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 .
We estimate that about 100 of these are cluster members , based on their spatial distribution and photometric redshifts .
For i \gtrsim 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 \lesssim 26.8 mag , there is an overpopulation of 15 \pm 5 UCD candidates with respect to the GC luminosity function .
For i \lesssim 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 } \simeq - 17 mag ) are slightly resolved .
At the distance of Abell 1689 , these two objects would have King-profile core radii of \simeq 35 pc and r _ { eff } \simeq 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 , but also allow for up to 4 foreground stars among the 6 brightest UCD candidates .
Our findings imply that in Abell 1689 there are \geq 10 UCDs with M _ { V } < -12.7 mag , probably created by stripping “ normal ” dwarf or spiral galaxies .
Compared to the UCDs in the Fornax cluster – the location of their original discovery – 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 .
Checking the photometric redshifts of the brightest UCD candidates with spectroscopy would be the next step in order to definitely confirm the existence of UCDs in Abell 1689 .