AGC198606: A gas-bearing dark matter minihalo?
  We present neutral hydrogen (HI) imaging observations with the Westerbork
Synthesis Radio Telescope of AGC198606, an HI cloud discovered in the ALFALFA
21cm survey. This object is of particular note as it is located 16 km/s and 1.2
degrees from the gas-bearing ultra-faint dwarf galaxy Leo T while having a
similar HI linewidth and approximately twice the flux density. The HI imaging
observations reveal a smooth, undisturbed HI morphology with a full extent of
23'x16' at the 5x10^18 atoms cm^-2 level. The velocity field of AGC198606 shows
ordered motion with a gradient of ~25 km/s across ~20'. The global velocity
dispersion is 9.3 km/s 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), and a cloud in the Galactic halo. At a distance of 420 kpc,
AGC198606 would have an HI mass of 6.2x10^5 Msun, an HI radius of 1.4 kpc, and
a dynamical mass within the HI extent of 1.5x10^8 Msun.
