3D outer bulge structure from near infrared star counts

Abstract

We attempt to study the characteristics of the different stellar populations present in the Galactic central region. A Monte Carlo method is used to simultaneously fit 11 thin disc and triaxial outer bulge density parameters on (Ks, J - Ks) star count data in almost 100 windows from the DENIS near infrared large scale survey at -8° < / < 12° and |b| < 4Æ. Various bulge density profiles and luminosity functions were tested using a population synthesis scheme. The best models, selected by a maximum likelihood test, give the following description: the outer bulge is boxy, prolate, and oriented 10.6° ±3° with respect to the Sun-center direction. It seems that the main bulge population is not older than 10 Gyr, but this preliminary result needs further work to be confirmed. A significant central hole is found in the middle of the thin disc. We discuss these results in regard to previous findings and the scenario of bulge formation.