Multiband photometric decomposition of nuclear stellar disks

Context: Small, bright stellar disks with scale lengths of a 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 during a merging (or acquisition) event or piled up by the secular evolution of a nuclear bar. Only a few of them have been studied in detail to date. Aims: Using archival Hubble Space Telescope (HST) imaging, we investigate the photometric parameters of the nuclear stellar disks hosted by three early-type galaxies in the Virgo cluster, NGC 4458, NGC 4478, and NGC 4570, to constrain the process that forms their stars. Methods: 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. Results: 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 Surveys 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 the properties of the host spheroid. Conclusions: We interpret the absence of color gradients in the stellar population of the nuclear disks as the signature that star formation homogeneously occurred along their length. An inside-out formation scenario is, instead, expected to produce color gradients and is therefore ruled out.