Origin of non-unique deprojection of the Galactic bar

Abstract

Non-uniqueness of deprojecting the integrated light distribution of a nearby or faraway triaxial body is reviewed in the context of deriving the volume density of the Galactic bar from the COBE/DIRBE maps of the Galactic plane. The exact origin of this non-uniqueness is studied. One can write down a sequence of triaxial bar models which appear identical in integrated light from the Sun's perspective, and the whole sequence is mapped out as a function of the Galactocentric distance of the observer from galactic to extragalactic distance scales. While mirror symmetries and perspective effects are compatible with any orientation of the bar in the sequence, weak upper and lower bounds can still be placed on the angles and axis ratios of the bar by positivity and other general requirements. Star count data of bulge giants are ideal for selecting a unique model from the sequence.