4 Summary and conclusions

The main contributions of this paper are:

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B- and R-band surface photometry for 25 Southern field dwarfs is presented, including the newly discoverd dwarf elliptical galaxy NGC 2784 DW1. An image gallery and the deduced surface brightness and colour profiles are shown; effective radii and surface brightnesses as well as the extrapolated central surface brightnesses and the scale lengths resulting from fitted exponential model profiles are listed. Total colors and formal color gradients are given.

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Our program of monitoring all dwarf galaxies in the nearby 10 Mpc volume is nearly completed, at least within a magnitude limit of $M_B\le-14$ mag, allowing for a statistical exploitation.

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Im galaxies, late-type spirals, and BCD/BCD-like galaxies are indistinguishable as concerns their basic photometric parameter relations, and thus can be lumped together to form a homogenous class of galaxies ("irregulars'').

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We introduce a clear-cut definition for the distinction of group and field galaxies: a group dwarf has a neighbour brighter than M_B = -17.5 mag within a distance of 1 Mpc.

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Our previously (Paper VI) claimed statistical dependence of structure on large-scale galaxy environment, the structure-environment relation, is confirmed for the dwarf irregulars: at a given luminosity a much denser environment is related to a higher mean scale length or fainter mean central surface brightness, and to sligthly redder B-R colours. See Bremnes (2000) for a similar finding concerning dwarf ellipticals.

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Group dwarfs obey the same structure-environment relation as field dwarfs.

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There seems to be a systematic trend from dwarf irregulars with shorter to longer scale lengths (i.e. from smaller/fainter to bigger/brighter galaxies) to change from positive to neglectable and even negative B-R colour gradients.

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Dwarf irregulars exhibit a mean outward reddening B-R of about 0.1 mag after one scale length or 0.16 mag after one kpc.

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For an individual dwarf irregular galaxy at a given luminosity the scatter in scale length or surface brightness is related to the kinematics of the galaxy's inner disk: a larger-than-mean scale length or fainter-than-mean central surface brightness is correlated with a faster-than-mean rotational velocity of the halo contribution at an inner disk radius of about two scale lengths; this is no more seen if circular velocity is measured at large radii where baryonic matter has lost dynamic importance.

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This structure-dynamics relation describing a linkage between photometric and kinematic properties has to be confirmed by means of a sample of galaxies with resolved velocity curves. If verified it may account for the structure-environment relation provided there is also a dynamics-environment relation, i.e. a systematic difference between the halos of cluster and field galaxies. This needs to be investigated.

Acknowledgements

B.R.P. thanks Markus Samland, Victor Debattista, and Alfonso Aguerri for stimulating conversations. Financial support by the Swiss National Science Foundation is gratefully acknowledged. This work has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, CalTech, under contract with NASA and NASA's ADS Abstract Service.