Dwarf galaxies are by now widely recognized as prime laboratories for the study of many key issues of astronomy, such as structure formation, galaxy evolution, star formation, and dark matter (e.g., Meylan & Prugniel 1994; Ferguson & Binggeli 1994). Up to the present, studies of dwarf galaxies have concentrated either on clusters of galaxies, such as Virgo and Fornax, or the Local Group (LG). The clusters, although rich in dwarfs, are relatively distant. The LG dwarfs can be studied in great detail, but there are only a few of them and these are much fainter, intrinsically, than the known cluster dwarfs. Hence there is a gap of data and knowledge between LG and cluster dwarfs.
To bridge (i.e., fill-in) that gap, we have started an observing programme, at the Observatoire de Haute Provence (OHP), to do systematic imaging of dwarf galaxies in nearby groups and the general field, by drawing on the "10 Mpc Catalogue'' of galaxies of Kraan-Korteweg & Tammann (1979), updated by Schmidt & Boller (1992). Our goal is to derive all relevant spectrophotometric parameters for these dwarfs of intermediate brightness and to compare them with existing data on the dwarf galaxy populations of the LG and the Virgo and Fornax clusters.
This paper is the first in a series dedicated to this purpose. A more detailed account of the scientific background and motivation of our project will be given in a later paper. Here we report on a first campaign of CCD photometry of dwarf galaxy members of the M 81 group of galaxies. This most prominent nearby galaxy group in the northern hemisphere is a natural starting point for our programme. It is quite nearby, with 4 Mpc (Karachentsev & Tikhonov 1994), and approximately 2 times as rich as the LG (above a given absolute magnitude). Our principal knowledge of the dwarf galaxy population of the M 81 group is due to the extensive photographic survey of Börngen et al. (1982)
In our first observing campaign we did B and R imaging for essentially all ( 35) known dwarf members from Börngen et al.'s list. Paper II will present deep photometry for a subsample of non-resolved (early-type) dwarfs. However, for the purpose of completeness we use here part of the material from Paper II, so that the principal data on the group members can be found in the present paper, including a CCD picture gallery which supplements/supersedes the photographic atlas of Karachentseva et al. (1985). The photometric data, i.e. surface brightness profiles and colour gradients presented here and in Paper II will be interpreted in Paper III, which is to appear in the main journal of A&A.
In Appendix A we discuss the possible nature of an unusual clustering of low-surface brightness objects in the south-east corner of the M 81 group. While the case for extra-galactic objects (Börngen et al. 1984) remains viable, we argue in favour of a galactic nature (cirrus clouds) for at least some of the cases.
In Appendix B we show the luminosity function for M 81 group galaxies.