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Register a new userMapping a stellar disk into a boxy bulge: The outside-in part of the Milky Way bulge formation
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By means of idealized, dissipationless N-body simulations which follow the formation and subsequent buckling of a stellar bar, we study the characteristics of boxy/peanut-shaped bulges and compare them with the properties of the stellar populations in the Milky Way bulge. The main results of our modeling, valid for the general family of boxy/peanut shaped bulges, are the following: (i) because of the spatial redistribution in the disk initiated at the epoch of bar formation, stars from the innermost regions to the outer Lindblad resonance of the stellar bar are mapped into a boxy bulge; (ii) the contribution of stars to the local bulge density depends on their birth radius: stars born in the innermost disk tend to dominate the innermost regions of the boxy bulge, while stars originating closer to the OLR are preferentially found in the outer regions of the boxy/peanut structure; (iii) stellar birth radii are imprinted in the bulge kinematics, the larger the birth radii of stars ending up in the bulge, the greater their rotational support and the higher their line-of- sight velocity dispersions (but note that this last trend depends on the bar viewing angle); (iv) the higher the classical bulge-over-disk ratio, the larger its fractional contribution of stars at large vertical distance from the galaxy mid-plane. (ABRIDGED) On the basis of their chemical and kinematic characteristics, the results of our modeling suggests that the populations A, B and C, as defined by the ARGOS survey, can be associated, respectively, with the inner thin disk, to the young thick and to the old thick disk, following the nomenclature recently suggested for stars in the solar neighborhood by Haywood et al. (2013).
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We analyzed the distribution of the RC stars throughout Galactic bulge using 2MASS data. We mapped the position of the red clump in 1 sq.deg. size fields within the area |l|<=8.5deg and $3.5deg<=|b|<=8.5deg, for a total of 170 sq.deg. The red clump seen single in the central area splits into two components at high Galactic longitudes in both hemispheres, produced by two structures at different distances along the same line of sight. The X-shape is clearly visible in the Z-X plane for longitudes close to $l=0 deg axis. Crude measurements of the space densities of RC stars in the bright and faint RC populations are consistent with the adopted RC distances, providing further supporting evidence that the X-structure is real, and that there is approximate front-back symmetry in our bulge fields. We conclude that the Milky Way bulge has an X-shaped structure within $|l|<~2deg, seen almost edge on with respect to the line of sight. Additional deep NIR photometry extending into the innermost bulge regions combined with spectroscopic data is needed in order to discriminate among the different possibilities that can cause the observed X-shaped structure.
A vertical X-shaped structure was recently reported in the Galactic bulge. Here we present evidence of a similar X-shaped structure in the Shen et al. (2010) bar/boxy bulge model that simultaneously matches the stellar kinematics successfully. The X-shaped structure is found in the central region of our bar/boxy bulge model, and is qualitatively consistent with the observed one in many aspects. End-to-end separations of the X-shaped structure in the radial and vertical directions are roughly 3 kpc and 1.8 kpc, respectively. The X-shaped structure contains about 7% of light in the boxy bulge region, but it is significant enough to be identified in observations. An X-shaped structure naturally arises in the formation of bar/boxy bulges, and is mainly associated with orbits trapped around the vertically-extended x_1 family. Like the bar in our model, the X-shaped structure tilts away from the Sun--Galactic center line by 20 degrees. The X-shaped structure becomes increasingly symmetric about the disk plane, so the observed symmetry may indicate that it formed at least a few billion years ago. The existence of the vertical X-shaped structure suggests that the formation of the Milky Way bulge is shaped mainly by internal disk dynamical instabilities.
