  The properties of Ultra Compact Dwarf (UCD) galaxy candidates in Abell 1689
(z=0.183) are investigated, based on deep high resolution ACS images. A UCD
candidate has to be unresolved, have i<28 (M_V<-11.5) mag and satisfy color
limits derived from Bayesian photometric redshifts. We find 160 UCD candidates
with 22<i<28 mag. It is estimated that about 100 of these are cluster members,
based on their spatial distribution and photometric redshifts. For i>26.8 mag,
the radial and luminosity distribution of the UCD candidates can be explained
well by Abell 1689's globular cluster (GC) system. For i<26.8 mag, there is an
overpopulation of 15 +/- 5 UCD candidates with respect to the GC luminosity
function. For i<26 mag, the radial distribution of UCD candidates is more
consistent with the dwarf galaxy population than with the GC system of Abell
1689. The UCD candidates follow a color-magnitude trend with a slope similar to
that of Abell 1689's genuine dwarf galaxy population, but shifted fainter by
about 2-3 mag. Two of the three brightest UCD candidates (M_V ~ -17 mag) are
slightly resolved. At the distance of Abell 1689, these two objects would have
King-profile core radii of ~35 pc and r_eff ~300 pc, implying luminosities and
sizes 2-3 times those of M32's bulge. Additional photometric redshifts obtained
with late type stellar and elliptical galaxy templates support the assignment
of these two resolved sources to Abell 1689. Our findings imply that in Abell
1689 there are at least 10 UCDs with M_V<-12.7 mag. Compared to the UCDs in the
Fornax cluster they are brighter, larger and have colors closer to normal dwarf
galaxies. This suggests that they may be in an intermediate stage of the
stripping process. Spectroscopy is needed to definitely confirm the existence
of UCDs in Abell 1689.
