H I -rich dwarf galaxies in the Hydra I cluster^*

I. Photometric and H I data

¹ Institute of Astronomy, Madingley Road, Cambridge, CB3 0HA, UK
² ESO, Karl–Schwarzschild–Str. 2, D–85748 Garching bei München, Germany
³ Universitäts-Sternwarte, Geismarlandstr. 11, D–37083 Göttingen, Germany
⁴ DAEC, URA 173 associée au CNRS et à l'Université Paris 7, Observatoire de Paris, F–92195 Meudon Cedex, France
⁵ Astronomy Department, University of Virginia, Charlottesville, VA 22903, U.S.A.
⁶ Unité Scientifique Nançay, CNRS USR B704, Observatoire de Paris, F–92195 Meudon Cedex, France

Send offprint request to: P.–A. Duc

Received: 7 December 1998
Accepted: 1 March 1999

Abstract

We present H I, optical and near-infrared photometric data of a sample of H I–selected dwarf galaxy candidates in the Hydra I cluster. This database has been compiled in order to study environmental effects on dwarf galaxies in clusters, in particular the role of interactions between them and either the intracluster-medium or companion galaxies, hitherto studied mainly in massive galaxies. The atomic gas is supposed to be very sensitive to any external perturbation, as it is more extended than the stellar disk especially in the case of dwarf star–forming galaxies. In that respect, the discovery of a large number of H I sources presumably associated with dwarf galaxies in a complete VLA survey of the Hydra I cluster by McMahon (1993) is intriguing since low-mass clouds might be very fragile in a cluster environment. In order to understand the origin and survival of these H I sources, we have carried out a multi–wavelength analysis of their host galaxies, which were all selected on 21 cm H I line widths smaller than 130 . Sixteen out of the 20 candidates in the initial VLA sample have been reobserved with much higher velocity resolution at Nançay. The H I detection has been confirmed for all of them. From the combined H I and optical data, 15 objects turn out to be true dwarf galaxies; for one H I source no optical counterpart was found, and the remaining 4 galaxies are in fact face-on spirals that were therefore rejected in our final analysis. The H I–selected dwarf galaxies show a large diversity in their morphologies and surface brightness profiles. They range from blue compact objects to red low surface brightness galaxies.