More connected, more active: galaxy clusters and groups at z~1 and the connection between their quiescent galaxy fractions and large-scale environments (1910.10975v1)

Abstract: High-redshift galaxy clusters, unlike local counterparts, show diverse star formation activities. However, it is still unclear what keeps some of the high-redshift clusters active in star formation. To address this issue, we performed a multi-object spectroscopic (MOS) observation of 226 high-redshift (0.8 < z < 1.3) galaxies in galaxy cluster candidates and the areas surrounding them. Our spectroscopic observation reveals six to eight clusters/groups at z ~ 0.9 and z ~ 1.3. The redshift measurements demonstrate the reliability of our photometric redshift measurements, which in turn gives credibility for using photometric redshift members for the analysis of large-scale structures (LSSs). Our investigation of the large-scale environment (~10 Mpc) surrounding each galaxy cluster reveals LSSs --- structures up to ~10 Mpc scale --- around many of, but not all, the confirmed overdensities and the cluster candidates. We investigate the correlation between quiescent galaxy fraction of galaxy overdensities and their surrounding LSSs, with a larger sample of ~ 20 overdensities including photometrically selected overdensities at 0.6 < z < 0.9. Interestingly, galaxy overdensities embedded within these extended LSSs show a lower fraction of quiescent galaxies (~ 20 %) than isolated ones at similar redshifts (with a quiescent galaxy fraction of ~ 50 %). Furthermore, we find a possible indication that clusters/groups with a high quiescent galaxy fraction are more centrally concentrated. Based on these results, we suggest that LSSs are the main reservoirs of gas and star-forming galaxies to keep galaxy clusters fresh and extended in size at z ~ 1.

• The paper uses spectroscopic observations to show that galaxy overdensities within large-scale structures at z~1 have a significantly lower fraction of quiescent galaxies (~20%) than isolated clusters (~50%) at the same redshift.
• This suggests that large-scale structures may sustain star formation in high-redshift galaxy clusters, acting as reservoirs that challenge typical environmental quenching mechanisms.
• The study underscores the critical role of the large-scale environment in understanding galaxy evolution and the properties of clusters at high redshift.

Overview of "More Connected, More Active: Galaxy Clusters and Groups at $z \sim 1$ and The Connection between Their Quiescent Galaxy Fractions and Large-scale Environments"

The paper by Lee et al. investigates the star formation activities in high-redshift galaxy clusters and groups, specifically focusing on their connection to large-scale structures (LSSs) at redshifts around $z \sim 1$. Through multi-object spectroscopic observations, the authors analyze a sample of 226 high-redshift galaxies to identify six to eight clusters or groups and examine their large-scale environments.

Key Findings and Analysis

The primary findings of the paper underscore the varied star formation activities among high-redshift galaxy clusters compared to their local counterparts. The paper identifies significant structures at $z \sim 0.9$ and $z \sim 1.3$, validating photometric redshift measurements with their spectroscopic data. The spectroscopic confirmation reveals multiple clusters and groups, demonstrating the effectiveness of combining photometric techniques with spectroscopic data to trace LSSs.

A critical aspect of the paper is the correlation between the quiescent galaxy fraction in galaxy overdensities and their surrounding LSSs. The analysis reveals that galaxy overdensities embedded within extended LSSs exhibit a lower fraction of quiescent galaxies (approximately 20%) compared to isolated clusters at similar redshifts, which exhibit around 50% quiescent fractions. This observation suggests that LSSs serve as reservoirs for gas and star-forming galaxies, sustaining the freshness and extended nature of galaxy clusters at high redshift.

Methodology

The paper employs a robust methodology utilizing the multi-object spectroscopy to trace high-redshift clusters, ensuring reliable redshift determination. The authors identify clusters by examining velocity gaps and applying a 3-$\sigma$ clipping to isolate significant structures. They further employ a friend-of-friends algorithm to analyze the spatial distribution of galaxies and discern the extent of LSS surrounding each cluster.

The authors effectively correlate the star formation rate with large-scale environmental conditions, providing a nuanced understanding of the relationship between galactic environments and star formation. The paper emphasizes the importance of considering large-scale environmental conditions when studying the evolution and properties of galaxy clusters.

Implications and Future Research

This paper has significant implications for understanding the mechanisms driving star formation in galaxy clusters and the role of LSSs in this process. The findings suggest that the presence of LSSs could facilitate gas inflow or sustained star formation activity, challenging the expected environmental quenching at high densities. The insight garnered from this research could inform models of galaxy evolution, highlighting the need to consider interactions with surrounding structures and the potential for galaxies to undergo rejuvenation in dynamic environments.

Future research could explore the mechanisms by which LSSs influence star formation activities within clusters, potentially investigating the role of dark matter and gravitational interactions. As more comprehensive datasets become available, leveraging machine learning techniques could uncover further correlations and underlying patterns in galaxy evolution processes. The paper sets a foundational understanding upon which further exploration can build, particularly in theorizing the complex interplay between galaxies and their large-scale cosmic environments.

In conclusion, Lee et al. contribute valuable insights into the connection between LSSs and galaxy cluster properties at high redshift, paving the way for more comprehensive interpretations of the cosmic web's role in galaxy formation and evolution.