AGC198606: A gas-bearing dark matter minihalo?

We present neutral hydrogen (H' i) imaging observations with the Westerbork Synthesis Radio Telescope of AGC198606, an H' i cloud discovered in the ALFALFA 21cm survey. This object is of particular note as it is located 16 km s^-1 and 1. 2 from the gas-bearing ultra-faint dwarf galaxy Leo T while having a similar H' i linewidth and approximately twice the flux density. The H' i imaging observations reveal a smooth, undisturbed H' i morphology with a full extent of 23′ × 16′ at the 5 × 10¹⁸ atoms' cm^-2 level. The velocity field of AGC198606 shows ordered motion with a gradient of ∼25 km s^-1 across ∼20′. The global velocity dispersion is 9.3 km s^-1 with no evidence for a narrow spectral component. No optical counterpart to AGC198606 is detected. The distance to AGC198606 is unknown, and we consider several different scenarios: physical association with Leo T, a minihalo at a distance of ∼150 kpc based on the models of Faerman et al. (2013, ApJ, 777, 119), and a cloud in the Galactic halo. At a distance of 420 kpc, AGC198606 would have an H' i mass of 6.2 × 10⁵ M_⊙, an H' i radius of 1.4 kpc, and a dynamical mass within the H' i extent of 1.5 × 10⁸ M_⊙.