Stellar ages through the corners of the boxy bulge^⋆,⋆⋆

¹ European Southern Observatory, Karl Schwarzschild-Straße 2, 85748 Garching bei München, Germany
e-mail: evalenti@eso.org
² Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile
³ INAF-Osservatorio Astronomico di Padova, Vicolo dell’ Osservatorio 5, 35122 Padova, Italy
⁴ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁵ European Southern Observatory, 19001 Casilla, Santiago, Chile
⁶ Jeremiah Horrocks Institute, University of Central Lancashire, Preston PR1 2HE, UK
⁷ Institute for Theoretical Physics, University of Zurich, Winterthurestrasse 190, 8057 Zurich, Switzerland

Received: 25 May 2013
Accepted: 13 September 2013

Abstract

Aims. In some scenarios for the formation of the Milky Way bulge, the stellar population at the edges of the boxy bulge may be younger than those on the minor axis or close to the Galactic center. So far the only bulge region where deep color–magnitude diagrams have been obtained is indeed along the minor axis. To overcome this limitation, we aim to age-date the bulge stellar populations far away from the bulge minor axis.

Methods. Color–magnitude diagrams and luminosity functions have been obtained from deep near-IR VLT/HAWK-I images taken at the two Southern corners of the boxy bulge, i.e., near the opposite edges of the Galactic bar. The foreground disk contamination has been statistically removed using a pure disk field observed with the same instrument and located approximately at similar Galactic latitudes of the two bulge fields and ~30° in longitude away from the Galactic center. For each bulge field, mean reddening and distance are determined using the position of red clump stars, and the metallicity distribution is derived photometrically using the color distribution of stars in the upper red giant branch.

Results. The resulting metallicity distribution function of both fields peaks around [Fe/H] ~ −0.1 dex, with the bulk of the stellar population having a metallicity within the range: −1 dex ≲ [Fe/H] ≲ +0.4 dex, quite similar to that of other inner bulge fields. As for the previously explored inner fields, the color–magnitude diagrams of the two bar fields are consistent with their stellar population being older than ~10 Gyr, with no obvious evidence of younger population.

Conclusions. The stellar population of the corners of the boxy bulge appears to be coeval with those within the innermost ~4° from the Galactic center.