Testing the universal stellar IMF on the metallicity distribution in the bulges of the Milky Way and M 31

¹ Dipartimento di Astronomia, Università di Trieste, via G. B. Tiepolo 11, 34121 Trieste, Italy e-mail: ballero@oats.inaf.it
² INAF – Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, 34121 Trieste, Italy
³ Argeland Institute for Astronomy, University of Bonn, Auf dem Hügel 71, 53121 Bonn, Germany

Received: 21 November 2006
Accepted: 22 December 2006

Abstract

Aims.We test whether the universal initial mass function (UIMF) or the integrated galaxial IMF (IGIMF) can be employed to explain the metallicity distribution (MD) of giants in the Galactic bulge.

Methods.We make use of a single-zone chemical evolution model developed for the Milky Way bulge in the context of an inside-out model for the formation of the Galaxy. We checked whether it is possible to constrain the yields above 80  by forcing the UIMF and required that the resulting MD matches the observed ones. We also extended the analysis to the bulge of M 31 to investigate a possible variation of the IMF among galactic bulges. Several parameters that have an impact on stellar evolution (star-formation efficiency, gas infall timescale) are varied.

Results.We show that it is not possible to satisfactorily reproduce the observed metallicity distribution in the two galactic bulges unless assuming a flatter IMF (x ≤ 1.1) than the universal one.

Conclusions.We conlude that it is necessary to assume a variation in the IMF among the various environments.