The Average Age Map of the Galactic Bulge

Abstract

The Galactic Bulge, as the center of the Galaxy, is the closest laboratory for studying galaxy formation and evolution. However, its study faces significant challenges due to heavy dust extinction. This paper is devoted to deriving the average age of the Galactic Bulge and investigating its spatial distribution. We utilize a high-precision PSF-fitting photometric catalogue in the $J$ and $K_{\mathrm{s}}$ bands observed by VISTA to study the average stellar ages within the Bulge. Red giant stars are employed as tracers, with their average distances determined using red clump stars as references. The average ages are fitted with stellar models. Our analysis reveals a systematic age gradient across the Galactic Bulge ($2^{\circ} < |b| < 8^{\circ}$). The mean stellar age increases significantly with galactic latitude, shifting from a younger population ($\sim 4.69^{+0.97}_{-0.81}$ Gyr) prevalent near the plane to a predominantly older population ($\sim 10.48^{+0.93}_{-0.85}$ Gyr) at higher latitudes. We hypothesize that the young stellar population at low latitudes is predominantly composed of a pseudo-bulge formed via disk/bar processes (incorporating contributions from recent star-forming activity in the Galactic center), whereas the older stellar population is associated with spheroidal bulges generated through early-stage collapse or accretion of debris from merged dwarf galaxies.

Figures (16)

• Figure 1: Schematic diagram of the Bulge region in the VVV survey, with numbers indicating the tile IDs.
• Figure 2: (a) Reddening map in the Bulge area from simion_parametric_2017, sampled at 1' $\times$ 1'; (b) Reddening error map.
• Figure 3: The CMD of tile b393. Left panel: all the sources in b393. Middle panel: the sources remaining after discarding the foreground stars. Right panel: the remaining sources after removing the foreground stars and those with poor photometric quality. $J'$ and $K_{\mathrm{s}}'$ are the apparent magnitudes after extinction correction. The red box contains samples used for calculating the average distance, and the black box covers the samples used for fitting the average age.
• Figure 4: (a) Apparent magnitude ($K_{\rm s}$ band) distribution of the RC stars in b393 tile ($l \sim 5.35^\circ$, $b \sim 4.49^\circ$); (b) An example of the bimodal distribution of the RC stars, b236 tile ($l \sim 1.05^\circ$, $b \sim -7.47^\circ$).
• Figure 5: The CMD for b393 tile. Similar to the right panel of Figure \ref{['fig:3.1']} but with absolute magnitude as the ordinate. The black box outlines the sample used for age fitting. The metallicities and logAge of the isochrones, shown in different colors, are provided in the legend. To better illustrate, we select only two sets of isochrones with different [M/H]. The isochrone for [M/H] = -1.4 is represented by the dashed line, and [M/H] = 0.4 is depicted by the solid line. The same age share same colors.