Structure and stellar content of dwarf galaxies^*,**

VII. and photometry of 25 southern field dwarfs and a disk parameter analysis of the complete sample of nearby irregulars

Astronomisches Institut der Universität Basel, Venusstrasse 7, 4102 Binningen, Switzerland

Corresponding author: B. R. Parodi, parodi@astro.unibas.ch

Received: 23 January 2002
Accepted: 12 March 2002

Abstract

We present B and R band surface photometry of 25 Southern field dwarf galaxies within a distance of 10 Mpc. For each galaxy we give the essential model-free photometric parameters and, by fitting exponentials to the surface brightness profiles, the central extrapolated surface brightness and the exponential scale length, in both colour bands. Surface brightness and colour profiles are shown. One of the objects, a very faint dwarf elliptical in the vicinity of NGC 2784, has been discovered in the course of this work. Drawing on the data from this and all previous papers of this series, we construct a complete sample of 72 late-type (“irregular”) dwarf galaxies in nearby groups and the field within the 10 Mpc volume, to study the exponential-disk parameter relations of these galaxies with respect to galaxy environment. We confirm our previous finding of statistically lower scale lengths/higher central surface brightnesses for field and group galaxies as compared to cluster galaxies. However, using a clear-cut definition of “group” versus “field” environment, we find no significant difference in the photometric structure of group and field irregulars. A difference in the star formation history may partly account for this structure-environment relation: for a given luminosity cluster dwarfs are on average redder than field and group galaxies. We also report evidence for the colour gradients of dwarf irregulars being roughly inversely proportional to the disk scale lengths. Supplementing our photometric data with kinematic data from the literature, we study possible relations with kinematic properties of the inner disk. Applying the dark matter scaling relations for a Burkert halo we show that for field and group galaxies of a given luminosity faster-than-mean disk rotational velocities at a radius of about two scale lengths are correlated with larger-than-mean disk scale lengths.