On the orbits that generate the X-shape in the Milky Way bulge

Abstract

The Milky Way bulge shows a box/peanut or X-shaped bulge (hereafter BP/X) when viewed in infrared or microwave bands. We examine orbits in an N-body model of a barred disk galaxy that is scaled to match the kinematics of the Milky Way (MW) bulge. We generate maps of projected stellar surface density, unsharp masked images, 3D excess-mass distributions (showing mass outside ellipsoids), line-of-sight number count distributions, and 2D line-ofsight kinematics for the simulation as well as co-added orbit families, in order to identify the orbits primarily responsible for the BP/X shape. We estimate that between 19-23% of the mass of the bar in this model is associated with the BP/X shape and that the majority of bar orbits contribute to this shape which is clearly seen in projected surface density maps and 3D excess mass for non-resonant box orbits, "banana" orbits, "fish/pretzel" orbits and "brezel" orbits. Although only the latter two families (comprising 7.5% of the total mass) show a distinct X-shape in unsharp masked images, we find that nearly all bar orbit families contribute some mass to the 3D BP/X-shape. All co-added orbit families show a bifurcation in stellar number count distribution with distance that resembles the bifurcation observed in red clump stars in the MW. However, only the box orbit family shows an increasing separation of peaks with increasing galactic latitude |b|, similar to that observed. Our analysis suggests that no single orbit family fully explains all the observed features associated with the MW's BP/X shaped bulge, but collectively the non-resonant boxes and various resonant boxlet orbits contribute at different distances from the center to produce this feature. We propose that since box orbits (which are the dominant population in bars) have three incommensurable orbital fundamental frequencies, their 3-dimensional shapes are highly flexible and, like Lissajous figures, this family of orbits is most easily able to adapt to evolution in the shape of the underlying potential.

Modelling the Milky Way as a dry Galaxy

“…The simulation data is presented by the green squares, red circles and blue triangles. In order to evaluate the match between observations and simulations, we adopt the method used in Abbott et al (2017) and Gardner et al (2014), in which the N -body simulations are compared with the BRAVA observations. We measure χ 2 for v los and σ los for b = −4 • , −6 • , and −8 • , respectively.…”

Signatures of the Galactic bar on stellar kinematics unveiled by APOGEE

“…Our work is inspired by Abbott et al (2017) who also took into account the dispersion for the fitting of the disc mass to match the results of the BRAVA survey (Rich et al 2007;Kunder et al 2012). Other statistical parameters, such as the skewness and kurtosis are not included in equation 2 because they are dimensionless.…”

Quantifying the (X/peanut)-shaped structure of the Milky Way – new constraints on the bar geometry

“…Multiple studies, based on stellar populations and numerical simulations, have shown evidence that the Milky Way's central 'bulge' is not (primarily) the remnant of past merger events, i.e., a 'classical' bulge, but rather it was built predominantly from disc stars through the buckling and secular evolution of the Galactic bar, the latter itself originating from the disc (Shen et al 2010, Ness et al 2012Di Matteo et al 2014;Di Matteo 2016;Abbott et al 2017; see also Fragkoudi et al 2017). This result is consistent with the X/P morphology and indicates that the X/P 'bulge' and bar are aligned, since one has formed from, and is still the thick central part of, the other (see also Martinez-Valpuesta & Gerhard 2011, Romero-Gómez et al 2011and Wegg, Gerhard & Portail 2015.…”