The GIRAFFE Inner Bulge Survey (GIBS)

We use APOGEE DR13 data to examine the metallicity trends in the Milky Way (MW) bulge and we explore their origin by comparing two N-body models of isolated galaxies that develop a bar and a boxy/peanut (b/p) bulge. Both models have been proposed as scenarios for reconciling a disc origin of the MW bulge with a negative vertical metallicity gradient. The first model is a superposition of cospatial, i.e. overlapping, disc populations with different scale heights, kinematics, and metallicities. In this model the thick, metal-poor, and centrally concentrated disc populations contribute significantly to the stellar mass budget in the inner galaxy. The second model is a single disc with an initial steep radial metallicity gradient; this disc is mapped by the bar into the b/p bulge in such a way that the vertical metallicity gradient of the MW bulge is reproduced, as has been shown already in previous works in the literature. However, as we show here, the latter model does not reproduce the positive longitudinal metallicity gradient of the inner disc, nor the metalpoor innermost regions seen in the data. On the other hand, the model with co-spatial thin and thick disc populations reproduces all the aforementioned trends. We therefore see that it is possible to reconcile a (primarily) disc origin for the MW bulge with the observed trends in metallicity by mapping the inner thin and thick discs of the MW into a b/p. For this scenario to reproduce the observations, the α-enhanced, metal-poor, thick disc populations must have a significant mass contribution in the inner regions, as has been suggested for the Milky Way.

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“…We see that M1 is compatible with the APOGEE data, which points to low metallicities of on average ∼ -0.1 dex (and see also Zoccali et al 2017).…”

Section: Resultssupporting

confidence: 81%

“…Calura et al 2012;Obreja et al 2013) and the origin of its various stellar populations, as revealed by its broad metallicity distribution function (MDF;McWilliam & Rich 1994;Hill et al 2011;Ness et al 2013;Rojas-Arriagada et al 2014;Zoccali et al 2017).…”

Section: Introductionmentioning

confidence: 99%

What the Milky Way bulge reveals about the initial metallicity gradients in the disc

Fragkoudi

Matteo

Haywood

et al. 2017

A&A

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“…Recently, Zoccali et al (2017) parametrized the metallicity distribution in each of the 26 fields of the GIBS survey into a metal-poor and a metal-rich component. They presented a density map for each of the two components for −10 • < l < 10 • and −10 • < b < 10 • showing that the metal-poor population is more centrally concentrated than the metal-rich one, and has a more axisymmetric spatial distribution.…”

Section: Trends In the Chemical Distributionmentioning

confidence: 99%

Separation of stellar populations by an evolving bar: implications for the bulge of the Milky Way

Debattista

Ness

Gonzalez

et al. 2017

Monthly Notices of the Royal Astronomical Society

157

255

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We present a novel interpretation of the previously puzzling different behaviours of stellar populations of the Milky Way's bulge. We first show, by means of pure N -body simulations, that initially co-spatial stellar populations with different in-plane random motions separate when a bar forms. The radially cooler populations form a strong bar, and are vertically thin and peanut-shaped, while the hotter populations form a weaker bar and become a vertically thicker box. We demonstrate that it is the radial, not the vertical, velocity dispersion that dominates this evolution. Assuming that early stellar discs heat rapidly as they form, then both the in-plane and vertical random motions correlate with stellar age and chemistry, leading to different density distributions for metal-rich and metal-poor stars. We then use a high-resolution simulation, in which all stars form out of gas, to demonstrate that this is what happens. When we apply these results to the Milky Way we show that a very broad range of observed trends for ages, densities, kinematics and chemistries, that have been presented as evidence for contradictory paths to the formation of the bulge, are in fact consistent with a bulge which formed from a continuum of disc stellar populations which were kinematically separated by the bar. For the first time we are able to account for the bulge's main trends via a model in which the bulge formed largely in situ. Since the model is generic, we also predict the general appearance of stellar population maps of external edge-on galaxies.

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“…Bulges of galaxies, once thought to be simple systems supported by velocity dispersion, have been reported to be more complex structures, with bars, pseudo-bulges, classical bulges, etc.,2013; Johnson et al 2012;Ness et al 2012;Zoccali et al 2008Zoccali et al , 2016. Recent observations have reported the existence of very old stars traced by RR Lyrae populations , as well as young stars (Dékány et al 2015), within the bulge region.…”

Section: Introductionmentioning

confidence: 99%

“…The Milky Way (MW) offers a unique perspective to test models of galaxy formation because stars can be individually observed and their properties, such as kinematics, dynamics, and chemical composition, can be more accurately derived in the four main stellar components: bulge, thin and thick discs and stellar haloes (see Bland-Hawthorn & Gerhard 2016, for a detailed discussion of the MW components). The copatricia.tissera@unab.cl existence of these components in the central region is now being explored in more detail as the result of larger and more precise observational data in the bulge (e.g., Gran et al 2016;Zoccali et al 2016), stellar halo (e.g., Santucci et al 2015;Carollo et al 2016;Helmi et al 2016) and the disc (e.g., Minniti et al 2017). …”

Section: Introductionmentioning

confidence: 99%

The central spheroids of Milky Way mass-sized galaxies

Tissera

Machado

Carollo

et al. 2017

Monthly Notices of the Royal Astronomical Society

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We study the properties of the central spheroids located within 10 kpc of the centre of mass of Milky Way mass-sized galaxies simulated in a cosmological context. The simulated central regions are dominated by stars older than 10 Gyr, mostly formed in situ, with a contribution of ∼ 30 per cent from accreted stars. These stars formed in well-defined starbursts, although accreted stars exhibit sharper and earlier ones. The fraction of accreted stars increases with galactocentric distance, so that at a radius of ∼ 8−10 kpc a fraction of ∼ 40 per cent, on average, are detected. Accreted stars are slightly younger, lower metallicity, and more α-enhanced than in situ stars. A significant fraction of old stars in the central regions come from a few (2 − 3) massive satellites (∼ 10 10 M ). The bulge components receive larger contributions of accreted stars formed in dwarfs smaller than ∼ 10 9.5 M . The difference between the distributions of ages and metallicities of old stars is thus linked to the accretion histories -those central regions with a larger fraction of accreted stars are those with contributions from more massive satellites. The kinematical properties of in situ and accreted stars are consistent with the latter being supported by their velocity dispersions, while the former exhibit clear signatures of rotational support. Our simulations demonstrate a range of characteristics, with some systems exhibiting a co-existing bar and spheroid in their central regions, resembling in some respect the central region of the Milky Way.

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The GIRAFFE Inner Bulge Survey (GIBS)

Cited by 170 publications

References 57 publications

What the Milky Way bulge reveals about the initial metallicity gradients in the disc

What the Milky Way bulge reveals about the initial metallicity gradients in the disc

Separation of stellar populations by an evolving bar: implications for the bulge of the Milky Way

The central spheroids of Milky Way mass-sized galaxies

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