ARGOS - III. Stellar populations in the Galactic bulge of the Milky Way

By M. Ness, Kenneth C. Freeman, E. Athanassoula, E. Wylie-de-Boer, Joss Bland-Hawthorn, Martin Asplund, Geraint F. Lewis, David Yong, R. Lane, László L. Kiss. 2012

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We present the metallicity results from the ARGOS spectroscopic survey of the Galactic bulge. Our aim is to understand the formation of the Galactic bulge: did it form via mergers, as expected from Lambda cold dark matter theory, or from disc instabilities, as suggested by its boxy/peanut shape, or both? Our stars are mostly red clump giants, which have a well-defined absolute magnitude from which distances can be determined. We have obtained spectra for 28 000 stars at a spectral resolution of R = 11 000. From these spectra, we have determined stellar parameters and distances to an accuracy of \textless1.5 kpc. The stars in the inner Galaxy span a large range in [Fe/H], -2.8 \textless= [Fe/H] \textless= +0.6. From the spatial distribution of the red clump stars as a function of [Fe/H], we propose that the stars with [Fe/H] \textgreater -0.5 are part of the boxy/peanut bar/bulge. We associate the lower metallicity stars ([Fe/H] \textless -0.5) with the thick disc, which may be puffed up in the inner region, and with the inner regions of the metal-weak thick disc and inner halo. For the bulge stars with [Fe/H] \textgreater -0.5, we find two discrete populations: (i) stars with [Fe/H] approximate to -0.25 which provide a roughly constant fraction of the stars in the latitude interval b = -5 degrees to -10 degrees, and (ii) a kinematically colder, more metal-rich population with mean [Fe/H] approximate to +0.15 which is more prominent closer to the plane. The changing ratio of these components with latitude appears as a vertical abundance gradient of the bulge. We attribute both of these bulge components to instability-driven bar/bulge formation from the thin disc. We associate the thicker component with the stars of the early less metal-rich thin disc, and associate the more metal-rich population concentrated to the plane with the colder more metal-rich stars of the early thin disc, similar to the colder and younger more metal-rich stars seen in the thin disc in the solar neighbourhood today. We do not exclude a weak underlying classical merger-generated bulge component, but see no obvious kinematic association of any of our bulge stars with such a classical bulge component. The clear spatial and kinematic separation of the two bulge populations (i) and (ii) makes it unlikely that any significant merger event could have affected the inner regions of the Galaxy since the time when the bulge-forming instabilities occurred.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/828e03e99811da92246ff6e518f95137a26ee0e8

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Citations in the corpus

Near-Field Cosmology with Extremely Metal-Poor Stars (2015, methodology): “We refer the reader to the works of Garćıa Pérez et al. (2013), Ness et al. (2013), and Howes et al. (2014), and references therein, for important recent advances in this area.”
CHEMICAL CARTOGRAPHY WITH APOGEE: METALLICITY DISTRIBUTION FUNCTIONS AND THE CHEMICAL STRUCTURE OF THE MILKY WAY DISK (2015, background): “…MDF has generally only been well characterized in the solar neighborhood (e.g., van den Bergh 1962; Nordström et al. 2004; Ak et al. 2007; Casagrande et al. 2011; Siebert et al. 2011; Schlesinger et al. 2012) and in the Galactic bulge (Zoccali et al. 2008; Gonzalez et al. 2013; Ness et al. 2013).”
Mapping the three-dimensional density of the galactic bulge with VVV red clump stars (2013, background): “…〈J−Ks〉RC,0, is an approximation: The intrinsic J−Ks colour of the red clump is expected to be a function of metallicity (Salaris & Girardi 2002; Cabrera-Lavers et al. 2007), and there is a gradient in the metallicity of the bulge (e.g. Zoccali et al. 2008; Ness et al. 2013; Gonzalez et al. 2013).”
Constraining the thick disc formation scenario of the Milky Way (2014, unknown): “The thick-disc contribution in bulge fields can explain population C described in the analysis of the metallicity distribution in the Argos survey by Ness et al. (2013a).”
The Galaxy in Context: Structural, Kinematic, and Integrated Properties (2016, methodology): “By contrast, the ARGOS stars with [Fe/H]<−1 rotate still fairly rapidly; whether they could be stars from the stellar halo or a low-mass classical bulge spun up by the b/p bulge (Saha et al. 2012) or whether they could include a component of thick disk stars must still be checked in detail.”
Target Selection for the SDSS-IV APOGEE-2 Survey (2017, background): “…the scientific proof of these conjectures until the late 19th and early 20th centuries. varying peaks in the bulge metallicity distribution (with ARGOS; Ness et al. 2013), and second-generation asymptotic giant branch (AGB) stars and multiple stellar populations in metal-poor and metal-rich…”
Dynamical modelling of the galactic bulge and bar: the Milky Way's pattern speed, stellar and dark matter mass distribution (2016, methodology): “In this paper, we extend the Made-to-Measure modelling to the entire bar region by taking advantage of the recent measurement of W15 on the bar outside the bulge together with stellar kinematics in the bar region from the ARGOS survey (Freeman et al. 2013; Ness et al. 2013).”
CHEMICAL CARTOGRAPHY WITH APOGEE: LARGE-SCALE MEAN METALLICITY MAPS OF THE MILKY WAY DISK (2013, background): “Existing measurements of bulge stars (e.g., Zoccali et al. 2008; Gonzalez et al. 2013; Ness et al. 2013) suggest that metal-rich stars are actually less prominent there than they are in the TRILEGAL simulations, so the bias in the actual data may be smaller than is present in our simulations.”
Chemical tagging with APOGEE: Discovery of a large population of N-rich stars in the inner Galaxy (2016, background): “As regards the structure of the bulge, a substantial part of its stellar mass is seemingly contained in the bar (e.g., Rojas-Arriagada et al. 2014; Ness et al. 2013a).”
ARGOS – IV. The kinematics of the Milky Way bulge (2013, background): “We have argued in ARGOS III that component B represents the stars formed out of the thin disk at early times.”
On the fine structure of the Cepheid metallicity gradient in the Galactic thin disk (2014, background): “This also meansth opportunity to constrain whether the high star formation rate of the innermost Galactic regions is driven by a disk instability that is dragging material from the inner disk into these regions (Freeman et al. 2013; Ness et al. 2013a,b).”