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1 Introduction

Intergalactic H  I clouds have been found to be extremely rare, although they were searched for rather intensively in the 21 cm line over the years, in the field as well as in environments with higher galaxy density, using both pointed and "blind'' surveys (see Briggs 1990 for a summary of the surveys made until 1989 and, e.g., Spitzak & Schneider 1998; Kraan-Korteweg et al. 1999; Zwaan 2001). Their space density was quantified by Briggs (1990) as less than 1% of that of galaxies in the H  I mass range of about 108 to 1010 ${M}_{\odot}$, based on data available at the time, while Zwaan (2001) estimated that in groups of galaxies their total H  I mass is less than 10% of the total H  I mass of the group.

Furthermore, almost all of the claimed cases turned out to have optical counterparts, e.g., the "isolated'' cloud H  I 1225+01 discovered at Arecibo by Giovanelli & Haynes (1989), which was later found to be associated with a blue dwarf galaxy (Djorgovski 1991; Salzer et al. 1992). On the other hand, the large intergalactic cloud discovered at Arecibo by Schneider et al. (1983) in the Leo group has a ring shape typical of tidal features and may well have been tidally stripped from one of the galaxies in its vicinity. An example of such a tidal cloud without an optical counterpart can also be found in the Local Group: the Magellanic Stream. A new H  I cloud without an optical counterpart found by Kilborn et al. (2000) could have been ejected from the interacting Magellanic Cloud - Galaxy system, and clouds once thought to be in the Sculptor group (Mathewson et al. 1975) were later shown to be part of the Magellanic Stream (Haynes & Roberts 1979). Although in the Hydra cluster we confirmed (Duc et al. 1999) the VLA H  I source H1032-2819, for which we did not find an optical counterpart in CCD images, it might be the result of an interaction between two cluster galaxies. In a VLA survey of the Coma cluster (Bravo-Alfaro et al. 2000, 2001) two H  I clouds without optical counterparts on the DSS were found with an ${M}_{\rm HI}$ of 1 and $4 \times 10^8$ ${M}_{\odot}$, respectively.

In a 21 cm H  I line survey of the Hercules Cluster carried out with the VLA by Dickey (1997, hereafter D97) the tentative detection was reported of 12 H  I clouds without optical counterparts on digitised Palomar Sky Survey (DSS) images - throughout this paper we will use the designations of H  I detections from D97: ce-70, ce-86, ce-102, ce-137, ce-224, sw-89, sw-103, sw-146, sw-174, sw-194, sw-201 and 47-52. Three of these (ce086, sw-103 and sw-194) will not be discussed in the present paper, for reasons explained below.

At the positions of two of these H  I detections invisible on the DSS, sw-103 and sw-194, faint optical counterparts were detected in the deeper B, V and i-band CCD images we obtained of all reported H  I clouds, except 47-52 (Duc et al. 2001; Iglesias-Páramo et al. 2002, hereafter IP02). Although we could not obtain optical redshifts of sw-103 and sw-194 they are the only galaxies seen within the H  I clouds and their optical centre positions are only 4'' to 11'' (one fourth and half the VLA HPBW, respectively) away from that of the H  I cloud centres. We therefore regard them as tentative optical detections of these two H  I clouds.

Given our non-confirmation at Arecibo of the reported H  I clouds, it should be noted that we did clearly confirm the D97 VLA H  I line signal from, e.g., sw-103, which is as weak as the faintest reported VLA H  I cloud profiles, during the same observing run at Arecibo (IP02).

A third H  I cloud, ce-86, although it shows a strong line in the VLA survey and remained undetected on our CCD images, is clearly part of the extended H  I distribution of the peculiar galaxy IC 1182, and it therefore cannot be grouped together with the other clouds. Our Arecibo and optical observations of these three objects are discussed in IP02.

We failed to detect any optical emission from the remaining 8 H  I clouds covered by our images (47-52 was not observed) to a limiting V-band surface brightness of about 27 mag arcsec-2. For a contour plot of the D97 VLA H  I column density maps superimposed on the V-band images of IP02, showing all of the H  I clouds under consideration in the present paper, except 47-52, see Fig. 1.

The present paper concerns the H  I line observations we made at Arecibo, the only other radiotelescope with the sensitivity required for checking the reality of the tentative D97 H  I detections without optical counterparts. The Arecibo observations are described in Sect. 2, and their results in Sect. 3. In Sect. 4 the results are discussed and the conclusions presented.


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Up: Non-confirmation of reported H

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