The Stellar Halo in the Inner MilkyWay: Predicted Shape and Kinematics

Angeles Pérez-Villegas, Matthieu Portail and Ortwin Gerhard, 2017, MNRAS, 460, L80

Spatial density profiles of RRLyrae as a function of spherical radius, for stars in the bulge as measured by Pietrukowicz et al. (2015) in red, compared to stars in the halo further out as measured from various studies indicated in the legend. The density of bulge RRLyrae naturally lines up with the outer halo, motivating the idea that bulge RRLyrae stars could just be the inner part of the stellar halo.

We have used N-body simulations for the Milky Way to investigate the kinematic and structural properties of the old metal-poor stellar halo in the barred inner region of the Galaxy. We find that the extrapolation of the density distribution for bulge RR Lyrae stars, ρ(r) ∼ r-3, approximately matches the number density of RR Lyrae in the nearby stellar halo. We follow the evolution of such a tracer population through the formation and evolution of the bar and box/peanut bulge in the N-body model. We find that its density distribution changes from oblate to triaxial, and that it acquires slow rotation in agreement with recent measurements. The maximum radial velocity is ∼15-25 km/s at |l| = 10°-30°, and the velocity dispersion is ∼120 km/s. Even though the simulated metal-poor halo in the bulge has a barred shape, just 12% of the orbits follow the bar, and it does not trace the peanut/X structure. With these properties, the RR Lyrae population in the Galactic bulge is consistent with being the inward extension of the Galactic metal-poor stellar halo.

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The structure of the Milky Way's bar outside the bulge

Christopher Wegg, Ortwin Gerhard and Matthieu Portail, 2015, MNRAS, 450, 4050

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 deg and | l |< 40 deg.

Surface density of stars in the Ks-band in the extinction-corrected magnitude range 12.25 < K0 < 12.75 from the combination of the VVV survey in the bulge, the UKIDSS in the plane and 2MASS elwhere. Asymmetric number counts in l close to the plane are a result of non-axisymmetry due to the long bar.

We find: (1) The bar extends to l~25deg at | b |~ 5 deg from the Galactic plane, and to l ~ 30 deg at lower latitudes. (2) The long bar has an angle to the line-of-sight in the range (28-33) deg, consistent with studies of the bulge at | l |< 10 deg. (3) The scale-height of RCG stars smoothly transitions from the bulge to the thinner long bar. (4) There is evidence for two scale heights in the long bar. We find a ~180pc thin bar component reminiscent of the old thin disk near the sun, and a ~45pc super-thin bar component which exists predominantly towards the bar end. (5) Constructing parametric models for the RC magnitude distributions, we find a bar half length of 5.0+-0.2kpc for the 2-component bar, and 4.6+-0.3kpc 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.

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Mapping the three-dimensional density of the Galactic bulge with VVV red clump stars

Chris Wegg & Ortwin Gerhard, 2013, MNRAS, 435, 1874

In the center we show an image of the 3D iso-density contours of the measured bulge density. Three projections are shown as surrounding plots: From above (i.e. from the North Galactic pole), from the side (i.e. along the intermediate axis of the bar) and the re-projected surface density from the sun. The three projections show the surface density of bulge red clump stars with isophotes spaced by 0.5 mag.

Using red clump giant stars identified in the VVV survey we produced the most accurate and high resolution map of the inner regions of the Milky Way. Our density map covers the inner (2.2x1.4x1.1)kpc of the bulge/bar. Line-of-sight density distributions were estimated by deconvolving extinction and completeness corrected K-band magnitude distributions. In constructing our measurement, we assumed that the three-dimensional bulge is 8-fold mirror triaxially symmetric, but the map is otherwise completely non-parametric. In doing so we measure the angle of the bar-bulge to the line-of-sight to be (27+- 2)deg, where the dominant error is systematic arising from the details of the deconvolution process. Our density distribution shows a highly elongated bar with projected axis ratios ~(1:2.1) for isophotes reaching ~2kpc 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 400pc 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.

Anyone is welcome to use the movie we made to vizualize the measured 3D structure. Just reference the original paper if you use the movie (Creative Commons attribution share-alike license).

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ESO Messenger Article.

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The inner Galactic bulge: evidence for a nuclear bar?

Gerhard, O., Martinez-Valpuesta, I.,  2012,  ApJ,  744,  L8

Starcount observations show evidence for a structural change in the Milky Way bulge inward of |l|~4◦. With an N-body barred galaxy simulation we showed that a boxy bulge formed through the bar and buckling instabilities matches these observations. The change in the slope of the model longitude profiles is caused by a transition from highly elongated to more nearly axisymmetric isodensity contours in the inner boxy bulge. This transition is confined to a few degrees from the Galactic plane.

The same simulation snapshot was earlier used to clarify the apparent boxy bulge—long bar dichotomy. Furthermore, the nuclear star count map derived from this simulation snapshot displays a longitudinal asymmetry similar to that observed in the TwoMicron All Sky Survey (2MASS) data. These combined results
    • support the interpretation that the Galactic bulge originated from disk evolution,
    • and question arguments based on star count data for the existence of a secondary nuclear bar in the Milky Way.

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