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Register a new userThe Gaia-ESO survey: Metal-rich bananas in the bulge
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We analyse the kinematics of $sim 2000$ giant stars in the direction of the Galactic bulge, extracted from the Gaia-ESO survey in the region $-10^circ lesssim ell lesssim 10^circ$ and $-11^circ lesssim b lesssim -3^circ$. We find distinct kinematic trends in the metal rich ($mathrm{[M/H]}>0$) and metal poor ($mathrm{[M/H]}<0$) stars in the data. The velocity dispersion of the metal-rich stars drops steeply with latitude, compared to a flat profile in the metal-poor stars, as has been seen previously. We argue that the metal-rich stars in this region are mostly on orbits that support the boxy-peanut shape of the bulge, which naturally explains the drop in their velocity dispersion profile with latitude. The metal rich stars also exhibit peaky features in their line-of-sight velocity histograms, particularly along the minor axis of the bulge. We propose that these features are due to stars on resonant orbits supporting the boxy-peanut bulge. This conjecture is strengthened through the comparison of the minor axis data with the velocity histograms of resonant orbits generated in simulations of buckled bars. The banana or 2:1:2 orbits provide strongly bimodal histograms with narrow velocity peaks that resemble the Gaia-ESO metal-rich data.
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We present the first results of the EMBLA survey (Extremely Metal-poor BuLge stars with AAOmega), aimed at finding metal-poor stars in the Milky Way bulge, where the oldest stars should now preferentially reside. EMBLA utilises SkyMapper photometry to pre-select metal-poor candidates, which are subsequently confirmed using AAOmega spectroscopy. We describe the discovery and analysis of four bulge giants with -2.72<=[Fe/H]<=-2.48, the lowest metallicity bulge stars studied with high-resolution spectroscopy to date. Using FLAMES/UVES spectra through the Gaia-ESO Survey we have derived abundances of twelve elements. Given the uncertainties, we find a chemical similarity between these bulge stars and halo stars of the same metallicity, although the abundance scatter may be larger, with some of the stars showing unusual [{alpha}/Fe] ratios.
We take advantage of the Gaia-ESO Survey iDR4 bulge data to search for abundance anomalies that could shed light on the composite nature of the Milky Way bulge. The alpha-elements (Mg, Si, and whenever available, Ca) abundances, and their trends with Fe abundances have been analysed for a total of 776 bulge stars. In addition, the aluminum abundances and their ratio to Fe and Mg have also been examined. Our analysis reveals the existence of low-alpha element abundance stars with respect to the standard bulge sequence in the [alpha/Fe] vs. [Fe/H] plane. 18 objects present deviations in [alpha/Fe] ranging from 2.1 to 5.3 sigma with respect to the median standard value. Those stars do not show Mg-Al anti-correlation patterns. Incidentally, this sign of the existence of multiple stellar populations is reported firmly for the first time for the bulge globular cluster NGC 6522. The identified low-alpha abundance stars have chemical patterns compatible with those of the thin disc. Their link with massive dwarf galaxies accretion seems unlikely, as larger deviations in alpha abundance and Al would be expected. The vision of a bulge composite nature and a complex formation process is reinforced by our results. The used approach, a multi-method and model-driven analysis of high resolution data seems crucial to reveal this complexity.
(Abridged) We analyzed the stellar parameters and radial velocities of ~1200 stars in five bulge fields as determined from the Gaia-ESO survey data (iDR1). We use VISTA Variables in The Via Lactea (VVV) photometry to obtain reddening values by using a semi-empirical T_eff-color calibration. 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. Our chemo-kinematical analysis is consistent with a varying field-to-field proportion 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 allow studying the detailed spatial dependence of the Galactic bulge populations, thanks to the analysis of individual fields with relatively high statistics.
Abridged: We used the fourth internal data release of the Gaia-ESO survey to characterize the bulge chemistry, spatial distribution, kinematics, and to compare it chemically with the thin and thick disks. The sample consist on ~2500 red clump stars in 11 bulge fields ($-10^circleq lleq+8^circ$ and $-10^circleq bleq-4^circ$), and a set of ~6300 disk stars selected for comparison. The bulge MDF is confirmed to be bimodal across the whole sampled area, with metal-poor stars dominating at high latitudes. The metal-rich stars exhibit bar-like kinematics and display a bimodality in their magnitude distribution, a feature which is tightly associated with the X-shape bulge. They overlap with the metal-rich end of the thin disk sequence in the [Mg/Fe] vs. [Fe/H] plane. Metal-poor bulge stars have a more isotropic hot kinematics and do not participate in the X-shape bulge. With similar Mg-enhancement levels, the position of the metal-poor bulge sequence knee is observed at [Fe/H]$_{knee}=-0.37pm0.09$, being 0.06 dex higher than that of the thick disk. It suggests a higher SFR for the bulge than for the thick disk. Finally, we present a chemical evolution model that suitably fits the whole bulge sequence by assuming a fast ($<1$ Gyr) intense burst of stellar formation at early epochs. We associate metal-rich stars with the B/P bulge formed from the secular evolution of the early thin disk. On the other hand, the metal-poor subpopulation might be the product of an early prompt dissipative collapse dominated by massive stars. Nevertheless, our results do not allow us to firmly rule out the possibility that these stars come from the secular evolution of the early thick disk. This is the first time that an analysis of the bulge MDF and $alpha$-abundances has been performed in a large area on the basis of a homogeneous, fully spectroscopic analysis of high-resolution, high S/N data.
We attempt to determine the relative fraction of stars that have undergone significant radial migration by studying the orbital properties of metal-rich ([Fe/H]$>0.1$) stars within 2 kpc of the Sun using a sample of more than 3,000 stars selected from iDR4 of the Gaia-ESO Survey. We investigate the kinematic properties, such as velocity dispersion and orbital parameters, of stellar populations near the sun as a function of [Mg/Fe] and [Fe/H], which could show evidence of a major merger in the past history of the Milky Way. This was done using the stellar parameters from the Gaia-ESO Survey along with proper motions from PPMXL to determine distances, kinematics, and orbital properties for these stars to analyze the chemodynamic properties of stellar populations near the Sun. Analyzing the kinematics of the most metal-rich stars ([Fe/H]$>0.1$), we find that more than half have small eccentricities ($e<0.2$) or are on nearly circular orbits. Slightly more than 20% of the metal-rich stars have perigalacticons $R_p>7$ kpc. We find that the highest [Mg/Fe], metal-poor populations have lower vertical and radial velocity dispersions compared to lower [Mg/Fe] populations of similar metallicity by $sim10$ km s$^{-1}$. The median eccentricity increases linearly with [Mg/Fe] across all metallicities, while the perigalacticon decreases with increasing [Mg/Fe] for all metallicities. Finally, the most [Mg/Fe]-rich stars are found to have significant asymmetric drift and rotate more than 40 km s$^{-1}$ slower than stars with lower [Mg/Fe] ratios. While our results cannot constrain how far stars have migrated, we propose that migration processes are likely to have played an important role in the evolution of the Milky Way, with metal-rich stars migrating from the inner disk toward to solar neighborhood and past mergers potentially driving enhanced migration of older stellar populations in the disk.
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