We present a photometric study of the Andromeda XVIII dwarf spheroidal galaxy associated with M31 , and situated well outside of the virial radius of the M31 halo .
The galaxy was resolved into stars with Hubble Space Telescope/Advanced Camera for Surveys revealing the old red giant branch and red clump .
With the new observational data we determined the Andromeda XVIII distance to be D = 1.33 _ { -0.09 } ^ { +0.06 } Mpc using the tip of red giant branch method .
Thus , the dwarf is situated at the distance of 579 kpc from M31 .
We model the star formation history of Andromeda XVIII from the stellar photometry and Padova theoretical stellar isochrones .
An ancient burst of star formation occurred 12–14 Gyr ago .
There is no sign of recent/ongoing star formation in the last 1.5 Gyr .
The mass fractions of the ancient and intermediate age stars are 34 and 66 per cent , respectively , and the total stellar mass is 4.2 \times 10 ^ { 6 } M _ { \odot } .
It is probable that the galaxy has not experienced an interaction with M31 in the past .
We also discuss star formation processes of dSphs KKR 25 , KKs 03 , as well as dTr KK 258 .
Their star formation histories were uniformly measured by us from HST/ACS observations .
All the galaxies are situated well beyond the Local Group and the two dSphs KKR 25 and KKs 03 are extremely isolated .
Evidently , the evolution of these objects has proceeded without influence of neighbours .