Unifying boxy bulge and planar long bar in the Milky Way

Abstract

It has been known for some time that the Milky Way is a barred disk galaxy. More recently several studies inferred from starcount observations that the Galaxy must contain a separate, new, flat long bar component, twisted relative to the barred bulge. Here we use a simulation with a boxy bulge and bar to suggest that these observations can be reproduced with a single structure. In this simulation a stellar bar evolved from the disk, and the boxy bulge originated from it through secular evolution and the buckling instability. We calculate starcount distributions for this model at different longitudes and latitudes, in a similar way as observers have done for resolved starcounts. Good agreement between the simulation and the observations can be achieved for a suitable snapshot, even though the simulation has a single boxy bulge and bar} structure. In this model, part of the long bar signature is due to a volume effect in the starcounts, and another part is due to chosing a snapshot in which the planar part of the boxy bulge and bar has developed leading ends through interaction with the adjacent spiral arm heads. We also provide predictions from this model for the line-of-sight velocity distributions at the longitudes with the long bar signature, for comparison with upcoming surveys.