Chemical radial gradients for the bulge-bar stellar populations from the APOGEE survey

Abstract

The Milky Way bulge-bar is composed of multiple populations. Using chemical and kinematical planes, we segregate six populations in a bulge-bar sample observed by the APOGEE survey: two with bar-driven orbits, two with eccentric orbits, and two with low-eccentricity orbits, each composed of low- and high-[Mg/Fe] stars. Our sample spans $-2.0\lesssim$[Fe/H]$\lesssim+0.5$ and Galactocentric distance $R_{Gal}$ $<6$ kpc. We use chemical abundances from APOGEE DR17 for the elements Mg, Si, Ca, Al, K, Mn, Co, Ni, and Fe, and from the BAWLAS catalog for Ce and Nd. We find that the low- and high-[Mg/Fe] stars with low-eccentricity orbits, which exhibit chemical and orbital characteristics similar to those of the low- and high-[$α$/Fe] disks, display slightly negative and positive metallicity gradients, respectively. This result for the low-[Mg/Fe] low-eccentricity stars indicates a break in the global thin disk metallicity gradient. The high eccentricity populations with both low- and high-[Mg/Fe] show approximately flat metallicity gradients. In general, the [X/H] gradients of all elements for all populations follow Fe, except for the neutron-capture elements Ce and Nd. For all elements, the high-[Mg/Fe] bar population shows a much steeper positive [X/H] gradient than the nearly flat gradient for the low-[Mg/Fe] bar stars. The positive [X/H] gradients observed among our high-[Mg/Fe] bar stars probably reflect an age variation along the peanut structure. This interpretation agrees with the N-body simulations. Such steep positive gradients have also been reported in some high-redshift (z$\sim$4--10) galaxies.

Figures (13)

• Figure 1: The [Mg/Fe]-[Fe/H] plane for the studied inner Galaxy sample. The top panel shows the low [Mg/Fe] sequence in blue and the high [Mg/Fe] sequence in red. Stars from our sample that have been identified as being from globular clusters, members of the Heracles substructure, N-rich, or chemically peculiar are marked with different symbols. The most metal-poor stars are also marked. The bottom panel shows the relative density distribution in this chemical plane, color-coded according to the probability density function. The dashed line separates the low- and high-[Mg/Fe] populations, as described in the text.
• Figure 2: Distribution for the elements Al, K, and Ni, color-coded according to the probability density function. The left and center panels show the red and blue distributions for the high- and low-[Mg/Fe] populations, respectively. The dashed lines in the center panel separate the multiple chemical populations of the low-[Mg/Fe] bulge-bar stars. The right panel presents the histogram for the low-[Mg/Fe] stars.
• Figure 3: $|Z_{max}|$-eccentricity ($ecc$) plane for the bulge-bar sample. Upper panels relating to the low-(right) and high-[Mg/Fe] (left) bar-member stars, and in the lower panels for the low-(right) and high-[Mg/Fe] (left) stars in the inner Galaxy non-bar member stars. The dashed lines limit the high and low eccentricity populations for the low-(right) and high-[Mg/Fe] (left) stars. Other stars not included in the bulge-bar sample (GC, Heracles, chemically peculiar and most metal-poor stars), are shown as filled and open symbols.
• Figure 4: Distribution of six stellar populations present in our sample in the X$_{Gal}$–Y$_{Gal}$ (left panels) and R$_{Gal}$–Z$_{Gal}$ (right panels) planes. Different point colors characterize distinct populations. Blue, green, and purple points represent the low-[Mg/Fe] bar stars, the low-[Mg/Fe] and high $ecc$, and the low-[Mg/Fe] and low $ecc$ populations, respectively, while red, dark yellow and orange represent the high-[Mg/Fe] bar stars, the high-[Mg/Fe] and high $ecc$, and the high-[Mg/Fe] and low $ecc$ populations, respectively.
• Figure 5: The gradient d[X/H]/dR$_{Gal}$ of Mg, Si, Ca, Al, K, Mn, Co, Ni, Ce, and Nd for different stellar populations. The asterisks at Ce and Nd indicate the use of the smaller sample in the gradient estimation. The legend indicates the stellar populations corresponding to each symbol. The lines in each circle represent the uncertainties of the gradients.