Kavli Affiliate: Risa H. Wechsler
| First 5 Authors: Monica Valluri, Parker Fagrelius, Sergey. E. Koposov, Ting S. Li, Oleg Y. Gnedin
| Summary:
We present 115 new spectroscopically identified members of the GD-1 tidal
stream observed with the 5000-fiber Dark Energy Spectroscopic Instrument
(DESI). We confirm the existence of a “cocoon” which is a broad (FWHM~2.932
deg ~ 460 pc) and kinematically hot (velocity dispersion, sigma ~ 5-8 km/s)
component that surrounds a narrower (FWHM~ 0.353 deg ~ 55) and colder (sigma =
3.09+/-0.76 km/s) thin stream component (based on a median per star velocity
precision of 2.7 km/s). The cocoon extends over at least a 30 degree segment of
the stream observed by DESI. The thin and cocoon components have similar mean
values of [Fe/H]: -2.54+/- 0.04 dex and -2.47+/- 0.06 dex suggestive of a
common origin. The data are consistent with the following scenarios for the
origin of the cocoon. The progenitor of the GD-1 stream was an accreted
globular cluster (GC) and: (a) the cocoon was produced by pre-accretion tidal
stripping of the GC while it was still inside its parent dwarf galaxy; (b) the
cocoon is debris from the parent dwarf galaxy; (c) an initially thin GC tidal
stream was heated by impacts from dark subhalos in the Milky Way; (d) an
initially thin GC stream was heated by a massive Sagittarius dwarf galaxy; or a
combination of some these. Future DESI spectroscopy and detailed modeling may
enable us to distinguish between these possible origins.
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