The kinematic signature of face-on peanut-shaped bulges

We present a kinematic diagnostic for peanut-shaped bulges in nearly face-on galaxies. The face-on view provides a novel perspective on peanuts that would allow study of their relation to bars and disks in greater detail than hitherto possible. The diagnostic is based on the fact that peanut shapes are associated with a flat density distribution in the vertical direction. We show that the kinematic signature corresponding to such a distribution is a minimum in the fourth-order Gauss-Hermite moment s(4). We demonstrate our method on N-body simulations of varying peanut strength, showing that strong peanuts can be recognized to inclinations i similar or equal to 30 degrees, regardless of the strength of the bar. We also consider compound systems in which a bulge is present in the initial conditions, as may happen if bulges form at high redshift through mergers. We show that in this case, because the vertical structure of the bulge is not derived from that of the disk, the signature of a peanut in s(4) is weakened. Thus the same kinematic signature of peanuts can be used to explore bulge formation mechanisms. The observational requirements for identifying peanuts with this method are challenging, but feasible.