  Small and bright stellar disks with scale lengths of few tens of parsec are
known to reside in the center of galaxies. They are believed to have formed in
a dissipational process as the end result of star formation in gas either
accreted in a merging (or acquisition) event or piled up by the secular
evolution of a nuclear bar. Only few of them have been studied in detail to
date. Using archival Hubble Space Telescope (HST) imaging, we investigated the
photometric parameters of the nuclear stellar disks hosted by three early-type
galaxies in the Virgo cluster, NGC 4458, NGC4478, and NGC4570. We aimed at
constraining the process of formation of their stars. The central surface
brightness, scale length, inclination, and position angle of the nuclear disks
were derived by adopting the photometric decomposition method introduced by
Scorza & Bender and assuming the disks to be infinitesimally thin and
exponential. The location, orientation, and size of the nuclear disks is the
same in all the images obtained with the Wide Field Planetary Camera 2 and
Advanced Camera for Survey and available in the HST Science Archive. The scale
length, inclination, and position angle of each disk are constant within the
errors in the observed U, B, V, and I passbands, independently of their values
and of the properties of the host spheroid. We interpret the absence of color
gradients in the stellar population of the nuclear disks as the signature that
star formation homogeneously occurred all through their extension. A inside-out
formation scenario is, instead, expected to produce color gradients and
therefore is ruled out.
