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Issue A&A
Volume 421, Number 1, July I 2004
Page(s) 157 - 174
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20040134



A&A 421, 157-174 (2004)
DOI: 10.1051/0004-6361:20040134

Detection of 20-30 h $\mathsf{^{-1}}$ Mpc-scale galaxy structures embedded in 100 h $\mathsf{^{-1}}$ Mpc-scale structures of quasars and MgII absorbers at $\textit{z}\simeq\mathsf{0.8}$ and $\textit{z} \simeq\mathsf{1.2}$

C. P. Haines1, 2, 3, L. E. Campusano2 and R. G. Clowes4

1  INAF - Osservatorio Astronomico di Capodimonte, v. Moiariello 16, 80131 Napoli, Italy
2  Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
3  Centre for Astrophysics, University of Central Lancashire, Preston PR1 2HE, UK
4  Computational Astrophysics, Department of Computing, University of Central Lancashire, Preston PR1 2HE, UK

(Received 24 January 2003 / Accepted 2 April 2004)

Abstract
We report the finding of two large-scale structures of galaxies in a $40\times35~{\rm arcmin}^{2}$ field embedded in a $25\deg^2$ area where two 100 $~h^{-1}\,{\rm Mpc}$-scale structures of quasars and $\ion{Mg}{ii}$ absorbers at $z\simeq0.8$ and $z\simeq1.2$ have been previously detected. Using deep optical ( V- and I-band) imaging, we are able to select high-redshift ( $0.5\la z\la 1.3$) early-type galaxies as those redder than the cluster red sequence at z=0.5 and having I<23.5. Through comparison with a $35\times35~{\rm arcmin}^{2}$ control field, we find a 30% excess of these red galaxies, corresponding to 563 galaxies across the field. The colour distribution of the excess galaxies shows a coherent peak at 2.7<V-I<3.1 and the magnitude distribution is well described by a Schechter function, showing unambiguously that the galaxy excess is primarily due to a population of early-type galaxies at $z=0.83\pm0.08$. In follow-up NIR imaging of the four best cluster candidates, we identify three clusters at $z=0.8\pm0.1$, each having well defined red sequences of ~ 20 galaxies at $I-K_{\rm s}\simeq3.4$, forming a structure $20~h^{-1}\,{\rm Mpc}$ across. The fourth candidate, corresponding to a previously identified cluster at $z=1.2\pm0.1$, is confirmed with 28  $K_{\rm s}<20.5$ galaxies having $\textit{VIJK}_{\rm s}$ colours consistent with passively-evolving galaxies at $z\simeq1.2$ located within 1 arcmin of the cluster centre. In all four NIR fields we find numerous galaxies with the $\textit{VIJK}_{\rm s}$ colours of passively-evolving galaxies both at $z\simeq0.8$ and $z\simeq1.2$, including regions well away from the clusters. The results of this study support the presence of two sheet-like large-scale galaxy structures extending across the 20-3 $0~h^{-1}\,{\rm Mpc}$ scales explored by the optical data, one "sheet" at $z\simeq0.8$ with three associated clusters, and a second "sheet" at $z\simeq1.2$ with one embedded cluster. Our results confirm that, at least inside the probed region, the two known 100 $~h^{-1}\,{\rm Mpc}$-scale structures at $z\sim1$ of quasars and $\ion{Mg}{ii}$ absorbers also mark out volumes with an enhanced density of galaxies. The finding of two galaxy structures at $z\simeq 1$ on scales of 20-30 $~h^{-1}\,{\rm Mpc}$ within much larger superstructures of quasars and $\ion{Mg}{ii}$ absorbers, firstly establishes the presence of galaxy sheet-like structures at $z\sim1$, and secondly, suggests that they may extend up to 100 $~h^{-1}\,{\rm Mpc}$-scales.


Key words: cosmology: large-scale structure of Universe -- galaxies: clusters: general -- galaxies: evolution -- quasars: general

Offprint request: C. P. Haines, chris@na.astro.it

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