An early attempt to list all galaxies out to a LG-centric velocity of 500 km s-1, corresponding roughly to a distance of 10 Mpc, by Kraan-Korteweg & Tammann (1979) has set the scene for all later efforts, including the present work, to improve on our knowledge of dwarf objects in that "10 Mpc volume''. The most recent update of the "10 Mpc Catalogue'', listing some 300 galaxies, is given by Karachentsev et al. (1999). Many of these objects have only recently been discovered by Karachentseva & Karachentsev (1998) and confirmed to be nearby dwarfs by Huchtmeier et al. (2000). The number of known galaxies within the 10 Mpc volume is bound to grow further in the future.
Follow-up work on the known nearby dwarfs, done primarily by Karachentsev and collaborators, has mainly focused on the distance determination for these objects from the brightest blue stars in late-type, and the tip of the red giant branch in early-type dwarfs (see Karachentsev et al. 1999 and refs. therein; Seitzer et al. 2001). In order to secure also a data base of surface photometry for these objects, we have started a programme of systematic CCD imaging, in at least two colour bands, of all known dwarf galaxies in the 10 Mpc volume for which no such photometric data was, or is, available. Our goal is not only to contribute to the "fair sample'' census mentioned above, but to study the systematic structural differences of dwarf galaxies in different environments. Clearly, the low-mass, low-density "dwarf'' galaxies are expected to show the strongest reactions to, and hence the most significant signs of, environmental influences. A large and homogeneous sample of nearby, well-resolved dwarfs will therefore provide an ideal test bed for evolutionary scenarios.
In previous papers of this series we have reported on our B and R-band observations of dwarf members of the M 81, M 101, and CVnI groups (Bremnes et al. 1998, 1999, 2000 = Papers I, III, IV) and of field dwarfs in the Northern hemisphere (Barazza et al. 2001 = Paper VI). Based on these data, supplemented by data from other studies, Bremnes (2000) did a preliminary comparison of the photometric scaling properties of nearby field and group dwarfs with those of cluster dwarfs, drawing on the cluster data of Binggeli & Cameron (1993) and Ferguson & Sandage (1990). Bremnes (2000) found an important, significant effect: at a given total magnitude, the central exponential (or also effective) surface brightness of a dwarf galaxy - of early or late type - is lower in the high-density environment of a cluster than in the low-density field or group. The same trend is mirrored in the exponential scale length or the effective radius. A sample of Northern field dwarfs was added by Paper VI, where the analysis of Bremnes (2000) was repeated and his results confiremd for late-type dwarfs.
In the present paper, number VII of the series, we present, in a first part (Sect. 2), B and R-band surface photometry for another 25 Southern field dwarfs. One of these objects, now called NGC 2784 DW1, was not listed before but has been discovered on one of our images in the course of this work. As in our previous papers we show surface brightness and colour profiles and give the conventional photometric parameters (total magnitudes, effective, and exponential parameters) for all objects.
As most dwarfs in the nearby field, including groups of galaxies, are low-luminosity spirals (Scd, Sd, Sm) and irregulars (Im, BCD), we have by now accumulated a fairly large sample of late-type dwarfs. In the second part of the paper (Sect. 3) we therefore present a statistical analysis of the photometric, but also kinematic, properties of all 72 odd late-type dwarfs (= "irregulars'' for short) for which we have done imaging so far. For the sake of homogeneity, the whole analysis is restricted to our own photometric data. This is in contrast to Bremnes (2000) and Paper VI, where data from the literature were mixed in.
One novelty of the present study is the use of a rather simple but clear-cut definition of "group'' versus "field'' dwarfs: a "field'' dwarf has no neighbour brighter than MB = -17.5 mag within a distance of 1 Mpc. Distance estimates are mostly provided by Karachentsev et al. (1999). In this way certain traditional group members lying in the outskirts of the group would become "field'' dwarfs, and certain dwarfs satellite to giants outside of the known groups would become "group'' dwarfs. A similar but continuous, and more physically motivated environmental parameter, the "tidal index'', was introduced before by Karachentsev & Makarov (1999).
We confirm, in Sect. 3.4, that there is a significant shift in exponential scale length, and consequently in all other photometric parameters as well, at a given total magnitude, between field and group and cluster irregulars; there is, however, no such shift between group and field objects alone. There is some support for Bremnes' (2000) interpretation of this effect in terms of a difference in star formation history, in that the higher surface brightness field and group irregulars are also bluer. However, the photometric difference between field/group and cluster dwarfs could primarily also be a structural difference: in a cluster environment the dwarfs could plausibly have been puffed up to larger scale length by frequent tidal encounters. Moreover, different halos in different environments may also cause a shift: by combining our photometry with H I rotational velocity data from Karachentsev et al. (1999) and that for the PGC (Bottinelli et al. 1990), in Sect. 3.6, we show that our irregulars are indeed rotating disk galaxies obeying a Tully-Fisher relation, and that photometric parameters are correlated with inner disk circular speeds.
We also report, in Sect. 3.5, perhaps the first clear evidence of a relation between colour gradient and exponential scale length, which has a simple physical meaning. A summary of our results can be found in Sect. 4.
Copyright ESO 2002