Surface Photometry of Low Surface Brightness Dwarf Irregular Galaxies

Abstract

We present CCD B and I surface brightness and color profiles for a sample of 51 dwarf and low surface brightness (LSB) galaxies. The galaxies were drawn mainly from the sample of UGC dwarf and LSB galaxies detected in H I by Schneider et al. We focus our attention on extreme dwarf irregulars (dIs) where turbulent motions are as important as or more important than rotational motions, so that the majority of the dwarfs were chosen to have narrow H I line widths ({DELTA}V_20_ < 100 km s^-1^), in addition to being nearby (v_0_ < 1500 km s^-1^).For each dI galaxy, we derive structural parameters for the underlying low surface brightness disk component on which are superimposed the star-forming regions. We found that the central surface brightness of the underlying exponential component of dIs is similar to that of dwarf ellipticals (dEs) but ~1.5 mag fainter than that of blue compact dwarfs (BCDs). As for the scale lengths, the dIs divide into two groups; one where, for a given B luminosity, the scale length of the underlying exponential component is comparable to that of dEs and a factor of ~2 larger than that of BCDs, and one group where the scale length of the dIs is comparable to that of BCDs but a factor of 2 smaller than that of dEs. These differences in structural parameters put strong constraints on evolutionary scenarios among the three types of dwarfs. We use the CCD images to set up a tentative morphological classification scheme for LSB dIs based of the shape of the underlying low surface brightness component and the morphology and location of the star-forming regions on top of it. The division of the dIs into two groups appears to be correlated with morphology. There are a few dwarf spirals in our sample, which show fragments of low surface brightness spiral arms. We found that Nilson and Zwicky magnitudes are systematically too faint for dwarfs and LSB galaxies by 1-2 mag. The underlying disks in dIs have B-I colors ~1.5, corresponding to a stellar population of G and K main-sequence or giant stars and are redder than those of BCDs. The color profiles of the galaxies in the sample are generally flat, with no strong gradient.