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Evidence for overdensity around z_(em) > 4 quasars from the proximity effect

Guimarães, R. and Petitjean, P. and Rollinde, E. and de Carvalho, R. R. and Djorgovski, S. G. and Srianand, R. and Aghaee, A. and Castro, S. (2007) Evidence for overdensity around z_(em) > 4 quasars from the proximity effect. Monthly Notices of the Royal Astronomical Society, 377 (2). pp. 657-666. ISSN 0035-8711. doi:10.1111/j.1365-2966.2007.11624.x.

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We study the density field around z_(em) > 4 quasars using high-quality medium spectral resolution Echelle Spectrograph and Imager–Keck spectra ( R∼ 4300 , signal-to-noise ratio (S/N) > 25) of 45 high-redshift quasars selected from a total of 95 spectra. This large sample considerably increases the statistics compared to previous studies. The redshift evolution of the mean photoionization rate and the median optical depth of the intergalactic medium (IGM) are derived statistically from the observed transmitted flux and the pixel optical depth probability distribution function, respectively. This is used to study the so-called proximity effect, that is, the observed decrease of the median optical depth of the IGM in the vicinity of the quasar caused by enhanced photoionization rate due to photons emitted by the quasar. We show that the proximity effect is correlated with the luminosity of the quasars, as expected. By comparing the observed decrease of the median optical depth with the theoretical expectation, we find that the optical depth does not decrease as rapidly as expected when approaching the quasar if the gas in its vicinity is part of the standard IGM. We interpret this effect as revealing gaseous overdensities on scales as large as ∼15 h^(−1) Mpc. The mean overdensity is of the order of 2 and 5 within, respectively, 10 and 3 h^(−1) Mpc. If true, this would indicate that high-redshift quasars are located in the centre of overdense regions that could evolve with time into massive clusters of galaxies. The overdensity is correlated with luminosity: brighter quasars show higher overdensities.

Item Type:Article
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de Carvalho, R. R.0000-0002-1283-3363
Djorgovski, S. G.0000-0002-0603-3087
Alternate Title:Evidence for overdensity around zem > 4 quasars from the proximity effect
Additional Information:© 2007 The Authors. Journal compilation © 2007 RAS. Accepted 2007 February 13. Received 2007 February 13; in original form 2006 November 3. RG is supported by a grant from the Brazilian government (CAPES/MEC). AA is supported by a PhD grant from the Ministry of Science, Research & Technology of Iran. PP and RS gratefully acknowledge support from the Indo-French Centre for the Promotion of Advanced Research (Centre Franco-Indien pour la Promotion de la Recherche Avancée) under contract no 3004-3. SGD is supported by the NSF grant AST-0407448, and the Ajax Foundation. Cataloguing of DPOSS and discovery of PSS QSOs were supported by the Norris Foundation and other private donors. We thank E. Thiebaut, and D. Munro for freely distributing his YORICK programming language (available at, which we used to implement our analysis. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. We acknowledge the Keck support staff for their efforts in performing these observations.
Funding AgencyGrant Number
Ajax FoundationUNSPECIFIED
Indo-French Centre for the Promotion of Advanced Research (Centre Franco-Indien pour la Promotion de la Recherche Avancée3004-3
Ministry of Science, Research and Technology (Iran)UNSPECIFIED
Kenneth T. and Eileen L. Norris FoundationUNSPECIFIED
Subject Keywords:data analysis; statistical methods; galaxy clusters; intergalactic medium; quasars; absorption lines; dark matter
Issue or Number:2
Record Number:CaltechAUTHORS:GUImnras07
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:11371
Deposited By: Tony Diaz
Deposited On:12 Aug 2008 16:36
Last Modified:08 Nov 2021 21:57

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