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

Ness, M. and Freeman, K. and Athanassoula, E. and Wylie-de-Boer, E. and Bland-Hawthorn, J. and Asplund, M. and Lewis, G. F. and Yong, D. and Lane, R. R. and Kiss, L. László (2013) ARGOS - III. Stellar populations in the Galactic bulge of the Milky Way. MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, 430 (2). pp. 836-857. ISSN 0035-8711

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Abstract

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 ΛCDM theory, or from disk instabilities, as suggested by its boxy/peanut shape, or both? Our stars are mostly red clump giants, which have a well defined absolute mag- nitude 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 param- eters and distances to an accuracy of < 1.5 kpc. The stars in the inner Galaxy span a large range in [Fe/H], –2.8 6 [Fe/H] 6 +0.6. From the spatial distribution of the red clump stars as a function of [Fe/H] (Ness et al. 2012a), we propose that the stars with [Fe/H] > −0.5 are part of the boxy/peanut bar/bulge. We associate the lower metallicity stars ([Fe/H] < −0.5) with the thick disk, which may be puffed up in the inner region, and with the inner regions of the metal-weak thick disk and inner halo. For the bulge stars with [Fe/H] > −0.5, we find two discrete populations; (i) stars with [Fe/H] ≈ −0.25 which provide a roughly constant fraction of the stars in the latitude interval b = −5◦ to −10◦, and (ii) a kinematically colder, more metal-rich population with mean [Fe/H] ≈ +0.15 which is more prominent closer to the plane. The changing ratio of these components with latitude appears as a vertical abun- dance gradient of the bulge. We attribute both of these bulge components to instability-driven bar/bulge formation from the thin disk. We associate the thicker component with the stars of the early less metal-rich thin disk, and associate the more metal-rich population concentrated to the plane with the colder more metal-rich stars of the early thin disk, similar to the colder and younger more metal-rich stars seen in the thin disk in the solar neighborhood today. We do not exclude a weak underlying classical merger–generated bulge component, but see no ob- vious 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.

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