OUR MILKY WAY AS A PURE-DISK GALAXY—A CHALLENGE FOR GALAXY FORMATION

By Juntai Shen, R. Rich, J. Kormendy, Christian D. Howard, R. Propris, A. Kunder. 2010

226 citations recorded by the source corpus.

Bulges are commonly believed to form in the dynamical violence of galaxy collisions and mergers. Here, we model the stellar kinematics of the Bulge Radial Velocity Assay (BRAVA) and find no sign that the Milky Way contains a classical bulge formed by scrambling pre-existing disks of stars in major mergers. Rather, the bulge appears to be a bar seen somewhat end-on, as hinted from its asymmetric boxy shape. We construct a simple but realistic N-body model of the Galaxy that self-consistently develops a bar. The bar immediately buckles and thickens in the vertical direction. As seen from the Sun, the result resembles the boxy bulge of our Galaxy. The model fits the BRAVA stellar kinematic data covering the whole bulge strikingly well with no need for a merger-made classical bulge. The bar in our best-fit model has a half-length of ∼4 kpc and extends 20° from the Sun–Galactic center line. We use the new kinematic constraints to show that any classical bulge contribution cannot be larger than ∼8% of the disk mass. Thus, the Galactic bulge is a part of the disk and not a separate component made in a prior merger. Giant, pure-disk galaxies like our own present a major challenge to the standard picture in which galaxy formation is dominated by hierarchical clustering and galaxy mergers.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/249e6717de2abfcdd19b699ea233a938e3dda147

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Citations in the corpus

• The Milky Way’s stellar disk (2013, background): “…for the Milky Way in particular, but lead generically to the question of how resilient stellar disks are to tidal interactions; it has been claimed (e.g., Kormendy et al. 2010; Shen et al. 2010) that the existence of large, thin stellar disks poses a challenge to the merger-driven ΛCDM picture.”
• ARGOS – IV. The kinematics of the Milky Way bulge (2013, background): “Shen et al. (2010) constrain the bulge to have no underlying merger-generated component to a level of6 8%.”
• REDDENING AND EXTINCTION TOWARD THE GALACTIC BULGE FROM OGLE-III: THE INNER MILKY WAY'S RV ∼ 2.5 EXTINCTION CURVE (2012, background): “We do recognise that these two words have different meanings, but the kinematic decomposition of the Galaxy’s central population remains a matter of active investigation and controversy at this time. with those of the bulge’s axis ratios (Stanek et al. 1997) and rotation speed (Shen et al. 2010).”
• Mean proper motions, space orbits, and velocity dispersion profiles of Galactic globular clusters derived fromGaiaDR2 data (2018, background): “However even if only 25% of the bulge are part of this classical bulge (Shen et al. 2010), disrupted globular clusters would c© 201x RAS, MNRAS 000, 1–13 still account for only a small fraction of the stars in the (classical) bulge and halo, unless most mass was in very low-mass clusters.”
• Coevolution (Or Not) of Supermassive Black Holes and Host Galaxies (2013, background): “There is no photometric or kinematic sign of a classical bulge (see Freeman 2008, Howard et al. 2009, Shen et al. 2010, and Kormendy et al. 2010 for reviews and for some of the evidence).”
• Made-to-measure models of the Galactic box/peanut bulge: stellar and total mass in the bulge region (2015, background): “The Galactic Bulge (GB) is regarded as the three-dimensional part of the bar seen nearly end-on (Shen et al. 2010; Martinez-Valpuesta & Gerhard 2011) with a semi-major axis of about 2kpc (Gerhard 2002; ?”
• The Galaxy in Context: Structural, Kinematic, and Integrated Properties (2016, methodology): “Li & Shen (2012) applied an unsharp masking technique to the side-on projection of the model of Shen et al. (2010), removing an elliptical bulge model from the total.”
• Alpha element abundances and gradients in the Milky Way bulge from FLAMES-GIRAFFE spectra of 650 K giants (2011, background): “This is in contrast with the recent literature claims of pure disk/bar origin of the Galactic bulge (Shen et al. 2010), which would result in shallower [α/Fe] decline, more similar to that of the thin disk.”
• The formation and assembly history of the Milky Way revealed by its globular cluster population (2018, background): “E-mail: kruijssen@uni-heidelberg.de formation and evolution (e.g. Wyse 2001; Stewart et al. 2008; Shen et al. 2010; Deason et al. 2013; Mackereth et al. 2018, Hughes et al. in prep.).”
• TWO RED CLUMPS AND THE X-SHAPED MILKY WAY BULGE (2010, methodology): “One major difficulty is that our results suggest a end-to-end bar length of ∼2.5 kpc, whereas the best fit model of Shen et al. (2010) indicates a bar half-length of 4kpc. X-shape, boxy, and peanut morphologies of extragalactic bulges are well known phenomena, e.g., IC 4767, NGC 128, NGC 4469, IC…”
• The GALAH survey: scientific motivation (2015, methodology): “This picture is supported by comparisons of radial velocity observations to N-body model predictions (e.g. Shen et al. 2010).”
• Reconstructing the star formation history of the Milky Way disc(s) from chemical abundances (2014, methodology): “The thick disc represents the only stellar population in the Galaxy which corresponds to this star formation phase, since the classical spheroid, if present, makes only a minor contribution to the total mass (Carney et al. 1990; Shen et al. 2010; Kunder et al. 2012; Di Matteo et al. 2014).”
• Dynamical modelling of the galactic bulge and bar: the Milky Way's pattern speed, stellar and dark matter mass distribution (2016, methodology): “This process has been used widely in order to build models that can then be compared to data (Martinez-Valpuesta et al. 2006; Athanassoula 2007; Shen et al. 2010) but suffers from three major limitations: (i) The evolution process is non-linear and very sensitive to initial conditions.”
• Mapping the three-dimensional density of the galactic bulge with VVV red clump stars (2013, methodology): “We hope that the 3D-density measurement of the Bulge pre- sented here will be useful for a variety of purposes, for example: (i) Constraining dynamical models of the Milky Way such as those produced in Shen et al. (2010) and Martinez-Valpuesta & Gerhard (2011).”