Proof that the Milky Way experienced a significant merger only 1.5 billion years after the Big Bang

Abstract

The merger history of the Galaxy has been traced back firmly to redshift 2 (10 Billion years ago). While there have been claims of the existence of at least one more significant merger before this time, supporting evidence has been indirect and contentious. Here we show that the population of globular clusters around the Galaxy depicts three distinct age-metallicity sequences, one associated with the merger with Gaia-Enceladus 10 billion years ago, one to the progenitor of the Milky Way and a third intermediate sequence associated to at least one merger which we estimate took place merely 1.5 billion years after the Big Bang. This discovery has been possible thanks to exquisite Hubble Space Telescope data and sophisticated analysis that enables very precise relative age determination of globular clusters. The newly identified sequence reveals that this merger took place with an object of stellar mass similar to that of Gaia-Enceladus (~5x10$^8$ M$_{\odot}$), and which deposited most of its mass in the inner 6 kpc of the Milky Way. The unambiguous identification of a third merger event in the inner Galaxy puts to rest earlier debates, and honoring previous work we name the progenitor system Low-energy-Kraken-Heracles, or LKH for short.

Figures (8)

• Figure 1: Chrono-dynamical properties of our sample of 39 GCs.Left panel: distribution in the E$_J$ vs J$_Z$ dynamical space of the 39 GCs with homogeneous age estimate (filled symbols, see the Methods Section). GCs are color-coded according to their association (with membership probability $>50$%) with GSE (cyan symbols), the MW (black symbols) or the LKH (red symbols), as determined by our Bayesian chrono/dynamical analysis. Uncertainties are derived as the 16th- and the 84th- percentile of each parameter distribution computed over 200 orbits. Middle panel: same in the E$_J$ vs circularity space. Right panel: distribution in the age-metallicity plane of the three populations of GCs. The relative age scale along the y-axis is defined by setting the age resulting from the isochrone fit of the oldest GC (t$_{iso}=14.48$ Gyr) as zero.
• Figure 2: AMR models fit.Left-hand panel: fit to the GSE GCs. Middle panel: fit to LKH GCs. Right-hand panel: fit to the in-situ MW GCs. The individual GC members of the three samples are selected as described in the Methods Section. The red line corresponds to the fit with the Maximum A Posteriori (MAP) value, while the blue line represents the median fit.
• Figure 3: AMR in simulated MW analogs. Median trend (solid lines) for the AMR of the main progenitor (black), a GSE-like merger (cyan) and a LKH-like event (red) in three MW analogs of the Auriga Level-4 simulations.
• Figure 4: Results of the isochrone fit for NGC 4372. Top left panel: best fit model in the ($\it{m_{\mathrm{F606W}}}$, $\it{m_{\mathrm{F606W}}} - \it{m_{\mathrm{F814W}}}$) CMD. Top right panel: best fit model in the ($\it{m_{\mathrm{F814W}}}$, $\it{m_{\mathrm{F606W}}} - \it{m_{\mathrm{F814W}}}$) CMD. Bottom left panel: posterior distributions for the output parameters and the best-fit solution, quoted in the labels, in the ($\it{m_{\mathrm{F606W}}}$, $\it{m_{\mathrm{F606W}}} - \it{m_{\mathrm{F814W}}}$) CMD. Bottom right panel: posterior distributions for the output parameters and the best-fit solution, quoted in the labels, in the ($\it{m_{\mathrm{F814W}}}$, $\it{m_{\mathrm{F606W}}} - \it{m_{\mathrm{F814W}}}$) CMD.
• Figure 5: Comparison with the literature. Differences between the output of our isochrone fit and literature values for distance modulus, color-excess and global metallicity. The mean difference is shown with a solid grey line, whereas the 1$\sigma$ dispersion around the mean is indicated with dashed grey lines.