Two sub-millimetre bright protoclusters bounding the epoch of peak star-formation activity

TL;DR

This study uses submillimetre observations to identify and analyze two high-redshift protoclusters, revealing significant galaxy over-densities and intense star formation activity concentrated in their cores, shedding light on early cluster formation.

Contribution

First detailed submillimetre survey of two protoclusters at z>2, quantifying galaxy over-densities and star formation rates within their dense cores.

Findings

Detected 56 SMGs with high significance in two protoclusters.

Over-densities of SMGs are 4-6 times higher than blank fields.

Total star formation rates in cores are around 12,500 M$_\ ext{\odot}$ yr$^{-1}$, much higher than the cosmic average.

Abstract

We present James Clerk Maxwell Telescope Submillimetre Common-User Bolometer Array 2 (SCUBA-2) 850 & 450 $\mu$m observations ($\sigma_{850}\sim0.5$ mJy, $\sigma_{450}\sim5$ mJy) of the HS1549+19 and HS1700+64 survey fields containing two of the largest known galaxy over-densities at $z=2.85$ and $2.30$, respectively. We detect 56 sub-millimetre galaxies (SMGs) with SNR $> 4$ over $\sim 50$ arcmin$^2$ at 850 $\mu$m with flux densities of 3 - 17 mJy. The number counts indicate over-densities in the $3$-arcmin diameter core region ($\sim 1.5$ Mpc at $z=2.5$) of $6^{+4}_{-2}\times$ (HS1549) and $4^{+6}_{-2}\times$ (HS1700) compared to blank field surveys. Within these core regions, we spectroscopically confirm that approximately one third of the SMGs lie at the protocluster redshifts for both HS1549 and HS1700. We use statistical identifications of other SMGs in the wider fields to constrain an additional four candidate protocluster members in each system. We combine multi wavelength estimates of the star-formation rates (SFRs) from Lyman-break dropout- and narrowband-selected galaxies, and the SCUBA-2 SMGs, to estimate total SFRs of 12,$500\pm2800$ M$_\odot$ yr$^{-1}$ ($4900\pm1200$ M$_\odot$ yr$^{-1}$) in HS1549 (HS1700), and SFR densities (SFRDs) within the central 1.5-Mpc diameter of each protocluster to be $3000\pm900$ M$_\odot$ yr$^{-1}$ Mpc$^{-3}$ ($1300\pm400$ M$_\odot$ yr$^{-1}$ Mpc$^{-3}$) in the HS1549 (HS1700) protocluster, $\sim10^4\times$ larger than the global SFRDs found at their respective epochs, due to the concentration of star-forming galaxies in the small volume of the dense cluster cores. Our results suggest centrally concentrated starbursts within protoclusters may be a relatively common scenario for the build up of mass in rich clusters assembling at $z\gtrsim2$.