NOAO >   Observing Info >   Approved Programs >   2017A-0254

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Program Information for 2017A-0254


PI: Kyoung-Soo Lee, Purdue U., soolee@purdue.edu
Address: Physics Department, 525 Northwestern Avenue, West Lafayette, Indiana 47907 United States

CoI: Ke Shi, Purdue U.
CoI: Rui Xue, Purdue U.
CoI: Arjun Dey, NOAO

Title: Measuring the True Extent of a Coma Cluster Analog at Z=3.78

Abstract: A recently discovered structure, dubbed as PC~217.96+32.3, is currently the highest redshift Coma cluster analog at $z=3.78$, observed when the universe was only 1.6 billion years old. A sensitive narrow- band imaging survey was conducted over a 1.2deg$times.6deg ($approx 150times75$ comoving Mpc) region enabling a complete mapping of the large-scale structure surrounding this `protocluster' traced by Ly$alpha$ emitters (LAEs). Based on the existing imaging and spectroscopic data, we have firmly established that: (1) the LAE overdensity is one of the largest known, with an estimated combined mass of $>10^M_odot$, which will evolve into a Coma-like cluster in the present-day universe; (2) the LAE selection is very robust; and (3) the kinematics of the known members suggests that the protocluster may be in the process of coalescing. The distribution of LBG candidates over the full field is remarkably similar to that of LAEs, suggesting that the two populations trace the same large-scale structure. Intriguingly, near the core of the protocluster, the LBG distribution is more extended than that traced by the LAEs by $approx-4~Mpc (physical) further west. Based on this fact, combined with the existing spectroscopic measurements, we speculate that the protocluster is even larger than that traced by LAEs. The lack of LAEs there may be explained if i) the structure extends at an angle from our viewpoint such that the western end falls out of our LAE selection function; or ii) there exists an age gradient or complex spatially varying formation history within the structure. The main limitation has been to robustly measure the level and extent of the LBG overdensity, which requires a higher source density to reduce shot noise and cosmic variance. We request Mayall Mosaic 3 time to improve the sensitivity of our existing optical dataset. The proposed data will play a central role in quantifying the spatial extent of this extremely rare structure, which will provide a remarkable crucible for the study of cluster formation.

Program Type: Standard/Extragalactic

Scheduled Nights:
Run 1 (2017A):  KP-4m/MOSA3 -- 3.5n on Jun 21 - Jun 27 2017


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