The inner Milky Way is dominated by a boxy, triaxial bulge which is believed to have formed through disk instability processes. Despite its proximity, its large-scale properties are still not very well known, due to our position in the obscuring Galactic disk.Here we make a measurement of the three-dimensional density distribution of the Galactic bulge using red clump giants identified in DR1 of the VVV survey. Our density map covers the inner (2.2 × 1.4 × 1.1) kpc of the bulge/bar. Line-of-sight density distributions are estimated by deconvolving extinction and completeness corrected K s -band magnitude distributions. In constructing our measurement, we assume that the three-dimensional bulge is 8-fold mirror triaxially symmetric. In doing so we measure the angle of the bar-bulge to the line-ofsight to be (27 ± 2) • , where the dominant error is systematic arising from the details of the deconvolution process.The resulting density distribution shows a highly elongated bar with projected axis ratios ≈ (1 : 2.1) for isophotes reaching ∼ 2 kpc along the major axis. Along the bar axes the density falls off roughly exponentially, with axis ratios (10 : 6.3 : 2.6) and exponential scale-lengths (0.70 : 0.44 : 0.18) kpc. From about 400 pc above the Galactic plane, the bulge density distribution displays a prominent X-structure. Overall, the density distribution of the Galactic bulge is characteristic for a strongly boxy/peanut shaped bulge within a barred galaxy.
While it is incontrovertible that the inner Galaxy contains a bar, its structure near the Galactic plane has remained uncertain, where extinction from intervening dust is greatest. We investigate here the Galactic bar outside the bulge, the long bar, using red clump giant (RCG) stars from UKIDSS, 2MASS, VVV, and GLIMPSE. We match and combine these surveys to investigate a wide area in latitude and longitude, |b| 9 • and |l| 40 • . We find: (i) The bar extends to l ∼ 25 • at |b| ∼ 5 • from the Galactic plane, and to l ∼ 30 • at lower latitudes. (ii) The long bar has an angle to the line-of-sight in the range (28 − 33) • , consistent with studies of the bulge at |l| < 10 • . (iii) The scale-height of RCG stars smoothly transitions from the bulge to the thinner long bar. (iv) There is evidence for two scale heights in the long bar. We find a ∼ 180 pc thin bar component reminiscent of the old thin disk near the sun, and a ∼ 45 pc super-thin bar component which exists predominantly towards the bar end. (v) Constructing parametric models for the RC magnitude distributions, we find a bar half length of 5.0 ± 0.2 kpc for the 2-component bar, and 4.6 ± 0.3 kpc for the thin bar component alone. We conclude that the Milky Way contains a central box/peanut bulge which is the vertical extension of a longer, flatter bar, similar as seen in both external galaxies and N-body models.
We construct a large set of dynamical models of the galactic bulge, bar and inner disk using the Made-to-Measure method. Our models are constrained to match the red clump giant density from a combination of the VVV, UKIDSS and 2MASS infrared surveys together with stellar kinematics in the bulge from the BRAVA and OGLE surveys, and in the entire bar region from the ARGOS survey. We are able to recover the bar pattern speed and the stellar and dark matter mass distributions in the bar region, thus recovering the entire galactic effective potential. We find a bar pattern speed of 39.0 ± 3.5 km s −1 kpc −1 , placing the bar corotation radius at 6.1 ± 0.5kpc and making the Milky Way bar a typical fast rotator. We evaluate the stellar mass of the long bar and bulge structure to be M bar/bulge = 1.88 ± 0.12 × 10 10 M , larger than the mass of disk in the bar region, M inner disk = 1.29 ± 0.12 × 10 10 M . The total dynamical mass in the bulge volume is 1.85 ± 0.05 × 10 10 M . Thanks to more extended kinematic data sets and recent measurement of the bulge IMF our models have a low dark matter fraction in the bulge of 17% ± 2%. We find a dark matter density profile which flattens to a shallow cusp or core in the bulge region. Finally, we find dynamical evidence for an extra central mass of ∼ 0.2 × 10 10 M , probably in a nuclear disk or disky pseudobulge.
We derive new constraints on the mass, rotation, orbit structure and statistical parallax of the Galactic old nuclear star cluster and the mass of the supermassive black hole. We combine star counts and kinematic data from Fritz et al. (2014), including 2'500 line-of-sight velocities and 10'000 proper motions obtained with VLT instruments. We show that the difference between the proper motion dispersions σ l and σ b cannot be explained by rotation, but is a consequence of the flattening of the nuclear cluster. We fit the surface density distribution of stars in the central 1000 ′′ by a superposition of a spheroidal cluster with scale ∼ 100 ′′ and a much larger nuclear disk component. We compute the self-consistent two-integral distribution function f (E, L z ) for this density model, and add rotation self-consistently. We find that: (i) The orbit structure of the f (E, L z ) gives an excellent match to the observed velocity dispersion profiles as well as the proper motion and line-of-sight velocity histograms, including the double-peak in the v l -histograms. (ii) This requires an axial ratio near q 1 = 0.7 consistent with our determination from star counts, q 1 = 0.73 ± 0.04 for r < 70 ′′ . (iii) The nuclear star cluster is approximately described by an isotropic rotator model. (iv) Using the corresponding Jeans equations to fit the proper motion and line-of-sight velocity dispersions, we obtain best estimates for the nuclear star cluster mass, black hole mass, and distance M * (r < 100 ′′ ) = (8.94±0.31| stat ±0.9| syst )×10 6 M ⊙ , M • = (3.86±0.14| stat ±0.4| syst )×10 6 M ⊙ , and R 0 = 8.27±0.09| stat ±0.1| syst kpc, where the estimated systematic errors account for additional uncertainties in the dynamical modeling. (v) The combination of the cluster dynamics with the S-star orbits around Sgr A * strongly reduces the degeneracy between black hole mass and Galactic centre distance present in previous S-star studies. A joint statistical analysis with the results of Gillessen et al. (2009) gives M • = (4.23±0.14)×10 6 M ⊙ and R 0 = 8.33±0.11 kpc.
