Overdensity of submillimeter galaxies around the $z\simeq 2.3$ MAMMOTH-1 nebula

TL;DR

This study uses submillimeter observations to identify and analyze an overdense region of galaxies around a high-redshift nebula, revealing a significant increase in obscured star-forming activity and pinpointing the protocluster core.

Contribution

First detailed submillimeter survey of a high-redshift protocluster region, confirming overdensity and characterizing obscured star formation and AGN activity.

Findings

Source counts at 850 μm are 4 times higher than blank fields.

Detected a ULIRG with significant infrared luminosity and obscured AGN.

Brightest sources are located at the density peak of the overdensity.

Abstract

In the hierarchical model of structure formation, giant elliptical galaxies form through merging processes within the highest density peaks known as protoclusters. While high-redshift radio galaxies usually pinpoint the location of these environments, we have recently discovered at z~2-3 three Enormous (>200 kpc) Lyman-Alpha Nebulae (ELANe) that host multiple AGN and that are surrounded by overdensities of Lyman-alpha Emitters (LAE). These regions are prime candidates of massive protoclusters in the early stages of assembly. To characterize the star-forming activity within these rare structures - both on ELAN and protocluster scales - we have initiated an observational campaign with the JCMT and the APEX telescopes. In this paper we report on sensitive SCUBA-2/JCMT 850 and 450 $\mu$m observations of a 128 arcmin$^2$ field comprising the ELAN MAMMOTH-1, together with the peak of the hosting BOSS1441 LAE overdensity at z=2.32. These observations unveil $4.0\pm1.3$ times higher source counts at 850 $\mu$m with respect to blank fields, likely confirming the presence of an overdensity also in obscured tracers. We find a strong detection at 850 $\mu$m associated with the continuum source embedded within the ELAN MAMMOTH-1, which - together with the available data from the literature - allow us to constrain the spectral energy distribution of this source to be of an ULIRG with a far-infrared luminosity of $L_{\rm FIR}^{\rm SF}=2.4^{+7.4}_{-2.1}\times10^{12}$ L$_{\odot}$, and hosting an obscured AGN. Such a source is thus able to power the hard photoionization plus outflow scenario depicted in Cai et al. (2017b) to explain the extended Lyman-alpha, HeII$\lambda1640$ and CIV$\lambda1549$ emission, and their kinematics. In addition, the two brightest detections at 850 $\mu$m ($f_{850}>18$ mJy) sit at the density peak of the LAEs overdensity, likely pinpointing the core of the protocluster.