Quantifying the (X/peanut)-shaped structure of the Milky Way – new constraints on the bar geometry

Ciambur, Bogdan C.; Graham, Alister W.; Bland-Hawthorn, Joss

doi:10.1093/mnras/stx1823

Cited by 32 publications

(39 citation statements)

References 98 publications

(132 reference statements)

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“…3&4, assuming a bar angle of ∼ 30 • ). Contrasting this with the value of ∼ 3.3 kpc derived from near-IR images (Ciambur et al 2017) or from red-clump stars (Wegg & Gerhard 2013) sets an upper limit (due to unknown increase with altitude) to the peanut spread factor of 4-5 between the respective BP structures. In our simulation the peanut spread factor was reaching a value of 2-3 between the oldest populations and those born 4-5 Gyr later.…”

Section: Theoretical Implicationscontrasting

confidence: 55%

The small boxy/peanut structure of the Milky Way traced by old stars

Semczuk

Dehnen

Schönrich

et al. 2021

Monthly Notices of the Royal Astronomical Society

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We analyse the positions of RR Lyrae stars in the central region of the Milky Way. In addition to the overall bar shape detected previously, we find evidence for a peanut shaped structure, in form of overdensities near ℓ = −2○ and 1○ at b ∼ −3.5○. The corresponding physical distance between the two peaks of the peanut is ∼0.7 kpc, significantly shorter than that found from near-IR images (3.3 kpc) and red-clump stars. Qualitatively this is expected from ‘fractionation’ of bar orbits, which we demonstrate to be matched in a simulation of an inside-out growing disc subsequently forming a bar.

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Section: Theoretical Implicationscontrasting

confidence: 55%

The small boxy/peanut structure of the Milky Way traced by old stars

Semczuk

Dehnen

Schönrich

et al. 2021

Monthly Notices of the Royal Astronomical Society

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“…This is much longer than what observations suggest. Estimates for the MW bar length, obtained from a model fit to near-IR star count, are 5.0 ± 0.2 kpc (Wegg et al 2015;Bland-Hawthorn & Gerhard 2016) and ∼ 4 kpc (Ciambur et al 2017). Our measured bar length (∼ 4 kpc) is consistent with these observations.…”

Section: Bar Length and Pattern Speedmentioning

confidence: 99%

Modelling the Milky Way as a dry Galaxy

Fujii

Bédorf

Baba

et al. 2018

Monthly Notices of the Royal Astronomical Society

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We construct a model for the Milky Way Galaxy composed of a stellar disc and bulge embedded in a dark-matter halo. All components are modelled as N -body systems with up to 8 billion equal-mass particles and integrated up to an age of 10 Gyr. We find that net angular-momentum of the dark-matter halo with a spin parameter of λ = 0.06 is required to form a relatively short bar (∼ 4 kpc) with a high pattern speed (40-50 km s −1 ). By comparing our model with observations of the Milky Way Galaxy, we conclude that a disc mass of ∼ 3.7 × 10 10 M and an initial bulge scale length and velocity of ∼ 1 kpc and ∼ 300 km s −1 , respectively, fit best to the observations. The disc-to-total mass fraction (f d ) appears to be an important parameter for the evolution of the Galaxy and models with f d ∼ 0.45 are most similar to the Milky Way Galaxy. In addition, we compare the velocity distribution in the solar neighbourhood in our simulations with observations in the Milky Way Galaxy. In our simulations the observed gap in the velocity distribution, which is expected to be caused by the outer Lindblad resonance (the so-called Hercules stream), appears to be a time-dependent structure. The velocity distribution changes on a time scale of 20-30 Myr and therefore it is difficult to estimate the pattern speed of the bar from the shape of the local velocity distribution alone.

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“…The Milky Way (hereafter MW) bulge is a boxy or peanut-shaped bulge (hereafter B/P; see Okuda et al 1977;Maihara et al 1978;Weiland et al 1994;Dwek et al 1995;Binney et al 1997;Babusiaux & Gilmore 2005;López-Corredoira et al 2005;Rattenbury et al 2007;Cao et al 2013;Wegg & Gerhard 2013;Portail et al 2015Portail et al , 2017aCiambur et al 2017). In its vast majority, it consists of stars that originated in its disk.…”

Section: Introductionmentioning

confidence: 99%

Disk origin of the Milky Way bulge: the necessity of the thick disk

Matteo

Fragkoudi

Khoperskov

et al. 2019

A&A

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In the Milky Way bulge, metal-rich stars form a strong bar and are more peanut-shaped than metal-poor stars. It has recently been claimed that this behavior is driven by the initial (i.e., before bar formation) in-plane radial velocity dispersion of these populations, rather than by their initial vertical random motions. This has led to the suggestion that a thick disk is not necessary to explain the characteristics of the Milky Way bulge. We discuss this issue again by analyzing two dissipationless N-body simulations of boxy or peanut-shaped bulges formed from composite stellar disks that consist of kinematically cold and hot stellar populations. These two models represent two extreme cases: one where all three components of the disk have a fixed vertical velocity dispersion and different in-plane radial dispersion, and another where they all have a fixed radial dispersion and different vertical random motions (thickness). This is intended to quantify the drivers of the main features that are observed in composite boxy or peanut-shaped bulges and their origin. We quantify the mapping into a boxy or peanut-shaped bulge of disk populations in these two cases, and we conclude that initial vertical random motions are as important as in-plane random motions in determining the relative contribution of cold-and hotdisk populations with height above the plane, the metallicity and age trends. Previous statements emphasizing the dominant role of in-plane motions in determining these trends are not confirmed. However, significant differences exist in the morphology and strength of the resulting boxy or peanut-shaped bulges. In particular, the model where disk populations initially have only different in-plane random motions, but similar thickness, results in a boxy or peanut-shaped bulge where all populations have a similar peanut shape, independent of their initial kinematics or metallicity. This is at odds with the trends observed in the Milky Way bulge. We discuss the reasons behind these differences, and also predict the signatures that these two extreme initial conditions would leave on the vertical age and metallicity gradients of disk stars outside the bulge region. As a consequence of this analysis, we conclude that given our current knowledge of the Milky Way bulge and of the properties of its main stellar components, a metal-poor, kinematically (radial and vertical) hot component, that is, a thick disk, is necessary in the Milky Way before bar formation. This supports the scenario that has been traced in previous works. Boxy or peanut-shaped bulges and their surrounding regions are fossil records of the conditions present at early times in disk galaxies, and by dissecting their stellar components by chemical compositions and/or age, it may be possible to reconstruct their early state.

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Quantifying the (X/peanut)-shaped structure of the Milky Way – new constraints on the bar geometry

Cited by 32 publications

References 98 publications

The small boxy/peanut structure of the Milky Way traced by old stars

The small boxy/peanut structure of the Milky Way traced by old stars

Modelling the Milky Way as a dry Galaxy

Disk origin of the Milky Way bulge: the necessity of the thick disk

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