MAHALO Deep Cluster Survey I. Accelerated and enhanced galaxy formation in the densest regions of a protocluster at z = 2.5

Abstract

We carried out deep Hα narrowband imaging with 10 hours net integrations towards the young protocluster, USS1558−003 at z = 2.53 with the Subaru Telescope. This system is composed of four dense groups with massive local overdensities, traced by 107 Hα emitters (HAEs) with stellar masses and dust-corrected star formation rates down to 1 × 10 8 M and 3 M yr −1 , respectively. We have investigated the environmental dependence of various physical properties within the protocluster by comparing distributions of HAEs in higher and lower densities with a standard Kolmogorov-Smirnov test. At 97% confidence level, we find enhanced star formation across the star-forming main sequence of HAEs living in the most extreme 'supergroup', corresponding to the top quartile of overdensities. Furthermore, we derive distribution functions of Hα luminosity and stellar mass in group and intergroup regions, approximately corresponding to 30 times and 8 times higher densities than the general field. As a consequence, we identify 0.7 and 0.9 dex higher cutoffs in Hα luminosity and stellar mass functions in the dense groups, respectively. On the other hand, HAEs in the intergroup environment of the protocluster show similar distribution functions to those of field galaxies despite residing in significant overdensities. In the early phase of cluster formation, as inferred from our results, the densest parts in the protocluster have had an accelerated formation of massive galaxies. We expect that these eventually grow and transform into early-type galaxies at the bright end of the red sequence as seen in present-day rich clusters of galaxies.

Extended star-forming regions within galaxies in a dense proto-cluster core at z = 2.53

“…NB imaging observations for this proto-cluster were conducted with Subaru/Multi-Object InfraRed Camera and Spectrograph (MOIRCS; Ichikawa et al 2006). So far, 107 Hα emitters are identified in this proto-cluster based on color-color selections (Hayashi et al 2012(Hayashi et al , 2016Shimakawa et al 2018).…”

“…The stellar mass-SFR relation of the Hα emitters in USS1558 is shown in figure 1. Here stellar masses were estimated from SED fitting using seven broad-band photometries, namely, B,r ′ ,z ′ ,J,H,F 160W and Ks, by Shimakawa et al (2018). Because we use a different method to estimate dust extinction for Hα (AHα) from previous studies (Hayashi et al 2012(Hayashi et al , 2016Shimakawa et al 2018), we re-calculate SFRs of the Hα emitters.…”

“…Here stellar masses were estimated from SED fitting using seven broad-band photometries, namely, B,r ′ ,z ′ ,J,H,F 160W and Ks, by Shimakawa et al (2018). Because we use a different method to estimate dust extinction for Hα (AHα) from previous studies (Hayashi et al 2012(Hayashi et al , 2016Shimakawa et al 2018), we re-calculate SFRs of the Hα emitters. SFRs are calculated with the relation between SFRs and Hα luminosities of Kennicutt (1998) considering the difference between Chabrier and Salpeter IMF (Salpeter 1955 .…”

“…However, environmental dependence of structural growth of star-forming galaxies at high redshift has still not been investigated. Some studies on the rest-frame optical or rest-frame UV structures of star-forming galaxies in (proto-)cluster environments at z > 1 have already been carried out (Peter et al 2007;Overzier et al 2008;Wang et al 2016;Allen et al 2017;Kubo et al 2017;Shimakawa et al 2018;Matharu et al 2018;Socolovsky et al 2019). However, it is yet required to directly trace the structures of star-forming regions as well as underlying stars (e.g., Nelson et al 2016b;Tacchella et al 2018;Belfiore et al 2018;Ellison et al 2018) to investigate how star-forming galaxies build up their structures and what physical processes are involved in their evolution.…”

More connected, more active: galaxy clusters and groups at z ∼ 1 and the connection between their quiescent galaxy fractions and large-scale environments

“…The evolution in the quiescent fraction, especially out to z ∼ 1, is closely related to the SF quenching mechanism (e.g., L15; Peng et al 2010), and close investigation of these high redshift clusters can possibly offer insight on what controls SF activities in dense environments. At z > 1, more clusters are being identified with strong SF activity (Brodwin et al 2013;Alberts et al 2014Alberts et al , 2016Shimakawa et al 2018). The insight gathered from z ∼ 1 clusters can help understand the underlying mechanism for the SF activity in clusters at z > 1.…”