This paper is the second in a series presenting the results of our deep Hα-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 Hα emitters (HAE). 17 out of the 68 are newly discovered protocluster members. First, a very high characteristic stellar mass of M * = 10 11.73 M 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 > 10 11 M ) 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 UV J plane. Moreover, our deep narrow-band data succeed in probing the bright Hα (+[Nii]) line nebula of the Spiderweb galaxy extending over ∼ 100 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.

Recent simulations predict that the presence of stellar bulge suppress the efficiency of star formation in early-type galaxies, and this "morphological quenching" scenario is supported by many observations. In this study, we discuss the net effect of galaxy morphologies on the star formation efficiency (SFE) during the phase of galaxy transition, on the basis of our CO(J = 1 − 0) observations of 28 local "green-valley" galaxies with the Nobeyama 45m Radio Telescope. We observed 13 "disk-dominated" and 15 "bulge-dominated" green-valley galaxies at fixed stellar mass (M * ) and star formation rate (SFR), supplemented by 1 disk-and 6 bulge-dominated galaxies satisfying the same criteria from the xCOLD GASS survey. By using a total of 35 green-valley galaxies, we reveal that the distributions of molecular gas mass, molecular gas fraction, and SFE of green-valley galaxies do not change with their morphologies, suggesting little impact of galaxy morphologies on their SFE, and interestingly this result is also valid for normal star-forming galaxies on the SF main-sequence selected from the xCOLD GASS galaxies. On the other hand, we find that ∼20% of bulge-dominated green-valley galaxies do not show significant CO emission line, showing high SFEs for their M * and SFR. These molecular gas deficient sources identified only in the bulge-dominated green-valley galaxies may represent an important population during the quenching phase under the influence of stellar bulge, but our results suggest that the presence of stellar bulge does not decrease the efficiency of on-going star formation, in contrast to the prediction of the morphological quenching scenario.

Studying the evolution of dwarf galaxies can provide insights into the characteristics of systems that can act as building blocks of massive galaxies. This paper discusses the history of star formation and gas flows (inflow and outflow) of a dwarf irregular galaxy in the Local Group, NGC 6822, from the viewpoint of gas-phase and stellar chemical abundance. Gas-phase oxygen abundance, stellar metallicity distribution and gas fraction data are compared to chemical evolution models in which continuous star formation and gas flows are assumed. If the galaxy is assumed to be a closed or an accretion-dominated system where steeper stellar initial mass functions are allowed, the observed gas-phase oxygen abundance and gas fraction can be explained simultaneously; however metallicity distributions predicted by the models seem to be inconsistent with the observed distribution, which suggests that the star formation, gas flows and/or chemical enrichment are more complex than assumed by the models. When NGC 6822 is assumed to be a system dominated by outflow, the observed values of gas-phase oxygen abundance and gas fraction can be explained, and the metallicity distributions predicted by some of the models are also roughly consistent with the observed distribution in the metallicity range of −2.0 [Fe/H] −0.5. It should be noted that this result does not necessarily mean that the accretion of gas is completely ruled out. More observables, such as chemical abundance ratios, and detailed modelling may provide deeper insight into the evolution of the system.