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

Abstract

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.

On the Gas Content, Star Formation Efficiency, and Environmental Quenching of Massive Galaxies in Protoclusters at z ≈ 2.0–2.5

“…All these results suggest that we are witnessing the first stages of the wellknown environmental quenching found in lower redshift and/or more evolved clusters (e.g. Tran et al 2009;Alberts et al 2014;Newman et al 2014;Nantais et al 2016;Ji et al 2018;Lee-Brown et al 2017;Noirot et al 2018;Paulino-Afonso et al 2018;Shimakawa et al 2018), which originates the excess of red sources in local galaxy clusters (Dressler 1984).…”

“…The effects of the environment are more evident when looking at the fraction of quenched galaxies. The larger number of massive gas-poor galaxies (ALMA non-detections) in the proto-clusters in comparison to the field (Figures 4 and 7) suggests that these protocluster galaxies are undergoing an accelerated evolution (see also Hayashi et al 2018;Shimakawa et al 2018;Wang et al 2018). These massive gas-poor systems have low gas mass fractions of f gas 6 − 10 % (Figure 4) and red colors ( Figures 5 and 6) which are in agreement with those found for post-starburst and quiescent galaxies at lower redshifts (e.g.…”

“…Although cluster-to-cluster variations, reflecting different evolutionary stages (e.g. Shimakawa et al 2018;Gómez-Guijarro et al 2019) are expected, it is still unclear if the bulk of the population in the aforementioned cluster also have enhanced gas fractions. Unfortunately, deeper ALMA spectroscopic observations would require several hours of on-source time per target, making spectroscopic observations of larger samples prohibitive.…”

“…Some of the typically cited quenching processes in high-density environments are AGN feedback (e.g. Kauffmann et al 2003;Fabian 2012;Shimakawa et al 2018), major and minor mergers (e.g. Davis et al 2018;Maltby et al 2018), ram-pressure stripping (meaning gas being removed from the galaxies by the IGM, e.g.…”

The High-redshift Clusters Occupied by Bent Radio AGN (COBRA) Survey: Follow-up Optical Imaging

“…More recently, radio loud AGNs have been used to trace both low-and high-z clusters and protoclusters (e.g. Blanton et al 2003;Galametz et al 2012;Wylezalek et al 2013Wylezalek et al , 2014Castignani et al 2014;Rigby et al 2014;Blanton et al 2015;Cooke et al 2015Cooke et al , 2016Noirot et al 2016;Paterno-Mahler et al 2017;Shen et al 2017;Noirot et al 2018;Shimakawa et al 2018;Croston et al 2019;Garon et al 2019;Moravec et al 2019;Golden-Marx et al 2019;Moravec et al 2020b). For example, the Clusters Around Radio Loud AGN (CARLA) survey found that ≈ 55% of their 387 radio loud AGNs at 1.3 < z < 3.2 were in overdense environments at the level of 2σ, with 10% of sources in overdense environments at the 5σ level when compared to a background field (Wylezalek et al 2013).…”