Galaxy archaeology for wet mergers: Globular cluster age distributions in the Milky Way and nearby galaxies
arxiv(2023)
摘要
Identifying past wet merger activity in galaxies has been a longstanding
issue in extragalactic formation history studies. Gaia's 6D kinematic
measurements in our Milky Way (MW) have vastly extended the possibilities for
Galactic archaeology, leading to the discovery of early mergers in the MW's
past. As recent work has established a link between young globular clusters
(GCs) and wet galaxy merger events, the MW provides an ideal laboratory for
testing how GCs can be used to trace galaxy formation histories. To test the
hypothesis that GCs trace wet mergers, we relate the measured GC age
distributions of the MW and three nearby galaxies to their merger histories and
interpret the connection with wet mergers through an empirical model for GC
formation. For the MW, we cross-match the GCs with their associated progenitor
host galaxies to disentangle the connection to the GC age distribution. We find
that the MW GC age distribution is bimodal, mainly caused by younger GCs
associated with Gaia-Sausage/Enceladus (GSE) and in part by unassociated
high-energy GCs. The GSE GC age distribution also appears to be bimodal. We
propose that the older GSE GCs were accreted together with GSE, while the
younger ones formed through the merger. For the nearby galaxies, we find that
peaks in the GC age distributions coincide with early gas-rich mergers. Even
small signatures in the GC age distributions agree well with the formation
histories of the galaxies inferred through other observed tracers. From the
models, we predict that the involved cold gas mass can be estimated from the
number of GCs found in the formation burst. Multimodal GC age distributions can
trace massive wet mergers as a result of GCs being formed through them. From
the laboratory of our own MW and nearby galaxies we conclude that the ages of
younger GC populations of galaxies can be used to infer the wet merger history
of a galaxy.
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