Age, Chemistry, and Kinematics of the Inner Galaxy Revealed by MUSE

TL;DR

This study leverages MUSE with adaptive optics to obtain precise ages, metallicities, and line-of-sight velocities for 98 MSTO-SGB stars within $R_{ m gc}<3.5$ kpc in Baade's Window, revealing multiple star-formation episodes with age peaks at $3.1$, $4.8$, $7.6$, and $10.8$ Gyr and a metallicity range spanning $-1.2<[Fe/H]<+0.6$. The data show both $oldsymbol{[ ext{alpha/Fe}]}$-rich and $oldsymbol{[ ext{alpha/Fe}]}$-poor sequences and a metal-rich dominant population, with a complex metallicity structure reproduced by multi-component Gaussian fits and comparative bulge studies. Kinematics show median $V_{ m LOS}$ near zero and sizeable dispersions ($oldsymbol{\sigma_{V_{ m LOS}}} obreak ext{~}80$–$120$ km s$^{-1}$), though trends with age or metallicity remain inconclusive due to sample size and projection effects; future proper-motion data will enable full 3D kinematics. The results underscore the viability of IFUs to probe crowded inner-Galaxy regions, constraining the chemodynamical evolution and indicating faster central enrichment and radial mixing beyond simple extrapolations of disk-based GCE models.

Abstract

The bar/bulge and inner disk are fundamental building blocks of the Milky Way, containing a large fraction of its stellar mass. However, stars in these regions are faint, crowded, and have high extinction, which makes studying their formation and evolution challenging. Using the integral-field spectrograph MUSE with adaptive-optics on the Very Large Telescope, we overcome these limitations and measure accurate ages, chemical abundances, and line-of-sight velocities for 98 main-sequence turn-off and subgiant branch stars with $R_{gc}<3.5$ kpc in Baade's Window. We find that 17% stars have ages younger than 5 Gyr, and the age distribution reveals multiple peaks at 3.1, 4.8, 7.6, and 10.8 Gyr, indicating that star formation in the inner Galaxy occurred in multiple episodes. These stars are predominantly metal-rich but span a broad metallicity range ($-1.2<$[Fe/H]$<+0.6$). The [$α$/Fe]-[Fe/H] distribution shows both $α$-rich and $α$-poor sequences, with most stars being metal-rich and low-[$α$/Fe]. Our results demonstrate that IFUs enable reliable measurements of stellar parameters even in the most crowded regions of the Milky Way, opening a new pathway to study the chemodynamical evolution of the inner Galaxy.

Figures (11)

• Figure 1: Overview of our MUSE observation towards Baade's window. Left: Pseudo-$gri$ color image of our MUSE field of view. The cyan and orange circles indicate the 98 MSTO–SGB stars and 77 giant stars with Galactocentric distance $R_{\mathrm{gc}}{}<3.5$ kpc, respectively. The circle sizes are related to MUSE spectral signal-to-noise ratio ($\rm{S/N}$). Right top: Absolute $V$-band magnitude ($M_V$) vs. $T_{\mathrm{eff}}$ for all 236 stars in the MUSE field after the quality cutoffs in Section \ref{['section:data_methods_cut_off']}, color-coded by $\rm{S/N}$. Right bottom: Kiel diagram of the same sample, color-coded by $R_{\mathrm{gc}}$ (in kpc). The cyan box shows the selection region for MSTO-SGB stars, and the orange line indicates the selection boundary for giants.
• Figure 1: Kiel diagram of 236 stars from our MUSE observation towards Baade's window. For each panel, the stars are color-coded by $V_{\rm LOS}$, $E(B-V)$, $[\mathrm{Fe/H}]$, and $[\mathrm{\alpha/Fe}]$, respectively.
• Figure 1: Line-of-sight velocity ($V_{\rm LOS}$) and velocity dispersion ($\sigma_{V_{\rm LOS}}$) of stars as a function of $[\mathrm{Fe/H}]$ in the inner Galaxy. Black solid lines show the results for 197 stars observed with MUSE at $R_{\mathrm{gc}}{}<3.5$ kpc (same as in Fig. \ref{['fig:vlos_sigma_feh_age']}). Blue dashed lines represent the Galaxia predictions for all stars ($>$400,000) in the same $(l, b)$ region as the MUSE observations, while grey dashed lines show the results for a subsample of 197 Galaxia stars randomly selected using the selection function of our MUSE sample. Error bars are estimated following the same procedure as in Fig. \ref{['fig:vlos_sigma_feh_age']}.
• Figure 2: Age distribution of the 98 MSTO-SGB stars in the inner Galaxy. The solid grey line shows the age histogram, and the black lines indicate the Gaussian mixture model (GMM) best-fit with peaks located at $3.1$, $4.8$, $7.6$ and $10.8$ Gyr. The shaded regions represent the uncertainties of these four peaks. We also show the four age peaks identified by B17 in blue for comparison.
• Figure 2: Distance-age relation for MSTO-SGB stars in our MUSE observation. The stars are color-coded by $[\mathrm{Fe/H}]$. The x- and y-axis error bars show the measurement error of age and distance, respectively.