We present observations of four nearby early-type galaxies with previously known nuclear stellar disks using two instruments on-board the Hubble Space Telescope .
We observed NGC 4128 , NGC 4612 , and NGC 5308 with the Wide Field Planetary Camera 2 , and the same three galaxies , plus NGC 4570 , with the Space Telescope Imaging Spectrograph .
We have detected a red nucleus in NGC 4128 , a blue nucleus in NGC 4621 , and a blue disk in NGC 5308 .
Additionally , we have discovered a blue disk-like feature with position angle \sim 15 \degr from the major axis in NGC 4621 .
In NGC 5308 there is evidence for a blue region along the minor axis .
We discovered a blue transient on the images of NGC 4128 at position 0 \aas@@fstack { \prime \prime } 14 west and 0 \aas@@fstack { \prime \prime } 32 north from the nucleus .
The extracted kinematic profiles belong to two groups : fast ( NGC 4570 and NGC 5308 ) and kinematically disturbed rotators ( NGC 4128 and NGC 4621 ) .
We report the discovery of a kinematically decoupled core in NGC 4128 .
Galaxies have mostly old ( 10-14 Gyr ) stellar populations with large spread in metallicities ( sub- to super-solar ) .
We discuss the possible formation scenarios , including bar-driven secular evolution and the influence of mergers , which can explain the observed color and kinematic features .