Relics of High-redshift Compaction in our Backyard: The Most Metal-poor Stars in the Proto-Galaxy

TL;DR

This work combines a large, photometrically derived map of metal-poor stars in the inner Milky Way with the Au18 Auriga simulation to argue that the proto-Galaxy formed through multiple high-redshift gas compaction (blue-nugget) events followed by quenching. The authors show consistent spatial concentration, a persistent very metal-poor tail in the MDF, and a transition from dispersion-dominated to rotating kinematics that mirror Au18’s chemo-dynamical evolution. This compaction-driven scenario unifies prior interpretations of the proto-Galaxy and links local fossil records to early, high-redshift galaxy processes, providing a coherent framework for the MW’s first 1–2 Gyr of assembly. The results motivate further high-resolution spectroscopy and simulations to refine the role of in-situ versus accreted components and the triggers of early compaction.

Abstract

The earliest assembly of the Milky Way (MW) remains poorly understood, yet the spatial, chemical, and kinematic properties of its most metal-poor stars provide a unique fossil record of its proto-Galaxy phase. Understanding how this ancient component formed is essential for linking near-field Galactic archaeology to high-redshift galaxy evolution. We construct the currently largest 3-D map of inner-Galaxy metal-poor giants by combining several narrow/medium-band photometric surveys, reaching metallicities down to [Fe/H]$\sim-$3.5. Comparing observational data with Auriga 18 (Au18) from the Auriga cosmological simulations, we find that the proto-Galaxy population ([Fe/H]$\lesssim-$1.4) is highly centrally concentrated within the Galactocentric distance $r_{\rm gc}\lesssim$15 kpc, and forms a dispersion-supported structure with negligible rotation. The spatial and chemo-dynamical properties of observed proto-Galaxy population closely match those of the metal-poor stars in Au18. Considering Au18 as an analog of the MW, we propose a new scenario in which the formation of the proto-Galaxy is linked, for the first time, to episodes of high-z (z$\gtrsim$3) gas compaction, blue-nugget phases, and quenching processes. This framework provides a unified physical picture for the first $\sim$1-2 Gyr of the MW's evolution, bridging local fossil records with future studies of early star-forming galaxies.

Figures (8)

