Kavli Affiliate: Alexander P. Ji
| First 5 Authors: Alexander P. Ji, Rohan P. Naidu, Kaley Brauer, Yuan-Sen Ting, Joshua D. Simon
| Summary:
We present the first high-resolution chemical abundances of seven stars in
the recently discovered high-energy dwarf galaxy stream Typhon. Typhon stars
have apocenters reaching to ${gtrsim}100$ kpc, making this the first detailed
chemical picture of the Milky Way’s very distant stellar halo. Typhon’s
chemical abundances are more like a dwarf galaxy than a globular cluster,
showing a metallicity dispersion and no presence of multiple stellar
populations. We find that Typhon stars display enhanced $alpha$-element
abundances and increasing $r$-process abundances with increasing metallicity.
The high-$alpha$ abundances suggest a short star formation duration for
Typhon, but this is at odds with expectations for the distant Milky Way halo
and the presence of delayed $r$-process enrichment. If the progenitor of Typhon
is indeed a new dwarf galaxy, possible scenarios explaining this apparent
contradiction include a dynamical interaction within the Milky Way that
increases Typhon’s orbital energy, a burst of enhanced late-time star formation
that raises [$alpha$/Fe], and/or group preprocessing by another dwarf galaxy
before infall into the Milky Way. An alternate explanation is that Typhon is
the high-energy tail of a more massive disrupted dwarf galaxy that lost energy
through dynamical friction. We cannot clearly identify a known low-energy
progenitor of Typhon, but cosmological simulations give about 2:1 odds that
Typhon is a high-energy tail of a massive galaxy instead of a new disrupted
galaxy. Typhon’s surprising combination of kinematics and chemistry thus
underscores the need to fully characterize the dynamical history and detailed
abundances of known substructures before identifying the origin of new
substructures.
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