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

Lacaille, Kevin M. and Chapman, Scott C. and Smail, Ian and Steidel, C. C. and Blain, A. W. and Geach, J. and Golob, A. and Gurwell, M. and Ivison, R. J. and Reddy, N. and Sawicki, M. (2019) 'Two sub-millimetre bright protoclusters bounding the epoch of peak star-formation activity.', Monthly notices of the Royal Astronomical Society., 488 (2). pp. 1790-1812.


We present James Clerk Maxwell Telescope Submillimetre Common-User Bolometer Array 2 (SCUBA-2) 850 & 450 μm observations (σ850 ∼ 0.5 mJy, σ450 ∼ 5 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 ∼50 arcmin2 at 850 μm with flux densities of 3 – 17 mJy. The number counts indicate over-densities in the 3-arcmin diameter core region (∼1.5 Mpc at z = 2.5) of 6+4−2× (HS1549) and 4+6−2× (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 ± 2800 M⊙ yr−1 (4900 ± 1200 M⊙ yr−1) in HS1549 (HS1700), and SFR densities (SFRDs) within the central 1.5-Mpc diameter of each protocluster to be 3000 ± 900 M⊙ yr−1 Mpc−3 (1300 ± 400 M⊙ yr−1 Mpc−3) in the HS1549 (HS1700) protocluster, ∼104 × 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 ≳ 2.

Item Type:Article
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Publisher statement:This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society. ©: 2019 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Date accepted:12 June 2019
Date deposited:18 July 2019
Date of first online publication:26 June 2019
Date first made open access:No date available

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