The age distribution of the inner Galaxy is less well known. Traditionally, from photometry, the bulge has been viewed as an old structure (e.g. Zoccali et al. 2003) but this was thrown into question by spectroscopic ages of microlensed dwarfs (Bensby et al. 2013), many of which are young. Recent work by Bernard et al. (2018) constrained the age distribution of the bulge ($-5\, \mathrm{deg}\lesssim b\lt -2\, \mathrm{deg}$) from Hubble Space Telescope photometry of the main sequence turn-off stars, concluding that, although the bulge is predominantly old, approximately $10\, \mathrm{per\, cent}$ of stars are younger than $5\, \mathrm{Gyr}$. This fraction increases to $\sim 20\, \mathrm{per\, cent}$ for more metal-rich ($[\mathrm{Fe}/\mathrm{H}]\gtrsim 0.2\, \mathrm{dex}$) stars, consistent with the Bensby et al. (2013) work. Further evidence for a predominantly old ($\gtrsim 8\, \mathrm{Gyr}$) bulge comes (indirectly) from [C/N] measurements of giant stars (Bovy et al. 2019; Hasselquist et al. 2020), although, as highlighted by Hasselquist et al. (2020), age appears to correlate with both metallicity and Galactic height of the populations. Nogueras-Lara et al. (2020a) have used the luminosity of red clump stars to conclude the majority ($\sim 95\, \mathrm{per\, cent}$) of the nuclear stellar disc formed more than $8\, \mathrm{Gyr}$ ago with some evidence of a more recent ($\lt 1\, \mathrm{Gyr}$ ago) star formation burst (Matsunaga et al. 2011). This is consistent with ongoing/recent star formation within the central molecular zone (Morris & Serabyn 1996) and is broadly consistent with the conclusions of Bernard et al. (2018) on the wider bar/bulge but possibly suggesting the nuclear stellar disc is on average older than the surrounding bulge. We have fitted by-eye a very simple star formation history to the ‘cleanest’ combined fit from Bernard et al. (2018) of the form $\mathrm{sech}^2((13.5\, \mathrm{Gyr}-\tau)/4.7\, \mathrm{Gyr})$ with a truncation at $14\, \mathrm{Gyr}$. Combining the metallicity distributions and star formation histories with the PARSEC isochrones and adopting a Kroupa (2001) initial mass function, we have computed the luminosity function of the giant branch stars in the inner bulge region. We show the results in the lower panel of Fig. 4 along with a simple double Gaussian plus quadratic fit to represent the red clump stars, the red giant branch bump stars and the red giant branch stars respectively. We find that the lowest latitude bin has a red clump magnitude of $M_{K_s,\mathrm{RC}}=-1.61\, \mathrm{mag}$. This agrees well with the mean solar neighbourhood result from Chan & Bovy (2020) of $M_{K_s,\mathrm{RC}}=-1.622\, \mathrm{mag}$ and more specifically using their relations adopting the mean (J − Ks) = 0.647 (see Appendix B1) and mean metallicity $-0.18\, \mathrm{dex}$ gives $M_{K_s,\mathrm{RC}}=-1.595\, \mathrm{mag}$. The metallicity gradient with latitude produces a red clump magnitude gradient of $0.032\, \mathrm{mag}\, \mathrm{deg}^{-1}$ whilst using the change in mean metallicity in combination with the results of Chan & Bovy (2020) we would expect $0.024\, \mathrm{mag}\, \mathrm{deg}^{-1}$. At all latitudes the red clump distribution is well reproduced by a Gaussian with standard deviation $\sim 0.11\, \mathrm{mag}$. Chan & Bovy (2020) measured the solar neighbourhood red clump to have an intrinsic standard deviation of $0.097\, \mathrm{mag}$ which combined in quadrature with that arising from the metallicity variance predicts a standard deviation of $\sim 0.13\, \mathrm{mag}$, similar to the PARSEC models. The red clump peaks from the PARSEC isochrones have a slight bimodal structure arising from the bimodal metallicity distributions such that the mode typically peaks $\sim 0.03\, \mathrm{mag}$ fainter than the Gaussian mean.
Citation: Matsunaga et al. 2011
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