DISCOVERY OF A VERY LARGE STRUCTURE ATZ= 3.78

Abstract

We report the discovery of a large-scale structure containing multiple protoclusters at z = 3.78 in the Boötes field. The spectroscopic discovery of five galaxies at z = 3.783±0.002 lying within 1 Mpc of one another led us to undertake a deep narrow-and broad-band imaging survey of the surrounding field. Within a comoving volume of 72 × 72 × 25 Mpc 3 , we have identified 65 Lyman alpha emitter (LAE) candidates at z = 3.795 ± 0.015, and four additional galaxies at z spec = 3.730, 3.753, 3.780, 3.835. The galaxy distribution within the field is highly non-uniform, exhibiting three large (≈ 3 − 5×) overdensities separated by 8 − 14 Mpc (physical) and possibly connected by filamentary structures traced by LAEs. The observed number of LAEs in the entire field is nearly twice the average expected in field environments, based on estimates of the Lyα luminosity function at these redshifts. We estimate that by z = 0 the largest overdensity will grow into a cluster of mass ≈ 10 15 M ⊙ ; the two smaller overdensities will grow into clusters of mass (2 − 6) × 10 14 M ⊙ . The highest concentration of galaxies is located at the southern end of the image, suggesting that the current imaging may not map the true extent of the large scale structure. Finding three large protocluster candidates within a single 0.3 deg 2 field is highly unusual; expectations from theory suggest that such alignments should occur less than 2% of the time. Searching for and characterizing such structures and accurately measuring their volume space density can therefore place constraints on the theory of structure formation. Such regions can also serve as laboratories for the study of galaxy formation in dense environments.

Discovery of a Protocluster Associated with a Lyα Blob Pair at z = 2.3

“…An overdensity is found by Saito et al (2015) around the radio galaxy TN J1338-1942 at redshift 3.1, with δ Σ = 2.8 ± 0.5. At z = 3.78, two or three overdensities with similar δ Σ values are found by Lee et al (2014); Dey et al (2016), with δ Σ = 2.5−2.8. The largest overdensity by far lies in the SSA22 field (Steidel et al 2000), with a surface density contrast δ Σ = 5 ± 2 (Steidel et al 2000; Matsuda et al 2004Matsuda et al , 2005Yamada et al 2012).…”

THE CANDIDATE CLUSTER AND PROTOCLUSTER CATALOG (CCPC) OF SPECTROSCOPICALLY IDENTIFIED STRUCTURES SPANNING 2.74 < z < 3.71

“…They found that galaxy overdensities peak at a radius ∼6−7 cMpc, with distributions that flattened out at R 10 cMpc (see their Figure 7). A protocluster identified by Prescott et al (2008) at z = 2.75 extended 20×50 cMpc, while Lee et al (2014) found an incredibly large structure of three protoclusters within 72 72 25´cMpc 3 at z 3.8 . These examples illustrate the importance of wide search radii in identifying these extended, perhaps filamentary, structures.…”

“…At these higher redshifts, there have been a number of amazing discoveries of groups of galaxies found at z 4 (Venemans et al 2002;Lee et al 2014), z 6 > (Utsumi et al 2010;Toshikawa et al 2014), and up to z 8 (Trenti et al 2012). The emergence of these protoclusters offer a test of ΛCDM, as simulations provide constraints on the number of clusters and on structure mass as a function of redshift and the dark energy equation of state (Vikhlinin et al 2009;Mortonson et al 2011), with greater leverage provided at larger redshifts.…”

THE CANDIDATE CLUSTER AND PROTOCLUSTER CATALOG (CCPC). II. SPECTROSCOPICALLY IDENTIFIED STRUCTURES SPANNING 2 < z < 6.6

“…This is a large structure, but it is not unique. For example, Lee et al (2014) found three overdensities at z = 3.78 within a 75 × 75 × 25 cMpc 3 volume. Dey et al (2016) provided follow-up spectroscopic coverage to that extended system, which may be part of a filament stretching ∼ 170 cMpc.…”