We construct dynamical models of the Milky Way's Box/Peanut (B/P) bulge, using the recently measured 3D density of Red Clump Giants (RCGs) as well as kinematic data from the BRAVA survey. We match these data using the NMAGIC Made-to-Measure method, starting with N-body models for barred discs in different dark matter haloes. We determine the total mass in the bulge volume of the RCGs measurement (±2.2 × ±1.4 × ±1.2 kpc) with unprecedented accuracy and robustness to be 1.84 ± 0.07 × 10 10 M . The stellar mass in this volume varies between 1.25 − 1.6 × 10 10 M , depending on the amount of dark matter in the bulge. We evaluate the mass-to-light and mass-to-clump ratios in the bulge and compare them to theoretical predictions from population synthesis models. We find a mass-to-light ratio in the K-band in the range 0.8 − 1.1. The models are consistent with a Kroupa or Chabrier IMF, but a Salpeter IMF is ruled out for stellar ages of 10 Gyr. To match predictions from the Zoccali IMF derived from the bulge stellar luminosity function requires ∼ 40% or ∼ 0.7 × 10 10 M dark matter in the bulge region. The BRAVA data together with the RCGs 3D density imply a low pattern speed for the Galactic B/P bulge of Ω p = 25 − 30 km s −1 kpc −1 . This would place the Galaxy among the slow rotators (R 1.5). Finally, we show that the Milky Way's B/P bulge has an off-centred X structure, and that the stellar mass involved in the peanut shape accounts for at least 20% of the stellar mass of the bulge, significantly larger than previously thought.
We propose a novel explanation for the Hercules stream consistent with recent measurements of the extent and pattern speed of the Galactic bar. We have adapted a made-to-measure dynamical model tailored for the Milky Way to investigate the kinematics of the solar neighborhood (SNd). The model matches the 3D density of the red clump giant stars (RCGs) in the bulge and bar as well as stellar kinematics in the inner Galaxy, with a pattern speed of 39 km s −1 kpc −1 . Cross-matching this model with the Gaia DR1 TGAS data combined with RAVE and LAMOST radial velocities, we find that the model naturally predicts a bimodality in the U-V-velocity distribution for nearby stars which is in good agreement with the Hercules stream. In the model, the Hercules stream is made of stars orbiting the Lagrange points of the bar which move outward from the bar's corotation radius to visit the SNd. While the model is not yet a quantitative fit of the velocity distribution, the new picture naturally predicts that the Hercules stream is more prominent inward from the Sun and nearly absent only a few 100 pc outward of the Sun, and plausibly explains that Hercules is prominent in old and metal-rich stars.
The second data release of the Gaia mission has revealed a very rich structure in local velocity space. In terms of in-plane motions, this rich structure is also seen as multiple ridges in the actions of the axisymmetric background potential of the Galaxy. These ridges are probably related to a combination of effects from ongoing phase-mixing and resonances from the spiral arms and the bar. We have recently developed a method to capture the behaviour of the stellar phase-space distribution function at a resonance, by reexpressing it in terms of a new set of canonical actions and angles variables valid in the resonant region. Here, by properly treating the distribution function at resonances, and by using a realistic model for a slowly rotating large Galactic bar with pattern speed Ω b = 39 km s −1 kpc −1 , we show that no less than six ridges in local action space can be related to resonances with the bar. Two of these at low angular momentum correspond to the corotation resonance, and can be associated to the Hercules moving group in local velocity space. Another one at high angular momentum corresponds to the outer Lindblad resonance, and can tentatively be associated to the velocity structure seen as an arch at high azimuthal velocities in Gaia data. The other ridges are associated to the 3:1, 4:1 and 6:1 resonances. The latter can be associated to the so-called 'horn' of the local velocity distribution. While it is clear that effects from spiral arms and incomplete phase-mixing related to external perturbations also play a role in shaping the complex kinematics revealed by Gaia data, the present work demonstrates that, contrary to common misconceptions, the bar alone can create multiple prominent ridges in velocity and action space.
We investigate interstellar extinction curve variations toward ∼4 deg 2 of the inner Milky Way in V IJK s photometry from the OGLE-III and V V V surveys, with supporting evidence from diffuse interstellar bands and F 435W, F 625W photometry. We obtain independent measurements toward ∼2,000 sightlines of A I , E(V − I), E(I − J), and E(J − K s ), with median precision and accuracy of 2%. We find that the variations in the extinction ratiosare large (exceeding 20%), significant, and positively correlated, as expected. However, both the mean values and the trends in these extinction ratios are drastically shifted from the predictions of Cardelli and Fitzpatrick, regardless of how R V is varied. Furthermore, we demonstrate that variations in the shape of the extinction curve has at least two degrees of freedom, and not one (e.g. R V ), which we conform with a principal component analysis. We derive a median value of < A V /A Ks >= 13.44, which is ∼60% higher than the "standard" value. We show that the Wesenheit magnitude W I = I − 1.61(I − J) is relatively impervious to extinction curve variations.Given that these extinction curves are linchpins of observational cosmology, and that it is generally assumed that R V variations correctly capture variations in the extinction curve, we argue that systematic errors in the distance ladder from studies of type Ia supernovae and Cepheids may have been underestimated. Moreover, the reddening maps from the Planck experiment are shown to systematically overestimate dust extinction by ∼100%, and lack sensitivity to extinction curve variations.
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