Compact stellar systems in the polar ring galaxies NGC 4650A and NGC
  3808B: Clues to polar disk formation
  Polar ring galaxies (PRGs) are composed of two kinematically distinct and
nearly orthogonal components, a host galaxy (HG) and a polar ring/disk (PR).
The HG usually contains an older stellar population than the PR. The suggested
formation channel of PRGs is still poorly constrained. Suggested options are
merger, gas accretion, tidal interaction, or a combination of both. To
constrain the formation scenario of PRGs, we study the compact stellar systems
(CSSs) in two PRGs at different evolutionary stages: NGC 4650A with
well-defined PR, and NGC 3808B, which is in the process of PR formation. We use
archival HST/WFPC2 imaging. PSF-fitting techniques, and color selection
criteria are used to select cluster candidates. Photometric analysis of the
CSSs was performed to determine their ages and masses using stellar population
models at a fixed metallicity. Both PRGs contain young CSSs ($< 1$ Gyr) with
masses of up to 5$\times$10$^6$M$_\odot$, mostly located in the PR and along
the tidal debris. The most massive CSSs may be progenitors of metal-rich
globular clusters or ultra compact dwarf (UCD) galaxies. We identify one such
young UCD candidate, NGC 3808 B-8, and measure its size of $r_{\rm
eff}=25.23^{+1.43}_{-2.01}$ pc. We reconstruct the star formation history of
the two PRGs and find strong peaks in the star formation rate (SFR $\simeq$
200M$_\odot$/yr) in NGC 3808B, while NGC 4650A shows milder (declining) star
formation (SFR $<$ 10M$_\odot$/yr). This difference may support different
evolutionary paths between these PRGs. The spatial distribution, masses, and
peak star formation epoch of the clusters in NGC 3808 suggest for a tidally
triggered star formation. Incompleteness at old ages prevents us from probing
the SFR at earlier epochs of NGC 4650A, where we observe the fading tail of CSS
formation. This also impedes us from testing the formation scenarios of this
PRG.
