We present observations of 77 early-type galaxies imaged with the PC1 CCD of HST +WFPC2 .
“ Nuker law ” parametric fits to the surface brightness profiles are used to classify the central structure into “ core ” or “ power-law ” forms .
Core galaxies are typically rounder than power-law galaxies .
Nearly all power-laws with central ellipticity \epsilon \geq 0.3 have stellar disks , implying that disks are present in most power-laws with \epsilon < 0.3 , but are not visible due to unfavorable geometry .
A few low-luminosity flattened core galaxies also have disks ; these may be transition forms from power-laws to more luminous core galaxies , which lack disks .
Several core galaxies have strong isophote twists interior to their break radii , although power-laws have interior twists of similar physical significance when the photometric perturbations implied by the twists are evaluated .
Central color gradients are typically consistent with the envelope gradients ; core galaxies have somewhat weaker color gradients than power-laws .
Nuclei are found in 29 % of the cores and 60 % of the power-laws .
Nuclei are typically bluer than the surrounding galaxy .
While some nuclei are associated with AGN , just as many are not ; conversely , not all galaxies known to have low-level AGN exhibit detectable nuclei in the broad-band filters .
NGC 4073 and 4382 , are found to have central minima in their intrinsic starlight distributions ; NGC 4382 resembles the double nucleus of M31 .
In general , the peak brightness location is coincident with the photocenter of the core to a typical physical scale < 1 pc .
Five galaxies , however , have centers significantly displaced from their surrounding cores ; these may be unresolved asymmetric double nuclei .
Lastly , as noted by previous authors , central dust is visible in about half of the galaxies .
The presence and strength of dust correlates with nuclear emission , thus dust may outline gas that is falling into the central black hole .
The prevalence of dust and its morphology suggest that dust clouds form , settle to the center , and disappear repeatedly on \sim 10 ^ { 8 } yr timescales .
We discuss the hypothesis that cores are created by the decay of a massive black hole binary formed in a merger .
Apart from their brightness profiles , there are no strong differences between cores and power-laws that demand this scenario ; however , the rounder shapes of cores , their lack of disks , and their reduced color gradients may be consistent with it .