Dependence of the Fundamental Plane of Early-type Galaxies on Age and
  Internal Structure
  We investigate the scatter in the fundamental plane (FP) of early-type
galaxies (ETGs) and its dependence on age and internal structure of ETGs, using
$16,283$ ETGs with $M_r\le-19.5$ and $0.025\le z<0.055$ in Sloan Digital Sky
Survey data. We use the relation between the age of ETGs and photometric
parameters such as color, absolute magnitude, and central velocity dispersion
of ETGs and find that the scatter in the FP depends on age. The FP of old ETGs
with age $\gtrsim9$ Gyrs has a smaller scatter of $\sim0.06$ dex ($\sim14\%$)
while that of young ETGs with age $\lesssim6$ Gyrs has a larger scatter of
$\sim0.075$ dex ($\sim17\%$). In the case of young ETGs, less compact ETGs have
a smaller scatter in the FP ($\sim0.065$ dex; $\sim15\%$) than more compact
ones ($\sim0.10$ dex; $\sim23\%$). On the other hand, the scatter in the FP of
old ETGs does not depend on the compactness of galaxy structure. Thus, among
the subpopulations of ETGs, compact young ETGs have the largest scatter in the
FP. This large scatter in compact young ETGs is caused by ETGs that have low
dynamical mass-to-light ratio ($M_\mathrm{dyn}/L$) and blue color in the
central regions. By comparing with a simple model of the galaxy that has
experienced a gas-rich major merger, we find that the scenario of recent
gas-rich major merger can reasonably explain the properties of the compact
young ETGs with excessive light for a given mass (low $M_\mathrm{dyn}/L$) and
blue central color.
