$ fields. The Galactic Center field reveals in contrast a mainly metal-rich population with a mean metallicity of $\rm +0.3\,dex$. We derived $\rm [Mg/Fe]$ and $\rm [Si/Fe]$ abundances which are consistent with trends from the outer bulge, with a gradually decreasing trend with increasing metallicity. We confirm for the supersolar metallicity stars the decreasing trend in \mgfe\ and \sife\ as expected from chemical evolution models. With the caveat of a relatively small sample, we do not find significant differences in the chemical abundances between the Northern and the Southern fields, hence the evidence is consistent with symmetry in chemistry between North and South.