Age and metallicity distribution of the Galactic bulge from extensive optical and near-IR stellar photometry

Abstract

Abstract. We present a new determination of the metallicity distribution, age, and luminosity function of the Galactic bulge stellar population. By combining near-IR data from the 2MASS survey, from the SOFI imager at ESO NTT and the NICMOS camera on board HST we were able to construct color-magnitude diagrams (CMD) and luminosity functions (LF) with large statistics and small photometric errors from the Asymptotic Giant Branch (AGB) and Red Giant Branch (RGB) tip down to ∼0.15 M . This is the most extended and complete LF so far obtained for the galactic bulge. Similar near-IR data for a disk control field were used to decontaminate the bulge CMDs from foreground disk stars, and hence to set a stronger constraint on the bulge age, which we found to be as large as that of Galactic globular clusters, or > ∼ 10 Gyr. No trace is found for any younger stellar population. Synthetic CMDs have been constructed to simulate the effect of photometric errors, blending, differential reddening, metallicity dispersion and depth effect in the comparison with the observational data. By combining the near-IR data with optical ones, from the Wide Field Imager at the ESO/MPG 2.2 m telescope, a disk-decontaminated (M K ,V-K) CMD has been constructed and used to derive the bulge metallicity distribution, by comparison with empirical RGB templates. The bulge metallicity is found to peak at near solar value, with a sharp cutoff just above solar, and a tail towards lower metallicity that does not appreciably extend below [M/H] ∼ −1.5.

Building blocks of the Milky Way's accreted spheroid

“…As pointed out by S13, if viewed as one component, the integrated or average values for the spheroids can best be compared with Zoccali et al 2003;Bensby et al 2013). S13 found that the bulge/disc ratios of the simulated galaxies in all Aquarius haloes except E and F are close to the estimated value of 0.2−0.3 (Bissantz et al 2004) for the Milky Way.…”

Separation of stellar populations by an evolving bar: implications for the bulge of the Milky Way

“…In the Milky Way, this fraction would be even lower, since observations based on turnoff studies show that such stars contribute < 5% of bulge stars (e.g. Ortolani et al 1995;Kuijken & Rich 2002;Zoccali et al 2003;Sahu et al 2006;Clarkson et al 2008Clarkson et al , 2011Brown et al 2010;Valenti et al 2013). The crucial point is that an age difference of just 2 Gyr is sufficient for the bulge red clump distributions to change from a single peak to a bimodal distribution.…”

“…Various lines of evidence favouring the existence of such a component in the Galactic bulge have been put forth. Photometric studies of resolved stellar populations have generally found uniformly old stars across the bulge, typically older than 10 Gyr (Ortolani et al 1995;Kuijken & Rich 2002;Zoccali et al 2003;Ferreras et al 2003;Sahu et al 2006;Clarkson et al 2008Clarkson et al , 2011Brown et al 2010;Valenti et al 2013;Calamida et al 2014). For instance, Clarkson et al (2011) estimate that any component younger than 5 Gyr must account for less than 3.4% of the bulge.…”

The Galactic Bulge

“…The Galactic bulge is old, with no evidence for a younger stellar population, as can be verified in the Colour-Magnitude Diagrams by Zoccali et al (2003), Clarkson et al (2008), Clarkson et al (2011), andGennaro et al (2015). Whereas Bensby et al (2013) claims from his sample of microlensed dwarfs towards the bulge, that 22% of bulge stars can reach ages as young as 5 Gyr, this is not confirmed in the CMDs.…”