Is GBT 1355+5439 a dark galaxy?
  We present HI imaging of GBT 1355+5439 performed with the Westerbork
Synthesis Radio Telescope. This is a dark HI object recently discovered close
to the nearby galaxy M101. We find GBT 1355+5439 to be an HI cloud 5x3 arcmin
in size. The total HI image and the kinematics show that the cloud consists of
condensations that have small (~10 km/s) motions with respect to each other.
The column densities of the HI are low; the observed peak value is 7.1x10^{19}
cm^{-2}. The velocity field shows a mild velocity gradient over the body of GBT
1355+5439, possibly due to rotation, but it may also indicate large-scale
radial motions. Although our data are limited in sensitivity, at all positions
the HI velocity dispersion is higher than 5 km/s and no narrow, cold, HI
component is seen. Because its distance is not known, we considered various
possibilities for the nature of GBT 1355+5439. Both the scenarios that it is a
tidal remnant near M101 and that it is a dark dwarf companion of M101 meet
difficulties. Neither do the data fit the properties of known compact
High-Velocity Clouds in the Galactic halo exactly, but we cannot entirely
exclude this option and deeper observations are required. We also considered
the possibility that GBT 1355+5439 is a gas-rich dark minihalo in the outer
regions of the Local Group. Interestingly, it would then have similar
properties as the clouds of a proposed Local Group population recently found in
the ALFALFA survey. In this case, the HI mass of GBT 1355+5439 would be about a
few times 10^5 Msol, its size about 1 kpc, and the dynamical mass M_dyn >
5x10^7 Msol. However, if GBT 1355+5439 is a dark Local Group object, the
internal kinematics of the HI appears to be different from that of
gas-dominated, almost dark galaxies of similar size.
