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Radiative Transfer of energetic photons: X-rays and helium ionization in C2-Ray
Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
2012 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 421, no 3, 2232-2250 p.Article in journal (Refereed) Published
Abstract [en]

We present an extension to the short-characteristic ray-tracing and non-equilibrium photoionization code C2-RAY. The new version includes the effects of helium and improved multifrequency heating. The motivation for this work is to be able to deal with harder ionizing spectra, such as from quasar-like sources during cosmic reionization. We review the basic algorithmic ingredients of C2-RAY before describing the changes implemented, which include a treatment of the full on-the-spot (OTS) approximation, secondary ionization, and multifrequency photoionization and heating. We performed a series of tests against equilibrium solutions from CLOUDY as well as comparisons to the hydrogen-only solutions by C2-RAY in the extensive cosmological radiative transfer code comparison project. We show that the full, coupled OTS approximation is more accurate than the simplified, uncoupled one. We find that also with helium and a multifrequency setup, long time-steps (up to ~10 per cent of the recombination time) still give accurate results for the ionization fractions. On the other hand, accurate results for the temperature set strong constraints on the time-step. The details of these constraints depend, however, on the optical depth of the cells. We use the new version of the code to confirm that the assumption made in many reionization simulations, namely that helium is singly ionized everywhere where hydrogen is, is indeed valid when the sources have stellar-like spectra.

Place, publisher, year, edition, pages
2012. Vol. 421, no 3, 2232-2250 p.
Keyword [en]
radiative transfer, methods: numerical, H ii regions, intergalactic medium
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
URN: urn:nbn:se:su:diva-74004DOI: 10.1111/j.1365-2966.2012.20449.xISI: 000302620500030OAI: oai:DiVA.org:su-74004DiVA: diva2:505922
Available from: 2012-02-27 Created: 2012-02-27 Last updated: 2017-12-07Bibliographically approved
In thesis
1. Simulations of Cosmic Reionization: Shapes & Sizes of H II regions around Galaxies and Quasars
Open this publication in new window or tab >>Simulations of Cosmic Reionization: Shapes & Sizes of H II regions around Galaxies and Quasars
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

After the era of recombination, roughly 360 000 years after the big bang (redshift 1100), the universe was neutral, continued to expand and eventually the first gravitationally collapsed structures capable of forming stars, formed. Observations show that approximately 1 billion years later (redshift 6), the Universe had become highly ionized. The transition from a neutral intergalactic medium to a highly ionized one, is called the epoch of Reionization (EoR). Although quasar spectra and polarization power-spectra from cosmic microwave background experiments set some time-constrains on this epoch, the details of this process are currently not known.

New radio telescopes operating at low frequencies aim at measuring directly the neutral hydrogen content between redshifts 6 - 10 via the HI spin-flip line at 21cm. The interpretation of these first measurements is not going to be trivial. Therefore, simulations of the EoR are useful to test the many ill-constrained parameters such as the properties of the sources responsible for reionization. This thesis contributes to such simulations.

It addresses different source models and discusses different measures to quantify their effect on the shapes and sizes of the emerging H II regions. It also presents a new version of the widely used radiative transfer code C2-Ray which is capable of handling the ionizing radiation produced by energetic sources such as quasars. Using this new version we study whether 21cm experiments could detect the signature of a quasar.

We find that different size measures of ionized regions can distinguish between different source models in the simulations and that a topological measure of the ionized fraction field confirms the inside-out (i.e. overdense regions ionize first) reionization scenario. We find that the HII regions from luminous quasars may be detectable in 21cm, but that it might not be possible to distinguish them from the largest HII regions produced by clustered galaxies.

Place, publisher, year, edition, pages
Stockholm: Department of Astronomy, Stockholm University, 2012. 47 p.
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-72409 (URN)978-91-7447-448-0 (ISBN)
Public defence
2012-03-29, lecture room FB42, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 3: Submitted. Available from: 2012-03-07 Created: 2012-02-10 Last updated: 2012-03-05Bibliographically approved

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Friedrich, Martina M.Mellema, Garrelt
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