* Astronomy

Title: Is GBT 1355+5439 a dark galaxy?
Authors: T.A. Oosterloo (1,2), G.H. Heald (2), W.J.G. de Blok (1,3) ((1) Netherlands Institute for Radio Astronomy (ASTRON), Dwingeloo, The Netherlands, (2) Kapteyn Astronomical Institute, University of Groningen, The Netherlands, (3) Astrophysics, Cosmology and Gravity Centre (ACGC), Department of Astronomy, University of Cape Town, South Africa)

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 solar masses, its size about 1 kpc, and the dynamical mass M_dyn > 5x10^7 solar masses. 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.

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