A search for galaxies in and around an HI overdense region at z = 5

Abstract

We present the discovery of a large‐scale structure of emission‐line galaxies at redshift z= 4.86 behind a massive cluster of galaxies, A1689. Previous spectroscopic observations of a galaxy, A1689−7.1 at z= 4.87, near this structure, revealed a possible overdense region of intergalactic medium (IGM) around the galaxy, which extends at least ∼80 comoving Mpc along the line of sight. In order to investigate whether this z∼ 5 IGM overdense region contains a galaxy overdensity, we undertook narrow‐ and broad‐band imaging observations around A1689−7.1 with Subaru/Suprime‐Cam. We detected 51 candidates as Lyα emitters at redshift z= 4.86 ± 0.03 in the 32 × 24 arcmin2 field of view. After correction for lensing by the foreground cluster, we found a large‐scale (∼20 × 60 comoving Mpc) overdense region of galaxies around A1689−7.1 in the source plane at z= 4.86. The densest peak in this region has an overdensity of δ∼ 4, suggesting that this structure is probably a good candidate for a protocluster which may evolve into a massive cluster of galaxies in the present‐day Universe. A1689−7.1 is located at the edge of this region, where the local galaxy density is ∼1.6 times the mean density and is close to the density contrast in the IGM along the line of sight to A1689−7.1 estimated from the optical depth. The overdensities of galaxies we have found may suggest that at least some parts of the IGM overdense region have already started to form galaxies and moreover they relate to the formation of a protocluster. Although we lack information on the three‐dimensional distributions of both IGM and galaxy overdense regions, the similarity of the scales of both regions may suggest that the two are parts of a single large‐scale structure, which would be an large edge‐on sheet along the line of sight with a size of ∼20 × 60 × 80 comoving Mpc.

Mapping the Most Massive Overdensity Through Hydrogen (Mammoth). I. Methodology

“…The progenitors of such galaxy clusters can be identified at high redshifts at z>2, given large-scale (∼10-40 h −1 Mpc) density contrasts compared to random fields (e.g., Hu et al 1996;Steidel et al 1998;Matsuda et al 2005Matsuda et al , 2010Ouchi et al 2005;Venemans et al 2007). At high redshift, galaxies are believed to interact with the surrounding IGM, and galaxies assemble their gas from the intergalactic H I gas Frye et al 2008;Matsuda et al 2010;Rudie et al 2012;Tejos et al 2014). A large-scale mass overdensity is associated with an H I reservoir in the IGM, which in turn traces a large galaxy overdensity.…”

“…We choose the scale of 15 h −1 Mpc because this scale gives a high correlation between the transmitted flux and mass overdensity (Figure 1). Furthermore, 15 h −1 Mpc is the typical extent of the large-scale galaxy overdensities at z>2 (e.g., Steidel et al 1998Steidel et al , 2005Ouchi et al 2005;Matsuda et al 2010;Chiang et al 2013). We also provide the results on the scales of 10 and 20 h −1 Mpc in the appendix.…”

“…The Lyα optical depth is twice higher than the mean optical depth at z = 4.9 on a large scale of 80 h −1 comoving Mpc. Matsuda et al (2010) conducted deep narrowband imaging centered on this unusual absorption, which indeed revealed a high concentration of Lyα galaxies at z = 4.87 on a large scale. The structure expands over a region of ∼20 comoving Mpc × 60 comoving Mpc on the sky with a galaxy overdensity of δ∼4.…”

An enhanced merger fraction within the galaxy population of the SSA22 protocluster atz = 3.1

“…Such protoclusters have been detected around massive high-redshift radio galaxies at z ≥ 2 (Le Fevre et al 1996;Venemans et al 2002Venemans et al , 2004Venemans et al , 2005Venemans et al , 2007Stern et al 2003;Hatch et al 2009;Matsuda et al 2009). However, protoclusters have also been identified 'blindly' as significant overdensities of Lyman-break galaxies (LBGs) and Ly α emitters (LAEs) in regions where no massive radio galaxy is detected (Steidel et al 1998;Steidel, Adelberger & Shapley 2000;Ouchi et al 2005;Matsuda et al 2010). Protoclusters could have formed due to the preferential accretion of gas collapsing on to dark matter filaments and nodes, leading to the more rapid formation of stars and galaxies compared to average density regions at a similar redshift (Matsuda et al 2005).…”

PANORAMIC SURVEY OF Lyα EMITTERS ATz= 3.1

“…The sensitive narrowband imaging observations have succeeded to detect a large number of the Lyα emitters at intermediate and high redshift toward z ∼ 7 (e.g., Hu & McMahon 1996;Steidel et al 2000;Rhoads & Malhotra 2001;Rhoads et al 2003;Kodaira et al 2003;Palunas et al 2004;Hayashino et al 2004;Matsuda et al 2004Matsuda et al , 2010Matsuda et al , 2011Taniguchi et al 2005;Gronwall et al 2007;Iye et al 2006;Ouchi et al 2008Ouchi et al , 2010.…”