A&A 399, 433-439 (2003)
DOI: 10.1051/0004-6361:20021684

Non-confirmation of reported clouds without optical counterparts in the Hercules cluster

W. van Driel¹, K. O'Neil², V. Cayatte¹, P.-A. Duc³, J. M. Dickey⁴, C. Balkowski¹, H. Hernández², J. Iglesias-Páramo⁵, P. Papaderos⁶, J. M. Vílchez⁷ and T. X. Thuan<sup>8

1</sup>  Observatoire de Paris, Section de Meudon, GEPI, CNRS UMR 8111, 5 place Jules Janssen, 92195 Meudon, France
    e-mail: wim.vandriel@obspm.fr;veronique.cayatte@obspm.fr;chantal.balkowski@obspm.fr
²  Arecibo Observatory, HC3 Box 53995, Arecibo, Puerto Rico 00612, USA
    e-mail: koneil@naic.edu;hhernand@naic.edu
³  CNRS URA 2052 and CEA, DSM, DAPNIA, Service d'Astrophysique, Centre d'Études de Saclay, 91911 Gif-sur-Yvette Cedex, France
    e-mail: paduc@cea.fr
⁴  Department of Astronomy, University of Minnesota, Minneapolis, USA
    e-mail: john@astro.umn.edu
⁵  Laboratoire d'Astrophysique de Marseille, Traverse du Siphon - Les Trois Lucs, 13376 Marseille, France
    e-mail: jorge.iglesias@oamp.fr
⁶  Universitäts-Sternwarte, Geismarlandstrasse 11, 37083 Göttingen, Germany
    e-mail: papade@uni-sw.gwdg.de
⁷  Instituto de Astrofísica de Andalucía (CSIC), Granada, Spain
    e-mail: jvm@iaa.es
⁸  Astronomy Department, University of Virginia, Charlottesville, VA 22903, USA
    e-mail: txt@astro.virginia.edu

(Received 30 August 2002 / Accepted 7 November 2002 )

Abstract
21 cm H I line observations were made with the Arecibo Gregorian telescope of 9 H I clouds in the Hercules Cluster which were reported as tentative detections in a VLA H I study of the cluster (Dickey 1997) and for which our deep CCD imaging failed to find any optical counterparts. No sensitive observations could be made of one of these (sw-174) due to the presence of a close-by strong continuum source. The other 8 tentative H I detections were not confirmed by the Arecibo H I measurements. The CCD images did reveal faint, low surface brightness counterparts near the centres of two other VLA H I sources invisible on the Palomar Sky Survey, sw-103 and sw-194.

Key words: galaxies: clusters: general -- galaxies: clusters: individual: Hercules cluster -- galaxies: ISM -- radio lines: galaxies

Offprint request: W. van Driel, wim.vandriel@obspm.fr

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