Hiding its age: the case for a younger bulge

¹ GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France
e-mail: Misha.Haywood@obspm.fr
² School of Physics, Korea Institute for Advanced Study, 85 Hoegiro, Dongdaemun-gu, 02455 Seoul, Republic of Korea
³ National Optical Astronomy Observatorie/AURA, Tucson, AZ 85719, USA

Received: 29 April 2016
Accepted: 8 June 2016

Abstract

The determination of the age of the bulge has led to two contradictory results. On the one side, the color–magnitude diagrams in different bulge fields seem to indicate a uniformly old (>10 Gyr) population. On the other side, individual ages derived from dwarfs observed through microlensing events seem to indicate a wide spread, from ~2 to ~13 Gyr. Because the bulge is now recognised as being mainly a boxy peanut-shaped bar, it is suggested that disk stars are one of its main constituents, and therefore also stars with ages significantly younger than 10 Gyr. Other arguments also point out that the bulge cannot be exclusively old, and in particular cannot be a burst population, as is usually expected if the bulge were the fossil remnant of a merger phase in the early Galaxy. In the present study, we show that given the range of metallicities observed in the bulge, a uniformly old population would be reflected in a significant spread in color at the turn-off, which is not observed. We demonstrate that the correlation between age and metallicity expected to hold for the inner disk would instead conspire to form a color–magnitude diagram with a remarkably narrow spread in color, thus mimicking the color–magnitude diagram of a uniformly old population. If stars younger than 10 Gyr are part of the bulge, as must be the case if the bulge has been mainly formed through dynamical instabilities in the disk, then a very narrow spread at the turn-off is expected, as seen in the observations.