2018/06/30 by J. M. Diederik Kruijssen, Joel L. Pfeffer, Marta Reina-Campos +2 · 1 citation
Physics and Astronomy · #astro-ph.GA
paper · pdf · doi:10.1093/mnras/sty1609
24 pages (including appendices), 6 figures, 5 tables; accepted by MNRAS (June 12, 2018), originally submitted on March 25, 2018; Figures 4 and 6 show the main results of the paper. Updated with published version, clarifying the intended historical use of "Canis Major" as referring to a group of GCs rather than a galaxy
arxiv created 2018/09/04 · arxiv updated 2018/09/05
We use the age-metallicity distribution of 96 Galactic globular clusters (GCs) to infer the formation and assembly history of the Milky Way (MW), culminating in the reconstruction of its merger tree. Based on a quantitative comparison of the Galactic GC population to the 25 cosmological zoom-in simulations of MW-mass galaxies in the E-MOSAICS project, which self-consistently model the formation and evolution of GC populations in a cosmological context, we find that the MW assembled quickly for its mass, reaching \25,50\% of its present-day halo mass already at z=\3,1.5\ and half of its present-day stellar mass at z=1.2. We reconstruct the MW's merger tree from its GC age-metallicity distribution, inferring the number of mergers as a function of mass ratio and redshift. These statistics place the MW's assembly rate among the 72th-94th percentile of the E-MOSAICS galaxies, whereas its integrated properties (e.g. number of mergers, halo concentration) match the median of the simulations. We conclude that the MW has experienced no major mergers (mass ratios >1:4) since z∼4, sharpening previous limits of z∼2. We identify three massive satellite progenitors and constrain their mass growth and enrichment histories. Two are proposed to correspond to Sagittarius (few 108~\rm M_\odot) and the GCs formerly associated with Canis Major (∼109~\rm M_\odot). The third satellite has no known associated relic and was likely accreted between z=0.6-1.3. We name this enigmatic galaxy Kraken and propose that it is the most massive satellite (M_*∼2×109~\rm M_\odot) ever accreted by the MW. We predict that ∼40% of the Galactic GCs formed ex-situ (in galaxies with masses M_*=2×107-2×109~\rm M_\odot), with 6±1 being former nuclear clusters.