NUCLEAR STELLAR DISCS IN EARLY-TYPE GALAXIES - II - PHOTOMETRIC PROPERTIES

Hubble Space Telescope images of two early-type galaxies harbouring bo th nuclear and outer stellar discs are studied in detail. By means of a photometric decomposition, the images of NGC 4342 and 4570 are analy sed and the photometric properties of the nuclear discs investigated. We find a continuity of properties in the parameter space defined by t he central surface brightness mu(0) and the scalelength R-d of discs i n spirals, S0s and embedded discs in ellipticals, in the sense that th e nuclear discs extend the observed disc properties even further towar ds smaller scalelengths and brighter central surface brightnesses. Whe n including the nuclear discs, disc properties span more than four ord ers of magnitude in both scalelength and central surface brightness. T he nuclear discs studied here are the smallest and brightest stellar d iscs known, and as such, they are as extreme in their photometric prop erties as Malin I, when compared with typical galactic discs that obey Freeman's law. We discuss a possible formation scenario in which the double-disc structure observed in these galaxies has been shaped by no w dissolved bars. Based on the fact that the black holes known to exis t in some of these galaxies have masses comparable to those of the nuc lear discs, we explore a possible link between the black holes and the nuclear discs.