2015A&A...573L...3A

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 16km/s and 1.2deg 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 5x10¹⁸ atoms/cm2 level. The velocity field of AGC198606 shows ordered motion with a gradient of ∼25km/s across ∼20'. The global velocity dispersion is 9.3km/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 ∼150kpc based on the models of Faerman et al. (2013ApJ...777..119F), and a cloud in the Galactic halo. At a distance of 420kpc, AGC198606 would have an HI mass of 6.2x10⁵M_☉, an HI radius of 1.4kpc, and a dynamical mass within the hI extent of 1.5x10⁸M_☉.