3D outer bulge structure from near infrared star counts^*

¹ CNRS UMR6091, Observatoire de Besançon, BP 1615, 25010 Besançon Cedex, France e-mail: [picaud;annie.robin]@obs-besancon.fr
² Astronomisches Rechen-Institut, Mönchhofstraße 12-14, 69120 Heidelberg, Germany e-mail: picaud@ari.uni-heidelberg.de

Received: 3 May 2004
Accepted: 13 June 2004

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 (K_s, ) star count data in almost 100 windows from the DENIS near infrared large scale survey at -8° < l < 12° and b < 4°. 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.