Isolated galaxies are important since they probe the lowest density regimes inhabited by galaxies .
We define a sample of 36 nearby isolated early-type galaxies for further study .
Our isolation criteria require them to have no comparable-mass neighbours within 2 B -band magnitudes , 0.67 Mpc in the plane of the sky and 700 km s ^ { -1 } in recession velocity .
New wide-field optical imaging of 10 isolated galaxies with the Anglo-Australian Telescope confirms their early-type morphology and relative isolation .
We also present imaging of 4 galaxy groups as a control sample .
The isolated galaxies are shown to be more gravitationally isolated than the group galaxies .
We find that the isolated early-type galaxies have a mean effective colour of ( B – R ) _ { e } = 1.54 \pm 0.14 , similar to their high-density counterparts .
They reveal a similar colour-magnitude relation slope and small intrinsic scatter to cluster ellipticals .
They also follow the Kormendy relation of surface brightness versus size for luminous cluster galaxies .
Such properties suggest that the isolated galaxies formed at a similar epoch to cluster galaxies , such that the bulk of their stars are very old .
However , our galaxy modelling reveals evidence for dust lanes , plumes , shells , boxy and disk isophotes in four out of nine galaxies .
Thus at least some isolated galaxies have experienced a recent merger/accretion event which may have induced a small burst of star formation .
We derive luminosity functions for the isolated galaxies and find a faint slope of –1.2 , which is similar to the ‘ universal ’ slope found in a wide variety of environments .
We examine the number density distribution of galaxies in the field of the isolated galaxies .
Only the very faintest dwarf galaxies ( M { { { { { } _ { R } \mathrel { \mathchoice { \vbox { \offinterlineskip \halign { \cr } $ \displaystyle > $% \cr$ \displaystyle \sim$ } } } { \vbox { \offinterlineskip \halign { \cr } $ \textstyle > $ \cr$% \textstyle \sim$ } } } { \vbox { \offinterlineskip \halign { \cr } $ \scriptstyle > $ \cr$% \scriptstyle \sim$ } } } { \vbox { \offinterlineskip \halign { \cr } $ \scriptscriptstyle > $% \cr$ \scriptscriptstyle \sim$ } } } } -15.5 ) appear to be associated with the isolated galaxies , whereas any intermediate-luminosity galaxies appear to lie in the background .
Finally we discuss possible formation scenarios for isolated early-type galaxies .
Early epoch formation and a merger/accretion of galaxies are possible explanations .
The collapse of a large , virialised group is an unlikely explanation , but that of a poor group remains viable .