Issue |
A&A
Volume 555, July 2013
|
|
---|---|---|
Article Number | L7 | |
Number of page(s) | 4 | |
Section | Letters | |
DOI | https://doi.org/10.1051/0004-6361/201321965 | |
Published online | 10 July 2013 |
Is GBT 1355+5439 a dark galaxy?
1
Netherlands Institute for Radio Astronomy (ASTRON),
Postbus 2,
7990 AA
Dwingeloo,
The Netherlands
e-mail:
oosterloo@astron.nl
2
Kapteyn Astronomical Institute, University of Groningen,
Postbus 800,
9700 AA
Groningen, The
Netherlands
3
Astrophysics, Cosmology and Gravity Centre (ACGC), Department of
Astronomy, University of Cape Town, Private Bag X3, 7701
Rondebosch, South
Africa
Received: 26 May 2013
Accepted: 14 June 2013
We present H i imaging of GBT 1355+5439 performed with the Westerbork Synthesis Radio Telescope. This is a dark H i object recently discovered close to the nearby galaxy M101. We find GBT 1355+5439 to be an H i cloud 5 × 3 arcmin in size. The total H i image and the kinematics show that the cloud consists of condensations that have small (~10 km s-1) motions with respect to each other. The column densities of the H i are low; the observed peak value is 7.1 × 1019 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 H i velocity dispersion is higher than 5 km s-1 and no narrow, cold, H i 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 H i mass of GBT 1355+5439 would be about a few times 105M⊙, its size about 1 kpc, and the dynamical mass Mdyn > 5 × 107M⊙. However, if GBT 1355+5439 is a dark Local Group object, the internal kinematics of the H i appears to be different from that of gas-dominated, almost dark galaxies of similar size.
Key words: galaxies: dwarf / radio lines: galaxies
© ESO, 2013
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