We report the discovery of a young ( \tau \sim 130 ~ { } \textrm { Myr } ) , low-mass ( M \sim 1200 ~ { } \textrm { M } _ { \odot } ) , metal-poor ( [ \textrm { Fe } / \textrm { H } ] \sim - 1.1 ) stellar association at a heliocentric distance D \approx 29 ~ { } \textrm { kpc } , placing it far into the Milky Way halo .
At its present Galactocentric position ( R,z ) \sim ( 23 , 15 ) ~ { } \textrm { kpc } , the association is ( on the sky ) near the leading arm of the gas stream emanating from the Magellanic cloud system , but is located \approx 60 ^ { \circ } from the Large Magellanic Cloud ( LMC ) center on the other side of the Milky Way disk .
If we assume that the cluster is co-located with H I gas in the stream , we directly measure the distance to the leading arm of the Magellanic stream .
The measured distance is inconsistent with stream predictions from models of the LMC/SMC interaction and infall into the Milky Way that do not account for ram pressure and gas interaction with Milky Way disk .
The estimated age of the cluster is consistent with the time of last passage of the leading arm gas through the Galactic midplane , and we therefore speculate that this star-formation event was triggered by its the last disk midplane passage .
Most details of this idea remain a puzzle : the Magellanic stream has low column density , the Milky Way disk at this large radius has low gas density , and the relative velocity of the leading arm gas and Milky Way gas is large .
However it formed , the discovery of a young stellar cluster in the Milky Way halo presents an interesting opportunity for study .
This cluster was discovered with Gaia astrometry and photometry alone , but folow-up DECam photometry was crucial for measuring its properties .