Star Formation in Massive Galaxies at Redshift $z \sim 0.5$
  It is believed that massive galaxies have quenched their star formation
because of active galactic nucleus feedback. However, recent studies have shown
that some massive galaxies are still forming stars. We analyze the morphology
of star formation regions for galaxies of stellar mass larger than 10$^{11.3}$
M$_{\odot}$ at around redshift $z_r=0.5$ using $u-z$ color images. We find that
about $20\%$ of the massive galaxies are star-forming (SF) galaxies, and most
of them ($\sim 85\%$) have asymmetric structures induced by recent mergers.
Moreover, for these asymmetric galaxies, we find that the asymmetry of the SF
regions becomes larger for bluer galaxies. Using the Illustris simulation, we
can qualitatively reproduce the observed relation between asymmetry parameter
and color. Furthermore, using the merger trees in the simulation, we find a
correlation between the color of the main branch galaxies at $z_r=0.5$ and the
sum of the Star Formation Rates (SFRs) of the recently accreted galaxies, which
implies that star formation of the accreted galaxies has contributed to the
observed star formation of the massive (host) galaxies (ex situ star
formation). Furthermore, we find two blue and symmetric galaxies, candidates
for massive blue disks, in our observed sample, which indicates that about
$\sim 10\%$ of massive SF galaxies are forming stars in the normal mode of disk
star formation (in situ star formation). With the simulation, we find that the
disk galaxies at $z_r \approx 0.5$ should have experienced few major mergers
during the last 4.3 Gyrs.
