MAHALO Deep Cluster Survey II. Characterizing massive forming galaxies in the Spiderweb protocluster at z=2.2

TL;DR

This study uses deep narrow-band imaging to identify and analyze massive Hα-emitting galaxies in the Spiderweb protocluster at z=2.2, revealing high stellar masses, AGN activity, and extended nebulae, shedding light on galaxy evolution in dense environments.

Contribution

First detailed characterization of massive galaxies in the Spiderweb protocluster using deep Hα imaging, including discovery of new members and insights into AGN activity and galaxy quenching.

Findings

High characteristic stellar mass of 10^{11.73} M_sun.

Four out of six massive HAEs host AGNs, indicated by X-ray emission.

Extended Hα nebula around the Spiderweb galaxy over 100 kpc.

Abstract

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.