MAHALO Deep Cluster Survey II. Characterizing massive forming galaxies
  in the Spiderweb protocluster at z=2.2
  This paper is the second in a series presenting the results of our deep
H$\alpha$-line survey towards protoclusters at $z>2$, based on narrow-band
imaging with the Subaru Telescope. This work investigates massive galaxies in a
protocluster region associated with a radio galaxy (PKS 1138$-$262), the
Spiderweb galaxy, at $z=2.2$. Our 0.5 mag deeper narrow-band imaging than
previous surveys collects a total of 68 H$\alpha$ emitters (HAE). 17 out of the
68 are newly discovered protocluster members. First, a very high characteristic
stellar mass of M$_\star^\ast=10^{11.73}$ M$_\odot$ is measured from a
Schechter function fit to the mass distribution of HAEs. Together with the
Chandra X-ray data, we find that four out of six massive HAEs
(M$_\star>10^{11}$ M$_\odot$) show bright X-ray emission, suggesting that they
host active galactic nuclei (AGNs). Their mass estimates, therefore, would be
affected by the nuclear emission from AGNs. Notably, the X-ray detected HAEs
are likely positioned near the boundary between star-forming and quiescent
populations in the rest-frame $UVJ$ plane. Moreover, our deep narrow-band data
succeed in probing the bright H$\alpha$ (+[Nii]) line nebula of the Spiderweb
galaxy extending over $\sim100$ physical kpc. These results suggest that the
massive galaxies in the Spiderweb protocluster are on the way to becoming the
bright red sequence objects seen in local galaxy clusters, where AGNs might
play an essential role in their quenching processes. Though a more statistical
database is needed to build a general picture.
