ARGOS – IV. The kinematics of the Milky Way bulge

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 and 2 more. 2013

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We present the kinematic results from our ARGOS spectroscopic survey of the Galactic bulge of the Milky Way. Our aim is to understand the formation of the Galactic bulge. We examine the kinematics of about 17 400 stars in the bulge located within 3.5 kpc of the Galactic Centre, identified from the 28 000 star ARGOS survey. We aim to determine if the formation of the bulge has been internally driven from disc instabilities as suggested by its boxy shape, or if mergers have played a significant role as expected from lambda cold dark matter simulations. From our velocity measurements across latitudes b=- 5 degrees,- 7 degrees. 5 and- 10 degrees we find the bulge to be a cylindrically rotating system that transitions smoothly out into the disc. From observations of 3 fields at b=+ 10, the kinematics of the bulge show North-South symmetry about the major axis. Within the bulge, we find a kinematically distinct metal-poor population ([ Fe/ H]\textless- 1.0) that is not rotating cylindrically. The 5 per cent of our stars with [ Fe/ H] \textless - 1.0 are a slowly rotating spheroidal population, which we believe are stars of the metal-weak thick disc and halo which presently lie in the inner Galaxy. The kinematics of the two bulge components that we identified in ARGOS Paper III ( mean [ Fe/ H]approximate to- 0.25 and [ Fe/ H]approximate to+ 0.15, respectively) demonstrate that they are likely to share a common formation origin and are distinct from the more metal-poor populations of the thick disc and halo which are co-located inside the bulge. We do not exclude an underlying merger generated bulge component but our results favour bulge formation from instabilities in the early thin disc.

Subfields: Astronomy and Astrophysics, Atomic and Molecular Physics, and Optics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/51286832f786b5366f86f982d9ad14ac1e749960

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Citations in the corpus

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).”
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).”
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.”
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).”
The GALAH survey: scientific motivation (2015, background): “The different components of the early inner disc (thick disc, old thin disc, younger thin disc) end up trapped dynamically within the bulge structure (Ness et al. 2013).”
Made-to-measure models of the Galactic box/peanut bulge: stellar and total mass in the bulge region (2015, methodology): “E-mail:portail@mpe.mpg.de Wegg & Gerhard 2013), as is also indicated by the near-cylindrical rotation of the bulge stars (Beaulieu et al. 2000; Kunder et al. 2012; Ness et al. 2013).”