Stellar populations in the Milky Way bulge region: towards solving the Galactic bulge and bar shapes using 2MASS data

By Annie Robin, D. Marshall, M. Schultheis, Céline Reylé. 2011

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Exploring the bulge region of our Galaxy is an interesting but challenging quest because of its complex structure and the highly variable extinction. We re-analyse photometric near-infrared data in order to investigate why it is so hard to reach a consensus on the shape and density law of the bulge, as witnessed in the literature. The apparent orientation of the bulge seems to vary with the range of longitude, latitude, and the population considered. To solve the problem we have used the Besancon galaxy model to provide a scheme for parameter fitting of the structural characteristics of the bulge region. The fitting process allows the shape of the bulge’s main structure to be determined. We explore various parameters and shapes for the bulge population, based on Ferrer’s ellipsoids, and fit the shape of the inner disc in the same process. The results show that the main structure has a standard triaxial boxy shape with an orientation of about 13° with respect to the Sun-Galactic centre direction. But the fit is greatly improved when we add a second structure,which is a longer and thicker ellipsoid. We emphasize that our first ellipsoid represents the main boxy bar of the Galaxy and that the thick bulge population could be either (i) a classical bulge slightly flattened by the effect of the bar’s potential; or (ii) an inner thick disc counterpart. With Ferrer’s ellipsoid, the model shows a general agreement with 2MASS data at the level of 10% in the whole bulge region but does not produce the “double clump” feature. However, we show that the double clump seen at intermediate latitudes can be reproduced by adding a slight flare to the bar. To characterize the populations better, we further simulate several fields that have been surveyed in spectroscopy and for which a metallicity distribution function (MDF) are available. The model agrees well with these MDF measured along the minor axis if we assume that the main bar has a mean solar metallicity and the second thicker population has a lower metallicity. It then naturally creates a vertical metallicity gradient by mixing the two populations. In the process of model fitting, we also determine the thin disc parameters. The thin disc is found to have a scale length of 2.2 kpc, in good agreement with previous estimates towards the anticentre, but with a large hole of scale length 1.3 kpc, giving a maximum density in the plane for this population at about 2.3 kpc from the Galactic centre. In the very central part of the bulge on top of our two populations, and by subtracting the fitted ellipsoids, we find evidence of an extra population in the nuclear region, at 2° in longitude and 1° of latitude from the Galactic centre. Its location corresponds well to the central molecular zone, and to the Alard nuclear bar.

Subfields: Astronomy and Astrophysics, Education, Instrumentation

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Citations in the corpus

DISCOVERY AND VALIDATION OF Kepler-452b: A 1.6 R⨁ SUPER EARTH EXOPLANET IN THE HABITABLE ZONE OF A G2 STAR (2015, methodology): “…for the blend frequency calculation included the number density of stars in the vicinity of the target from the Besançon Galactic structure model (Robin et al. 2003, 2012),12 and the rates of occurrence of eclipsing bina- 12http://model.obs-besancon.fr . ries (Slawson et al. 2011) and of…”
ARGOS - III. Stellar populations in the Galactic bulge of the Milky Way (2012, background): “Robin et al. (2012) estimates a peakin the thin disk number density at∼ 2.3 kpc from the centre of the Galaxy which may contribute to the larger number of metal rich starsseen at low Galactic latitudes on the near side of the bulge.”
REDDENING AND EXTINCTION TOWARD THE GALACTIC BULGE FROM OGLE-III: THE INNER MILKY WAY'S RV ∼ 2.5 EXTINCTION CURVE (2012, methodology): “Robin et al. (2012) estimated 1.1×1010M by fitting 2MASS data to various parametric models.”
BANYAN. VII. A NEW POPULATION OF YOUNG SUBSTELLAR CANDIDATE MEMBERS OF NEARBY MOVING GROUPS FROM THE BASS SURVEY (2015, methodology): “We performed a Monte Carlo simulation based on the Besançon galactic model (A. C. Robin et al. in preparation, Robin et al. 2012) and the SKMs of YMGs to obtain a field contamination probability for each individual target in our sample, which allows for a more absolute interpretation in terms of…”
Chemical tagging with APOGEE: Discovery of a large population of N-rich stars in the inner Galaxy (2016, methodology): “For that purpose, we adopt the Besançon models by Robin et al. (2012, 2014), which match stellar counts in 2MASS and SDSS by considering a combination of four components: a thick disk, a thin disk, a halo, and a bar.”
The structure of the Milky Way's bar outside the bulge (2015, background): “The functional form given in equation (9) is equivalent to the exponential shape function fitted in Robin et al. (2012).”
Reddening and metallicity maps of the Milky Way bulge from VVV and 2MASS II. The complete high resolution extinction map and implications for Galactic bulge studies (2012, methodology): “This model is described in detail in Robin et al. (2012).”
Predictions of the WFIRST Microlensing Survey. I. Bound Planet Detection Rates (2018, background): “It is intermediate to the original, publicly available version (Robin et al. 2003), and a more recent version (Robin et al. 2012).”
Constraining the thick disc formation scenario of the Milky Way (2014, unknown): “The inner part of the thick disc significantly contributes to the stellar populations in the inner Galaxy and was misinterpreted as a thick bulge or a flattened classical bulge in Robin et al. (2012).”
StarHorse : a Bayesian tool for determining stellar masses, ages, distances, and extinctions for field stars (2017, methodology): “Finally, as our default model we included a bar-like bulge model from Robin et al. (2012).”
The Fermi Galactic Center GeV Excess and Implications for Dark Matter (2017, methodology): “The stellar population of the Galactic bulge is older than 5 Gyr (e.g., Robin et al. 2012, and references therein).”
BANYAN. XI. The BANYAN Σ Multivariate Bayesian Algorithm to Identify Members of Young Associations with 150 pc (2018, methodology): “It is based on the Besançon model (Robin et al. 1996, 2003, 2012, 2014, 2017)11 of the Galactic disk in the Solar neighborhood (with a very small contribution from the Galactic halo), and uses the multivariate Gaussian formalism described in Section 3.1 for it to be compatible with the solution of…”
Photo-astrometric distances, extinctions, and astrophysical parameters for Gaia DR2 stars brighter than G = 18 (2019, result): “The significance of the result lies in the fact that although we are using a prior for the Galactic bulge-bar (Robin et al. 2012), its shape and inclination angle are quite different from our bar prior (see Fig.”
BANYAN. II. VERY LOW MASS AND SUBSTELLAR CANDIDATE MEMBERS TO NEARBY, YOUNG KINEMATIC GROUPS WITH PREVIOUSLY KNOWN SIGNS OF YOUTH (2013, methodology): “• We use a Besançon Galactic model (Robin et al. 2012) to build the young and old field hypotheses.”
The Milky Way’s stellar disk (2013, background): “…into a consistent—if not always dynamically consistent—stellar-populations-based model that can be used to simulate the expected content of new surveys (e.g., Robin et al. 2012), test Disk analyses on realistic mock data, and provide fiducial or background models for the interpretation of new data.”
The Bulge Radial Velocity Assay (BRAVA): II. Complete Sample and Data Release (2011, methodology): “Secondly, the Shen et al. (2010) model still allows a relatively small merger-built classical bulge (about 10% of the disk mass), the mixing of two bulge populations could conceivably produce a vertical metallicity gradient (also see the Besancon model, Robin et al. 2011).”