The Smith Cloud is a gaseous high-velocity cloud ( HVC ) in an advanced state of accretion , only 2.9 kpc below the Galactic plane and due to impact the disk in \approx 27 Myr .
It is unique among HVCs in having a known distance ( 12.4 \pm 1.3 kpc ) and a well-constrained 3D velocity ( 296 km s ^ { -1 } ) , but its origin has long remained a mystery .
Here we present the first absorption-line measurements of its metallicity , using HST /COS UV spectra of three AGN lying behind the Cloud together with Green Bank Telescope 21 cm spectra of the same directions .
Using Voigt-profile fitting of the S ii \lambda \lambda 1250 , 1253 , 1259 triplet together with ionization corrections derived from photoionization modeling , we derive the sulfur abundance in each direction ; a weighted average of the three measurements gives [ S/H ] = - 0.28 \pm 0.14 , or 0.53 ^ { +0.21 } _ { -0.15 } solar metallicity .
The finding that the Smith Cloud is metal-enriched lends support to scenarios where it represents recycled Galactic material , rather than the remnant of a dwarf galaxy or accreting intergalactic gas .
The metallicity and trajectory of the Cloud are both indicative of an origin in the outer disk .
However , its large mass and prograde kinematics remain to be fully explained .
If the cloud has accreted cooling gas from the corona during its fountain trajectory , as predicted in recent theoretical work , its current mass would be higher than its launch mass , alleviating the mass concern .