We use the extensive $Gaia$ Data Release 2 set of Long Period Variables to select a sample of Oxygen-rich Miras throughout the Milky Way disk and bulge for study. Exploiting the relation between Mira pulsation period and stellar age/chemistry, we slice the stellar density of the Galactic disk and bulge as a function of period. We find the morphology of both components evolves as a function of stellar age/chemistry with the stellar disk being stubby at old ages, becoming progressively thinner and more radially extended at younger stellar ages, consistent with the picture of inside-out and upside-down formation of the Milky Ways disk. We see evidence of a perturbed disk, with large-scale stellar over-densities visible both in and away from the stellar plane. We find the bulge is well modelled by a triaxial boxy distribution with an axis ratio of $sim [1:0.4:0.3]$. The oldest of the Miras ($sim$ 9-10 Gyr) show little bar-like morphology, whilst the younger stars appear inclined at a viewing angle of $sim 21^{circ}$ to the Sun-Galactic Centre line. This suggests that bar formation and buckling took place 8-9 Gyr ago, with the older Miras being hot enough to avoid being trapped by the growing bar. We find the youngest Miras to exhibit a strong peanut morphology, bearing the characteristic X-shape of an inclined bar structure.
Recent observational programmes are providing a global view of the Milky Way bulge that serves as template for detailed comparison with models and extragalactic bulges. A number of surveys (i.e. VVV, GIBS, GES, ARGOS, BRAVA, APOGEE) are producing comprehensive and detailed extinction, metallicity, kinematics and stellar density maps of the Galactic bulge with unprecedented accuracy. However, the still missing key ingredient is the distribution of stellar ages across the bulge. To overcome this limitation, we aim to age-date the stellar population in several bulge fields with the ultimate goal of deriving an age map of the Bulge. This paper presents the methodology and the first results obtained for a field along the Bulge minor axis, at $b=-6^circ$. We use a new PSF-fitting photometry of the VISTA Variables in the V{i}a L{a}ctea (VVV) survey data to construct deep color-magnitude diagrams of the bulge stellar population down to $sim$ 2 mag below the Main Sequence turnoff. We find the bulk of the bulge stellar population in the observed field along the minor axis to be at least older than $sim$ 7.5 Gyr. In particular, when the metallicity distribution function spectroscopically derived by GIBS is used, the best fit to the data is obtained with a combination of synthetic populations with ages in between $sim$ 7.5 Gyr and 11 Gyr. However, the fraction of stars younger than $sim$ 10 Gyr strongly depends upon the number of Blue Straggler Stars present in the bulge. Simulations show that the observed color-magnitude diagram of the bulge in the field along the minor axis is incompatible with the presence of a conspicuous population of intermediate-age/young (i.e. $lesssim 5$ Gyr) stars.
The detailed study of the Galactic bulge stellar population necessarily requires an accurate representation of the interstellar extinction particularly toward the Galactic plane and center, where the severe and differential reddening is expected to vary on sub-arcmin scales. Although recent infrared surveys have addressed this problem by providing extinction maps across the whole Galactic bulge area, dereddened color-magnitude diagrams near the plane and center appear systematically undercorrected, suggesting the need for higher resolutions. These undercorrections affect any stellar study sensitive to color (e.g. star formation history analysis via color-magnitude diagram fitting), either making them inaccurate or limiting them to small low/stable extinction windows where this value is better constrained. We aim at providing a high-resolution (2 arcmin to $sim$ 10 arcsec) color excess map for the VVV bulge area, in $mathrm{J}-mathrm{K}_s$ color. We use the MW-BULGE-PSFPHOT catalogs sampling $sim$ 300 deg$^2$ across the Galactic bulge ($|l| < 10^circ$ and $-10^circ < b < 5^circ$) to isolate a sample of red clump and red giant branch stars, for which we calculate average $mathrm{J}-mathrm{K}_s$ color in a fine spatial grid in $(l, b)$ space. We obtain a E$(mathrm{J}-mathrm{K}_s)$ map spanning the VVV bulge area of roughly 300 deg$^2$, with the equivalent to a resolution between $sim$ 1 arcmin for bulge outskirts ($l < -6^circ$) to below 20 arcsec within the central $|l| < 1^circ$, and below 10 arcsec for the innermost area ($|l| < 1^circ$ and $|b| < 3^circ$). The result is publicly available at http://basti-iac.oa-teramo.inaf.it/vvvexmap/
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