A systematic search for galaxy protocluster cores at the transition epoch of their star formation activity

Abstract

The redshift of z ∼ 1.5 is the transition epoch of protoclusters (PCs) from the star-forming phase into the quenching phase, and hence an appropriate era to investigate the build up of the quenched population. We define a ‘core’ as the most massive halo in a given PC, where environmental effects are likely to work most effectively, and search for cores at 1 < z < 1.5. We use a photometric redshift catalogue of a wide (effective area of ∼22.2 deg2) and deep (i ∼ 26.8 mag) optical survey with Subaru Hyper-Suprime Cam. Regarding galaxies with log (M*/M⊙) > 11.3 as the central galaxies of PC cores, we estimate their average halo mass by clustering analysis and find it to be log (Mh/M⊙) ∼ 13.7. An expected mass growth by the IllustrisTNG simulation and the observed overdensities around them suggest that the PC cores we find are progenitors of present-day clusters. Classifying our galaxy sample into red and blue galaxies, we calculate the stellar mass function (SMF) and the red galaxy fraction. The SMFs in the PC cores are more-top heavy than field, implying early high-mass galaxy formation and disruption of low-mass galaxies. We also find that the red fraction increases with stellar mass, consistent with stellar-mass dependent environmental quenching recently found at z > 1. Interestingly, although the cores with red and blue centrals have similar halo masses, only those with red centrals show a significant red fraction excess compared to the field, suggesting a conformity effect. Some observational features of PC cores may imply that the conformity is caused by assembly bias.

The physical origin of galactic conformity: from theory to observation

“…Hartley et al (2015) found strong evidence of small scale galactic conformity at high redshifts (𝑧 ≲ 2) in the UKIRT Infrared Deep Sky Survey (UKIDSS; Lawrence et al 2007). Kawinwanichakij et al (2016) analysed data from Lawrence et al (2007); McCracken et al (2012) and found small scale conformity at high redshifts (see also Ando et al 2022). Overall, the small scale galactic conformity is considered relatively well understood, although some studies challenge its presence (e.g.…”

Groups and Protocluster Candidates in the CLAUDS and HSC-SSP Joint Deep Surveys

“…The galaxies that we link to each group are supposed to be mostly located within the halo's virial radius (r 180 ). Those systems in our S1, S2, and S3 samples can be regarded as the cores of the protoclusters, as also shown in Ando et al (2022), so we check the available matter (or galaxies) surrounding them. If the mass and/or number of galaxies within and surrounding the group are sufficient to build a redshift z = 0 cluster with mass 10 14 h −1 M e , we regard the group as a protocluster candidate.…”

“…Although a few of the group catalogs using HSC data have been constructed with different methods, such as red-sequence galaxies (Oguri et al 2017) or weaklensing techniques (Miyazaki et al 2018;Hamana et al 2020;Oguri et al 2021), and a system of galaxy protoclusters at z ∼ 4 has been searched using a sample of g-dropout galaxies selected from the wide fields (Toshikawa et al 2018), only a few group or cluster catalogs have been constructed for the deep fields in particular (e.g., the updated data version of Oguri et al 2017). Ando et al (2022) searched for cores of protoclusters at 1 < z < 1.5, using photometric data from the HSC-SSP wide and deep fields. In addition to the grizy fiveband photometries, the U band contained in the CFHT Large Area U-band Deep Survey (CLAUDS; Sawicki et al 2019) allows the bracketing of the Balmer and 4000 Å breaks at intermediate redshift, which improves the performance of the photometric redshift obtained from spectral energy distribution (SED) fitting (e.g., Connolly et al 1995;Sawicki et al 1997Sawicki et al , 2019.…”

Groups and protocluster candidates in the CLAUDS and HSC-SSP joint deep surveys

“…The galaxies we linked to each group are supposed to be mostly located within the halo virial radius (r 180 ). Those systems in our S1, S2 and S3 samples can be regarded as the cores of protoclusters as also shown in Ando et al (2022), then we check the available matter (or galaxies) surrounding them. If the mass and/or the number of galaxies within and surrounding the group are sufficient to build a redshift z = 0 cluster with mass 10 14 h −1 M , we regard it as a protocluster candidate.…”

“…Although a few group catalogs using HSC data have been constructed with different methods like redsequence galaxies (Oguri et al 2017) or weak lensing technique (Miyazaki et al 2018;Hamana et al 2020;Oguri et al 2021), and a system of galaxy protoclusters at z ∼ 4 was searched using g-dropout galaxies sample selected from the Wide fields (Toshikawa et al 2018), a few group or cluster catalogs were constructed particularly for the Deep fields (e.g., an updated data version of Oguri et al 2017). Ando et al (2022) searched the cores of protoclusters at 1 < z < 1.5 using a photometric data from HSC-SSP wide and deep fields. In addition to the grizy five band photometries, the U -band contained in CLAUDS surveys (Sawicki et al 2019) allows bracketing the Balmer and 4000 Å breaks at intermediate redshift that improve the performances of photometric redshift obtained from spectral energy distribution (SED) fitting (e.g., Connolly et al 1995;Sawicki et al 1997Sawicki et al , 2019.…”