Hiding its age: the case for a younger bulge

TL;DR

This paper argues that the apparent uniform old age of the galactic bulge is a result of observational biases, and that the bulge likely contains a significant population of stars younger than 10 Gyr, consistent with formation through disk instabilities.

Contribution

It demonstrates that the observed narrow color spread at the bulge's turn-off can be explained by age-metallicity correlations, challenging the view of a uniformly old bulge.

Findings

A uniformly old population would show a wide color spread, which is not observed.

The age-metallicity correlation in the inner disk can produce a narrow color spread.

The bulge likely contains stars younger than 10 Gyr, supporting a formation via disk instabilities.

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 large spread, from $\sim$ 2 to $\sim$ 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 as well point to the fact that the bulge cannot be exclusively old, and in particular cannot be a burst population, as it is usually expected if the bulge was 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 into a significant spread in color at the turn-off which is not observed. Inversely, we demonstrate that the correlation between age and metallicity expected to hold for the inner disk would conspire to form a color-magnitude diagram with a remarkably small 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 small spread at the turn-off is expected, as seen in the observations.