Chronology of our Galaxy from Gaia colour-magnitude diagram fitting (ChronoGal): IV. On the inner Milky Way stellar age distribution

TL;DR

This study uses Gaia-based CMD fitting (CMDft.Gaia) within a solar-neighborhood volume to infer the inner Milky Way's stellar age distribution by exploiting metal-rich stars presumed to have formed near the center and migrated outward. The analysis uncovers a discrete set of star-formation episodes at roughly 13.5, 10, 7, 4, 2, and <1 Gyr ago, with metal-rich populations decreasing with height above the plane. Comparisons with Auriga Superstars simulations and Gaia DR3 GSP-Spec data support a scenario where bar dynamics and satellite interactions drive global, episodic star formation and radial migration that populates the solar vicinity. The work positions metal-rich solar-neighborhood stars as tracers of the inner Galaxy's past, offering a framework to connect extinction-corrected Gaia CMDs with the Galaxy's formation history and guiding future high-resolution age–metallicity mapping. ChronoGal thus provides a powerful approach to decoding the Milky Way's central evolution through precise, discretized stellar ages and metallicities.

Abstract

The Milky Way's inner region is dominated by a stellar bar and a boxy-peanut shaped bulge. However, which stellar populations inhabit the inner Galaxy or how star formation proceeded there is still unknown. The difficulty in studying these stars stems from their location in dense regions that are strongly impacted by extinction and crowding effects. In this work, we use star formation histories computed in the solar neighbourhood using Gaia Colour-Magnitude Diagram fitting to shed light onto the evolution of the central regions of our Galaxy. For that, we have obtained precise age distributions for the non-negligible amount of super metal-rich stars ([M/H] $\sim$ 0.5) in the solar neighbourhood (more than 5$\%$ of the total stars within 400 pc of the plane). Assuming that these stars were born in the inner Galaxy and migrated outwards, those distributions should be indicative of the true stellar age distribution in the inner Galaxy. Surprisingly, we find that these age distributions are not continuous but show clear signs of episodic star formation ($\sim$~13.5, 10.0, 7.0, 4.0, 2.0 and less than 1~Gyr ago). Interestingly, with the exception of the 4~Gyr event, the timings of the detected events coincide with the formation of the primitive Milky Way and with known merging events or satellite encounters (Gaia-Enceladus-Sausage, Sagittarius dwarf galaxy, and the Magellanic Clouds), suggesting that these could have induced enhanced and global star-forming episodes. These results are compatible with a scenario in which Gaia-Enceladus-Sausage is responsible for the formation of the bar 10 Gyr ago. However, we cannot associate any accretion counterpart with the 4-Gyr-ago event, leaving room for a late formation of the bar, as previously proposed. A qualitative comparison with the Auriga Superstars simulations suggesting a possible link to bar dynamics and satellite accretion. [Abridged]

Figures (6)

• Figure 1: Stellar density distribution in the age-metallicity plane for volumes 1, 5, and 8 (i.e. from the plane to 50 pc, 0.2 to 0.3 kpc, and 0.5 to 0.6 kpc, respectively above and below the plane) as representative examples of the reported super metal-rich populations (for these we use the 2022AA...661A.147L dust map). Coloured polygons delimit the areas in the age-metallicity plane used to quantify the z-profiles in Fig. \ref{['fig:prof']}. Note that a logarithmic scale has been used to represent the number of stars, in order to enhance these relatively low intensity features.
• Figure 2: z-profile of number density of stars for the five different events of super metal-rich star formation. Such events are defined using the polygons depicted in Fig. \ref{['fig:SFH_blobs']}. We show the profiles using two different extinction maps, Bayestar map Green2019, and 2022AA...661A.147L map (solid lines, L22). Given incompleteness affecting the observed samples together with quality cuts, absolute values for this number density should be taken with caution. A normalisation has been applied to the Bayestar densities to account for the missing quadrant in the Bayestar coverage Green2019. Colours follow Fig. \ref{['fig:SFH_blobs']}.
• Figure 3: Stellar age-metallicity distribution of stellar particles for a solar neighbourhood-like selection of stars from AuS18. The distribution of stars are colour-coded according to number density (left) and birth radius (right). Pericentric passages of subhalo 6281 are shown as pink squares.
• Figure 4: Kiel diagram of the disc super metal-rich population with [M/H]=0.4$\pm$0.1 dex. We divide the sample in two based on their value of v$_\phi$: slow (purple) and fast (orange) stars (see Fig. \ref{['fig:vphi']}). Solar-scaled BaSTI isochrones of 13.5, 10, 7, 4, 2, 1, and 0.6 Gyr (red, orange, pistacho, green, cyan, blue, and purple, respectively) are overlaid on the data. Left: Whole diagram. Right: Focusing only on the turn-off region.
• Figure 5: Orbital properties of the metal-rich stars detected in the high-quality Gaia DR3 GSP-Spec sub-catalogue from alejandra2024, compared with those of a subset of solar-metallicity stars. Left: Distribution of v$_\phi$ velocities for metal-rich (red, empty histogram) and solar-metallicity (grey histogram) stars. From the shape of the metal-rich stars histogram we divide the sample in slow (|v$_\phi$| below 205 km/s) and fast (|v$_\phi$| above 205 km/s) stars. Middle: Distribution of guiding radius alejandra2024. Right: Distribution of eccentricity alejandra2024. For these last panels, we divide the sample in solar metallicity (grey), slow, metal-rich stars (purple) and fast, metal-rich stars (orange).