8.4 Completeness of Survey
The frequency function of apparent diameters for the DDO dwarfs is as follows:
|N = n||1||7||15||27||56||102||222|
A plot of log N versus log approximates closely the N -3 distribution expected for a statistically uniform space density in the range 11' < < 20', but falls off rapidly for < 10'. Either incompleteness begins at = 10' or there is a local excess of dwarfs associated with the Local Group. The seven largest DDO dwarfs ( 8'.0) are listed in Table 9: six (DDO 008, DDO 074, DDO 199, DDO 208, DDO 209, DDO 221) are in the Local Group and one (DDO 081) is in the M81 group.
|DDO 008 = IC 1613||15'.0||0.66||3.1|
|DDO 074 = Leo I||8'.5||0.23||0.6|
|DDO 081 = IC 2574||11'.5||2.5||8.5|
|DDO 199 = UMi||20'.0||0.08||0.5|
|DDO 208 = Draco||8'.0||0.06||0.14|
|DDO 209 = NGC 6822||12'.5||0.50||1.8|
|DDO 221 = WLM||11'.0||0.87||2.8|
If the smallest DDO dwarfs ( = 1'.0) have the same mean linear diameter = 2.5 kpc as the seven above, the limiting range of the survey is 8.5 Mpc on the average; half the total ( 1'.5) is within an average range of 5.5 Mpc. However, the range of linear diameters is large (8.5-0.14 kpc or a 60-1 ratio in the first seven), and the range of the survey in depth is necessarily indefinite.
For a survey of dwarf galaxies in the Virgo cluster, see Reaves (1956).