Letter to the Editor

An enigmatic H I cloud

¹ Argelander Institut für Astronomie (AIfA), University of Bonn, Auf dem Hügel 71, 53121 Bonn, Germany e-mail: [ldedes;pkalberla]@astro.uni-bonn.de
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 4 September 2008
Accepted: 14 October 2008

Abstract

Aims. The discovery of an H I cloud with peculiar properties at equatorial coordinates (J2000) , is presented.

Methods. The H I object was detected at 21 cm using the 300-m NAIC Arecibo telescope. Subsequent follow-up high-resolution observations with the NRAO Very Large Array (VLA) telescope at L-Band revealed more details about its morphology.

Results. The most intriguing aspect of the object is the clear velocity gradient of 1 km s^-1, which is present in the data, an indication of either rotation or expansion. The gas appears to be cold, and its morphology is somewhat elliptical with clumpy substructure. Assuming disk rotation, the dynamical mass could be determined as a function of distance.

Conclusions. Depending on the exact nature of the velocity gradient in the H I cloud, we can reach some preliminary conclusions about the nature of the object. Expansion would imply association with a circumstellar envelope of an evolved AGB star, while in the case of rotation, a comparison between the visible and the dynamical mass can lead to some preliminary conclusions. A common feature of those conclusions is the presence of a gravitational potential well, which is required to account for the rotation of the trapped H I gas. This potential well could be associated with a dark galaxy or some other exotic object.