There is a rich variation in morphologies of SFDGs. The local group galaxy NGC6 822 is in many respects a prototype of the normal dIs. It is the nearest SFDG without a massive neighbor. In the central area it contains a rich number of H II regions and star clusters of various ages. On larger scales the stars are embedded in an 1.8 108 M H I disk (de Blok and Walter 2000). Young stars trace the H I structure out to large distances (Komiyama et al. 2003). Apparently also NGC 6822 has been influenced by tidal interaction that has caused tidal tails and enhanced star formation in the outer regions. Older stars are more spherically distributed. This demonstrates that also dIs may have had an initial intense star formation epoch during which the galaxy was taken shape but still in the collapse state. The mean SFR over the past 100 Myr is estimated to 1.4 × 10-2 M yr-1 (Efremova et al. 2011). Correcting for He, H2 (Gratier et al. 2010) and heavy elements, we arrive at a gas consumption timescale of about 19 Gyr, rather typical for its type (Hunter and Elmegreen 2004).
While the luminosity profiles of dIs follow a simple exponential law, the more enhanced SF activity in the centres of BCGs, possibly linked to dynamical instabilities, show a more complex behavior (Kunth et al. 1988, Meurer et al. 1995, Cairós et al. 2001). BCGs often have morphologies reminiscent of mergers and sometimes experience strong starbursts. In an effort to develop a classification system for BCGs, Loose and Thuan (1986) proposed four different types with the most common one having several star forming regions near the centre, embedded in a regular halo. Kunth et al. (1988) associated the structure with that of dEs. Papaderos et al. (1996) found that the luminosity distribution in BCGs in the optical region could be represented by 1) an exponential component at large radii describing an underlying older stellar population; 2) a plateau at intermediate radii and 3) a gaussian component at small radii. Infrared observations reveal in many cases a flattening of the exponential profile (Noeske et al. 2003) towards the centre. The faint component emerges at very low surface brightness, and supports a high age. The presence of a faint extended red component indicates that the host has similarities to dEs. In some cases, as in the extremely metal poor SFDG SBS 0335-052, we observe faint extended emission that is strongly influenced by nebular emission. After correction, the remaining stellar component agrees with a fairly young population (Izotov et al. 1997 Papaderos et al. 1998) as if the galaxy formed recently.