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Large-scale opacity fluctuations in the Ly$\alpha$ forest: evidence for QSOs dominating the ionizing UV background at $z$ ~ 5.5–6?

Published version
Peer-reviewed

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Authors

Chardin, J 
Puchwein, E 
Haehnelt, MG 

Abstract

Lyα forest data probing the post-reionization Universe show surprisingly large opacity fluctuations over rather large (≥50 h−1 comoving Mpc) spatial scales. We model these fluctuations using a hybrid approach utilizing the large-volume Millennium simulation to predict the spatial distribution of QSOs combined with smaller scale full hydrodynamical simulation performed with RAMSES and post-processed with the radiative transfer code ATON. We produce realistic mock absorption spectra that account for the contribution of galaxies and QSOs to the ionizing UV background. These improved models confirm our earlier findings that a significant ($\gtrsim$50 per cent) contribution of ionizing photons from QSOs can explain the large reported opacity fluctuations on large scales. The inferred QSO luminosity function is thereby consistent with recent estimates of the space density of QSOs at this redshift. Our simulations still somewhat struggle, however, to reproduce the very long (110 h−1 comoving Mpc) high-opacity absorption through observed in ULAS J0148+0600, perhaps suggesting an even later end of reionization than assumed in our previously favoured model. Medium-deep/medium area QSO surveys as well as targeted searches for the predicted strong transverse QSO proximity effect would illuminate the origin of the observed large-scale opacity fluctuations. They would allow us to substantiate whether UV fluctuations due to QSO are indeed primarily responsible, or whether significant contributions from other recently proposed mechanisms such as large-scale fluctuations in temperature and mean free path (even in the absence of rare bright sources) are required.

Description

Keywords

methods: numerical, galaxies: evolution, quasars: general, cosmology: theory, diffuse radiation

Journal Title

Monthly Notices of the Royal Astronomical Society

Conference Name

Journal ISSN

0035-8711
1365-2966

Volume Title

465

Publisher

Oxford University Press
Sponsorship
Science and Technology Facilities Council (ST/N000927/1)
Science and Technology Facilities Council (ST/J005673/1)
Science and Technology Facilities Council (ST/K00333X/1)
Science and Technology Facilities Council (ST/M007065/1)
European Research Council (320596)
Science and Technology Facilities Council (ST/P000673/1)
Science and Technology Facilities Council (ST/L000636/1)
STFC (ST/M007073/1)
Science and Technology Facilities Council (ST/P002315/1)
Science and Technology Facilities Council (ST/R002452/1)
This work was supported by the ERC Advanced Grant 320596 ‘The Emergence of Structure during the epoch of Reionization’. This work was performed using the COSMOS and Darwin Supercomputers of the University of Cambridge High Performance Computing Service (http://www.hpc.cam.ac.uk/), provided by Dell Inc. using Strategic Research Infrastructure Funding from the Higher Education Funding Council for England and funding from the Science and Technology Facilities Council. This work furthermore used the Wilkes GPU cluster at the University of Cambridge High Performance Computing Service (http://www.hpc.cam.ac.uk/), provided by Dell Inc., NVIDIA and Mellanox, and part funded by STFC with industrial sponsorship from Rolls Royce and Mitsubishi Heavy Industries. Some of the hydrodynamical simulations used in this work were performed with supercomputer time awarded by the Partnership for Advanced Computing in Europe (PRACE) 8th Call. We acknowledge PRACE for enabling access to the Curie supercomputer, based in France at the Tres Grand Centre de Calcul (TGCC). EP acknowledges support by ` the Kavli foundation. The research of JC and MH was supported by the Munich Institute for Astro- and Particle Physics (MIAPP) of the DFG cluster of excellence ‘Origin and Structure of the Universe’.