Lost in secular evolution: the case of a low mass classical bulge

Abstract

The existence of a classical bulge in disk galaxies holds important clue to the assembly history of galaxies. Finding observational evidence of very low mass classical bulges particularly in barred galaxies including our Milky Way, is a challenging task as the bar driven secular evolution might bring significant dynamical change to these bulges alongside the stellar disk. Using high-resolution N-body simulation, we show that if a cool stellar disk is assembled around a non-rotating low-mass classical bulge, the disk rapidly grows a strong bar within a few rotation time scales. Later, the bar driven secular process transform the initial classical bulge into a flattened rotating stellar system whose central part also have grown a bar-like component rotating in sync with the disk bar. During this time, a boxy/peanut (hereafter, B/P) bulge is formed via the buckling instability of the disk bar and the vertical extent of this B/P bulge being slightly higher than that of the classical bulge, it encompasses the whole classical bulge. The resulting composite bulge appears to be both photometrically and kinematically identical to a B/P bulge without any obvious signature of the classical component. Our analysis suggest that many barred galaxies in the local universe might be hiding such low-mass classical bulges. We suggest that stellar population and chemodynamical analysis might be required in establishing the evidence for such low-mass classical bulges.