8.3 Survey of Nearby Dwarfs

The only systematic search for nearby dwarf galaxies was made by van den Bergh (1959) on the 48-inch Sky Survey prints. Criteria for identification depend mainly on low surface brightness and density gradient in an image of diameter 1' or larger. Compact dE systems are therefore excluded. Most objects must be dIm and low-density dE systems except for a few resolved Local Group ``pygmies'' and some occasional misidentifications. The apparent distribution in supergalactic coordinates of the DDO dwarfs is shown in figure 6; the large gap is the unobserved southern sky ( < -23°). The symbols correspond to van den Bergh's classification:

DIr. - Dwarf Magellanic irregulars of the NGC 6822, IC 1613, WLM type.

DSp. - Dwarf spirals which from van den Bergh's description are clearly late-type barred spirals of the SBd-SBm types or Magellanic irregulars of the SMC-LMC type.

DEl. - Dwarf ellipticals of the NGC 0205 type of which only a few of the nearer and larger ones can be identified on the Sky Survey prints.

DSph. - Dwarf spheroidal galaxies of the IC 3475 type in the Virgo cluster (van den Bergh notes that resolved systems of this type in the Local Group such as Draco are difficult to distinguish from clusters of distant galaxies on the Sky Survey prints).

Local Group members and objects larger than 1'.0 are identified by open circles in figure 6.

Figure 6. Apparent distribution of DDO dwarf galaxies in supergalactic coordinates.

The nearby dwarfs are clearly concentrated to the supergalactic equator, especially in the northern (galactic) hemisphere, and several condensations corresponding to the nearby groups can be recognized (compare with figs. 3 and 4), in particular the M81 and M101 groups, the Virgo I cluster, and the CVn I, CVn II, Vir II clouds. Probable or possible associations of DDO dwarfs with some individual groups are noted in Table 3. In most cases, however, definite proof of association will require more detailed studies.