A catalogue of potential dwarf galaxies in the vicinity of strongly
interacting galaxies is presented. The catalogue is based on the
imaging of 12 nearby interacting galaxies with V and R filters. After
careful cleaning of the images and applying a restrictive S/N
criterion we have measured and cataloged all possible extended objects (EO's)
in each field. On the frames, covering 11
12 arcmins, we have found
typically 100-200 non-stellar faint, extended objects (EOs), the vast
majority 
of which had not been previously identified.
Absolute V and R magnitudes of each EO have been measured, and magnitude errors are given for each object. Reliable statistics of the colors of the galaxies can be made with the provided information. The coordinates for each EO were derived with an accuracy better than 1 arcsec.
For each field we have derived the magnitude distribution of the EOs so as to determine the magnitude up to which the search can be considered complete. To give another example of a sample field, in Fig. 6 (click here) the sizes and magnitudes of SOs and EOs around NGC 1023 are shown. The maximum in the size distribution is used to classify all identified objects into stellar or extended. As mentioned before, the SO distribution is similar to that of the other fields (see Fig. 5 (click here) for NGC 3226/7). However, the EO magnitude distribution function is very different from that of NGC 3226/7. Among the 12 sample fields, the one of NGC 1023 has the lowest number of EO identifications. Only 21 dwarf candidates are found. No direct comparison can be done with this number, given the large range in linear size covered by our target fields. However, these 21 objects identified are far from following any particular distribution function.
Figure 6: Left: Distribution of 
of all identified
objects in the field around NGC 1023. The dashed line is a fit (a
combination of a Gaussian with a linear quadratic function) whose peak
was used to determine the final size (
) of the stellar psf.
The units of are in pixels; 1 pixels = 0.55 arcsec. Right:
Distribution of R magnitudes in the field of NGC 1023 among
identified extended objects (solid line) and stellar objects (dashed
line)
The method used for the identification of the EOs cannot exclude the possibility that some barely resolved double or multiple star systems may be listed as EOs. On the other hand, EO's with near stellar psf's will have been discarded as stellar objects. Among the faintest objects identified, there may also be some globular clusters (GC's) of the central galaxy. In the case of the field around NGC 3226/7, which is the closest sample galaxy with a significant number of EOs, the apparent magnitude of the brightest GC's would be V=21.5, and the angular size of the largest GC's around this object would be on the order of the psf. GC's can therefore not be of significant numbers among the EOs identified; there may however be a large number of them among the faintest SOs.
Metcalfe et al. (1991) found in their galaxy-star separations,
performed in a similar way to ours, that later spectroscopy revealed
29% of the star-like objects to be compact galaxies, but only
of the galaxy identifications turned out to be
stars. Our separation algorithm is based on the fitting of elliptical
Gaussians to each objects, and the FWHM was the major parameter for
inclusion into the list of EOs. Objects with a concentrated nucleus,
and faint extended wings close to disappearance in the noise of the
CCD frame were therefore generally classified as stellar-like
objects. We also note that the indentification of such small extended
objects is very sensitive to the seeing conditions. The algorithm
used in this catalogue did however find reliably objects with an
extended elevated surface brightness, providing a photometric list of
possible dwarf galaxy candidates around a sample of interacting
galaxies.
The number of EOs varies strongly within the sample, and with larger fluctuations than the number of stellar objects (SOs). The number of expected objects in the magnitude range R = 18 - 19.5 exceeds the expected count of background galaxies. At the catalog's completeness limit, the measured galaxy counts agree with the expected one within its errors. This is not surprising, given the definition of the completeness limit we used. The excess of EOs supports the possibility that a density enhancement of extended objects around some interacting galaxies results from the addition of a locally formed dwarf galaxy population.
In order to compare the distributions of extended objects within each field, the densities of the EOs (N(EO)/area) in dependence of the distances from the central galaxies, and deviations of the angular 2-point correlation functions from that of background galaxies needs to be calculated. All the required information is provided in this paper, although a discussion and a further study in this sense will be the topic of a forthcoming communication.
Acknowledgements
We are grateful to Prof. J. Lequeux for a careful refereeing of this paper. We are pleased to thank M. Balcells for his valuable comments. We thank A. Robin for making her models of the Galactic stellar population available to the community. The Isaac Newton Telescope is operated on the island of La Palma by the Royal Greenwich Observatory in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautic and Space Administration. This work has been partially supported by the Spanish DGICYT (Dirección General de Investigación Cientifıca y Técnica) Grant No. PB94-1106. HJD acknowledges a fellowship (Programa Nacional de Formación de Personal Investigador) from the Ministerio de Educación y Cultura of Spain. GTT gratefully acknowledges the BBV foundation for a visiting professorship at the University of Cambridge