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The Milky Way's 11.8 billion year merger was dated; titanium-44 raised supernova yields

Hubble and Gaia show the dwarf galaxy LKH merged with the young Milky Way 11.8 billion years ago; an Argonne measurement raised titanium-44 production by up to 35 per cent, updating supernova models.

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A dense white-gold globular cluster hangs below an amber Milky Way band crossing black space, with two smaller distant clusters and a faint filamentary wisp.

Hubble dated the Milky Way's first big merger to 11.8 billion years ago

Combining NASA's Hubble Space Telescope with measurements from ESA's Gaia mission, a team derived the age and metal content of 39 globular clusters inside the Milky Way's inner 20,000 light-years. The clusters trace back to a dwarf galaxy called LKH, carrying roughly 500 million solar masses in stars, that merged with the young Milky Way 11.8 billion years ago, about 2 billion years after the Big Bang. The peer-reviewed study in Nature Astronomy pushes the known merger history back by 1.8 billion years, placing the event roughly 1.8 billion years before the Gaia-Sausage-Enceladus merger. Early mergers are hard to spot: young galaxies were small and similar in size to their victims, and traces can fade over billions of years. In work led by Davide Massari, Hubble also imaged clusters never studied before. Earlier studies had disputed whether such an early merger occurred; the new data separate three distinct age-metal sequences and clarify LKH's presence.[1]

The LKH merger carried a significant fraction of the young Milky Way's mass

LKH, short for Low-energy-Kraken-Heracles, was a dwarf galaxy with roughly 500 million solar masses in stars, a significant fraction of the young Milky Way's mass at the time. Most of its clusters settled within about 6,000 light-years of the galactic centre, showing that much of the material landed in the inner Galaxy. The authors argue that the Milky Way's earliest phases were shaped not only by stars born in situ but also by stars accreted from external galaxies. The team plans to keep mapping every major merger by treating globular clusters as cosmic archaeological sites.[1]

A titanium-44 measurement raised supernova production estimates by up to 35 per cent

A measurement led by the University of Surrey team appeared in Physical Review Letters. The experiment at Argonne National Laboratory provided the first data showing that the 45V(p,gamma)46Cr reaction controlling titanium-44 production in core-collapse supernovae proceeds more slowly than predicted, raising production estimates by up to 35 per cent. Titanium-44 is produced in the layers closest to the centre of the explosion, which makes it one of the few observational handles that test supernova models directly: its abundance, measured by gamma-ray telescopes, is compared against calculated production. A second measurement at the Facility for Rare Isotope Beams in Michigan narrowed uncertainty on the nickel-copper cycle in type I X-ray bursts by more than tenfold, while confirming the trapped fraction stays small. Both results replace values that entered stellar models by estimate with reaction rates measured in the laboratory.[2]

References

  1. News sourceNASAThe Milky Way's first big meal is dated to 11.8 billion years ago
    Primary paperNature AstronomyEvidence of a massive accretion event 1.8 billion years before the Gaia-Sausage-Enceladus merger
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  2. News sourcePhys.orgTitanium-44 production in supernovae runs slower than predicted, raising yields by up to 35 per cent
    Primary paperPhysical Review LettersIdentification of Resonant States in the 45V(p,gamma)46Cr Reaction and Their Influence on the Production of Ti-44 in Core-Collapse Supernovae
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