Mapping a stellar disk into a boxy bulge: The outside-in part of the Milky Way bulge formation

Abstract

By means of idealized, dissipationless N-body simulations that 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 (MW) 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 (OLR) 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 preferably 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 midplane. Comparing these results with the properties of the stellar populations of the MW bulge recently revealed by the ARGOS survey, we conclude that (I) the two most metal-rich populations of the MW bulge, labeled A and B in the ARGOS survey, originate in the disk, with the population of A having formed on average closer to the Galaxy center than the population of component B; (II) a massive (B/D ∼ 0.25) classical spheroid can be excluded for the MW, thus confirming previous findings that the MW bulge is composed of populations that mostly have a disk origin. On the basis of their chemical and kinematic characteristics, the results of our modeling suggest 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 that we recently suggested for stars in the solar neighborhood.

TheGaia-ESO Survey: metallicity and kinematic trends in the Milky Way bulge

“…This scenario has recently been examined by Di Matteo et al (2014), who argued that the characteristics (kinematics, chemistry) of the metal-poor bulge component agree better with a thick-disk nature than with a spheroid. In particular, they argued that a classical bulge spheroid with the mean metallicity of ARGOS C cannot be larger than 10 8 M (Gallazzi et al 2005), which is incompatible with the number of stars in that component.…”

Clues to the formation of the Milky Way’s thick disk

“…Thick disks, however, are found to be ubiquitous, and are detected on nearly all nearby edge-on disks galaxies (Comerón et al 2011). Recent results (Ness et al 2013a,b;Di Matteo et al 2014 have confirmed the view that the Milky Way (MW) is also a pure disk galaxy (Shen et al 2010;Kunder et al 2012), i.e., it has no substantial classical bulge. Furthermore, measurements of its SFH have shown that during the peak of star formation in the universe, the MW was a star-bursting galaxy and was forming its thick disk (Snaith et al , 2015Lehnert et al 2014).…”

“…Furthermore, measurements of its SFH have shown that during the peak of star formation in the universe, the MW was a star-bursting galaxy and was forming its thick disk (Snaith et al , 2015Lehnert et al 2014). The structural parameters of this population, with a short scale length (Bensby et al 2011;Cheng et al 2012a;Bovy et al 2012) suggest that it constitutes most of the mass in the MW's central region (Ness et al 2013a,b;Di Matteo et al 2014Haywood et al, in prep. ).…”

Constraining the thick disc formation scenario of the Milky Way

“…These data show that different populations overlap in the inner region of the Milky Way, and at moderate latitudes their component C has metallicity, alpha abundances, and velocities very similar to the old thick disc we found, while component B could be a counterpart of our younger thick disc. Most recently, Di Matteo et al (2014) analysed simulations of the bulge evolution and showed that components B and C identified by ARGOS survey can also be identified with the young and old thick discs. A more detailed analysis of the metallicity and velocities distributions in the inner region with newly available spectroscopic surveys will be performed in the near future using the new population model proposed here.…”