TheGaia-ESO Survey: metallicity and kinematic trends in the Milky Way bulge

Abstract

Aims. Observational studies of the Milky Way bulge are providing increasing evidence of its complex chemo-dynamical patterns and morphology. Our intent is to use the iDR1 Gaia-ESO Survey (GES) data set to provide new constraints on the metallicity and kinematic trends of the Galactic bulge, exploring the viability of the currently proposed formation scenarios. Methods. We analyzed the stellar parameters and radial velocities of ∼1200 stars in five bulge fields wich are located in the region −10 • < l < 7 • and −10 • < b < −4 • . We use VISTA Variables in the Via Lactea (VVV) photometry to verify the internal consistency of the atmospheric parameters recommended by the consortium. As a by-product, we obtained reddening values using a semi-empirical T eff -color calibration. We constructed the metallicity distribution functions and combined them with photometric and radial velocity data to analyze the properties of the stellar populations in the observed fields. Results. From a Gaussian decomposition of the metallicity distribution functions, we unveil a clear bimodality in all fields, with the relative size of components depending of the specific position on the sky. In agreement with some previous studies, we find a mild gradient along the minor axis (−0.05 dex/deg between b = −6 • and b = −10 • ) that arises from the varying proportion of metal-rich and metal-poor components. The number of metal-rich stars fades in favor of the metal-poor stars with increasing b. The K-magnitude distribution of the metal-rich population splits into two peaks for two of the analyzed fields that intersects the near and far branches of the X-shaped bulge structure. In addition, two lateral fields at (l, b) = (7, −9) and (l, b) = (−10, −8) present contrasting characteristics. In the former, the metallicity distribution is dominated by metal-rich stars, while in the latter it presents a mix of a metal-poor population and and a metal-intermediate one, of nearly equal sizes. Finally, we find systematic differences in the velocity dispersion between the metal-rich and the metal-poor components of each field. Conclusions. The iDR1 bulge data show chemo-dynamical distributions that are consistent with varying proportions of stars belonging to (i) a metal-rich boxy/peanut X-shaped component, with bar-like kinematics; and (ii) a metal-poor more extended rotating structure with a higher velocity dispersion that dominates far from the Galactic plane. These first GES data already allow studying the detailed spatial dependence of the Galactic bulge populations, thanks to the analysis of individual fields with relatively high statistics.

The GIRAFFE Inner Bulge Survey (GIBS)

“…The origin of the observed metallicity gradients would then be the natural consequence of the different contribution of each of these components as a function of Galactic latitude. A similar conclusion was reached by Rojas-Arriagada et al (2014) based on the Gaia-ESO survey data.…”

“…The good number statistics of the total sample allows us to investigate in detail the metallicity distribution. In particular, we are interested in evaluating the decomposition of the distribution in two populations, as suggested in previous studies (Hill et al 2011;Rojas-Arriagada et al 2014). We note that Ness et al (2013a) suggested that the bulge metallicity distribution can be decomposed into five Gaussian components, each of them corresponding to different populations from the bulge and foreground disc.…”

Apogee Kinematics. I. Overview of the Kinematics of the Galactic Bulge as Mapped by Apogee

“…There is a known metallicity gradient with latitude in the bulge, for latitudes b 5 | | > , of about −0.45 dex kpc −1 (e.g., Zoccali et al 2008;Ness et al 2013a;Rojas-Arriagada et al 2014) and a smaller gradient in longitude (e.g., Gonzalez et al 2013). The flattening of the metallicity gradient with latitude near the plane has been detected (Ramírez et al 2000;Rich et al 2007Rich et al , 2012, and the APOGEE survey now demonstrates that flattening is present not only at the minor axis but also at the entire extent of the boxy bulge (l 15°) within b | | < 2°.…”

Separation of stellar populations by an evolving bar: implications for the bulge of the Milky Way

“…Di instead consider pure N -body simulations including a classical bulge and find that stars initially in the disc as far out as the outer Lindblad resonance of the bar contribute to the boxy bulge, comprising up to 30% of stars at high latitudes. In these scenarios, assuming that the initial disc has a declining metallicity profile, a puzzling observation is that the X-shape is weak or absent in the metal-poor stars (Ness et al 2012;Uttenthaler et al 2012;Rojas-Arriagada et al 2014). Bekki & Tsujimoto (2011) instead considered the evolution of a thin+thick disc model, where the thick disc was metal-poor and the thin disc was metal-rich.…”