• Figure 1: Panel (a): The universal SFMS of star-forming Au18 as a function of $M_\star$ and redshift $z$. The black-solid line indicates the SFMS ridge, while the gray-shaded region denotes a $\pm$0.3 dex scatter of the SFMS Tacchella2016. The labeled look-back times $t_{\mathrm{lb1}}$, $t_{\mathrm{lb2}}$, $t_{\mathrm{lb3}}$, and $t_{\mathrm{lb4}}$, mark the approximate durations of three compaction events identified in the Au18 evolution, using the same colors as the associated redshifts. Panel (b): The edge-on stellar number-density map of stars formed during the earliest episode of compaction and quenching events, corresponding to ages between $t_{\mathrm{lb2}}$ and $t_{\mathrm{lb1}}$ at $z=0$. Panels (c) and (d): The same as panel (b), but for the second and third compaction episodes, respectively. The border colors of panels (b)-(d) match the colors of the relevant redshifts. Panel (e): The edge-on stellar number-density distribution at $z=0$ for all stars formed during the three compaction episodes shown in panel (a). This figure indicates the possible connection between proto-Galaxy formation and multiple high-$z$ ($z\gtrsim3$) compaction episodes.
• Figure 2: Spatial number-density distributions of the final observed sample and the Au18 star particles in three metallicity bins. The [Fe/H] values of Au18 star particles have been calibrated using the cumulative metallicity distribution function of the final sample (see Appendix \ref{['sec:appendix_Au18']} for details). The three rows of panels correspond to different metallicity bins: $-2.0 \le \mathrm{[Fe/H]} < -1.0$ (top), $-3.0 \le \mathrm{[Fe/H]} < -2.0$ (middle), and $-4.0 \le \mathrm{[Fe/H]} < -3.0$ (bottom). The first column shows the all-sky distribution in Galactic coordinates $(l, b)$, and the second column presents the distribution in Galactocentric Cartesian coordinates $(X, Z)$ for the final sample. The open circles and open star symbols in the second column indicate the positions of the Galactic center and the Sun, respectively. The third column shows the edge-on view of the Au18 star particles. In the second and third columns, both axes are divided into $500\times500$ bins. Note that the maximum values of color bars vary across rows: $10^4$ for the top row, $10^3$ for the middle row, and $10^2$ for the bottom row. The number-density contour levels are $[10^{2.0},10^{2.5},10^{3.0},10^{3.5}]$ for the top row, $[10^{1.5},10^{2.0},10^{2.5}]$ for the middle row, and $[10,10^{1.5}]$ for the bottom row. This figure illustrates that stars in the MW and Au18 exhibit similar three-dimensional spatial distributions across all metallicities of $-4.0\le$[Fe/H] $<-1.0$.
• Figure 3: MDFs of the observed MW and Au18 within $r_{\mathrm{gc}}<15$ kpc and the GMM fits of the observed MW. Different panels correspond to different $r_{\mathrm{gc}}$ bins, with the actual number of observed stars in each bin shown in the top-left corner. The gray and magenta histograms are the MDFs of the observed MW stars and Au18 star particles, respectively. For the observed MW, the density values shown in the vertical axis are corrected by the weights derived from the selection function. The histograms of Au18 are constructed from stars with calibrated [Fe/H] $<-0.6$ to exclude artificial structures, and a Gaussian noise of 0.4 dex is added to the [Fe/H] of each star particle. The optimal number of GMM components in each bin is determined by the BIC, which favors four components for all bins. Colored dashed curves represent the individual GMM components, and the black solid curve shows their sum. For each component, the corresponding weight, mean, and standard deviation are indicated as $(w,\mu,\sigma)$. The VMP component is present throughout the inner 15 kpc and is relatively prominent in the 1-3 kpc bin.
• Figure 4: Kinematic and age trends as functions of metallicity. Top panel: Galactocentric rotation velocity $V_{\phi}$ versus [Fe/H] for observational and simulated stars within $r_{\mathrm{gc}}<15$ kpc and $|Z|>1$ kpc. Blue curves represent observed stars in the final sample with available RVs, and orchid curves represent Au18 star particles at redshift $z=0$. Solid lines show the median $V_{\phi}$, with dark- and light-shaded regions indicating the 16$^{\rm th}$-84$^{\rm th}$ and 2.5$^{\rm th}$-97.5$^{\rm th}$ inter-percentile ranges, respectively. Middle panel: Rotational support, $V_{\phi}/\sigma$, where $V_{\phi}$ median is adopted and $\sigma$ is estimated as half the 16$^{\rm th}$-84$^{\rm th}$ inter-percentile range of $V_{\phi}$ in each [Fe/H] bin. The spatial selection is identical to the top panel. Bottom panel: Stellar age-metallicity relation for Au18 star particles in the same spatial region. The solid line indicates the median age, with shaded regions showing the 16$^{\rm th}$-84$^{\rm th}$ and 2.5$^{\rm th}$-97.5$^{\rm th}$ inter-percentile ranges. The right vertical axis gives the corresponding redshift of star formation. All panels include only Au18 stars with calibrated [Fe/H]$< -0.6$ to exclude artificial structures at higher metallicity. This figure demonstrates that Au18 exhibits a strong similarity to the MW in chemo-dynamical space.
• Figure 5: Stellar number-density distribution of the proto-Galaxy sample in the plane of orbital eccentricity $e$ versus angular momentum along the $Z$-axis, $L_Z$. The sample includes 116,319 stars from the final sample with $-3.5 < \mathrm{[Fe/H]} < -1.4$, $r_{\mathrm{gc}} < 15$ kpc, and $|Z| > 1$ kpc, selected to represent the proto-Galaxy population. The upper metallicity threshold is motivated by the spin-up metallicity inferred from Figure \ref{['fig:Vphi_FEH_sigma_age']}. Theblack-dashed box shows the kinematically in-situ region defined by Conroy2022 ($e < 0.8$ and $L_Z > 0$ kpc km s$^{-1}$), which contains 56,757 stars, 48.8% of the total proto-Galaxy sample shown in this figure.