Effects of the Sources of Reionization on 21Cm Redshiftspace Distortions
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Effects of the sources of reionization on 21-cm redshift-space distortions Article (Published Version) Majumdar, Suman, Jensen, Hannes, Mellema, Garrelt, Chapman, Emma, Abdalla, Filipe B, Lee, Kai-Yan, Iliev, Ilian T, Dixon, Keri L, Datta, Kanan K, Ciardi, Benedetta, Fernandez, Elizabeth R, Jelić, Vibor, Koopmans, Léon V E and Zaroubi, Saleem (2015) Effects of the sources of reionization on 21-cm redshift-space distortions. Monthly Notices of the Royal Astronomical Society, 456 (2). pp. 2080-2094. ISSN 0035-8711 This version is available from Sussex Research Online: http://sro.sussex.ac.uk/id/eprint/69098/ This document is made available in accordance with publisher policies and may differ from the published version or from the version of record. If you wish to cite this item you are advised to consult the publisher’s version. Please see the URL above for details on accessing the published version. 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Copies of full text items generally can be reproduced, displayed or performed and given to third parties in any format or medium for personal research or study, educational, or not-for-profit purposes without prior permission or charge, provided that the authors, title and full bibliographic details are credited, a hyperlink and/or URL is given for the original metadata page and the content is not changed in any way. http://sro.sussex.ac.uk MNRAS 456, 2080–2094 (2016) doi:10.1093/mnras/stv2812 Effects of the sources of reionization on 21-cm redshift-space distortions Suman Majumdar,1,2‹ Hannes Jensen,1,3‹ Garrelt Mellema,1 Emma Chapman,4 Filipe B. Abdalla,4,5 Kai-Yan Lee,1 Ilian T. Iliev,6 Keri L. Dixon,6 Kanan K. Datta,7 Benedetta Ciardi,8 Elizabeth R. Fernandez,9 Vibor Jelic,´ 9,10,11 Leon´ V. E. Koopmans9 and Saleem Zaroubi9 1Department of Astronomy and Oskar Klein Centre, Stockholm University, AlbaNova, SE-10691 Stockholm, Sweden 2Department of Physics, Blackett Laboratory, Imperial College, London SW7 2AZ, UK 3Department of Physics and Astronomy, Uppsala University, 75120 Uppsala, Sweden 4Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK 5Department of Physics and Electronics, Rhodes University, PO Box 94, Grahamstown 6140, South Africa 6Astronomy Centre, Department of Physics & Astronomy, Pevensey II Building, University of Sussex, Falmer, Brighton BN1 9QH, UK 7Department of Physics, Presidency University, 86/1 College Street, Kolkata 700073, India 8Max-Planck-Institut fur¨ Astrophysik, Karl-Schwarzschild-Strasse 1, D-85748 Garching b. Munchen,¨ Germany 9Kapteyn Astronomical Institute, University of Groningen, PO Box 800, NL-9700 AV Groningen, the Netherlands 10ASTRON – the Netherlands Institute for Radio Astronomy, PO Box 2, NL-7990 AA Dwingeloo, the Netherlands 11Ruđer Boskoviˇ c´ Institute, Bijenickaˇ cesta 54, 10000 Zagreb, Croatia Accepted 2015 November 27. Received 2015 November 8; in original form 2015 September 24 ABSTRACT The observed 21 cm signal from the epoch of reionization will be distorted along the line of sight by the peculiar velocities of matter particles. These redshift-space distortions will affect the contrast in the signal and will also make it anisotropic. This anisotropy contains information about the cross-correlation between the matter density field and the neutral hydrogen field, and could thus potentially be used to extract information about the sources of reionization. In this paper, we study a collection of simulated reionization scenarios assuming different models for the sources of reionization. We show that the 21 cm anisotropy is best measured by the quadrupole moment of the power spectrum. We find that, unless the properties of the reionization sources are extreme in some way, the quadrupole moment evolves very predictably as a function of global neutral fraction. This predictability implies that redshift- space distortions are not a very sensitive tool for distinguishing between reionization sources. However, the quadrupole moment can be used as a model-independent probe for constraining the reionization history. We show that such measurements can be done to some extent by first-generation instruments such as LOFAR, while the SKA should be able to measure the reionization history using the quadrupole moment of the power spectrum to great accuracy. Key words: methods: numerical – dark ages, reionization, first stars. White et al. 2003;Gotoetal.2011). These observations suggest 1 INTRODUCTION that reionization was an extended process, spanning over the red- One of the periods in the history of our Universe about which we shift range 6 z 15 (see e.g. Alvarez et al. 2006; Mitra, Ferrara know the least is the epoch of reionization (EoR). During this epoch & Choudhury 2013;Bouwensetal.2015; Mitra, Choudhury & Fer- the first sources of light formed and gradually ionized the neutral rara 2015;Robertsonetal.2015). Such indirect observations are, hydrogen (H I) in the intergalactic medium (IGM). Our present un- however, limited in their ability to answer several important ques- derstanding of this epoch is mainly constrained by observations of tions regarding the EoR. These unresolved issues include the precise the cosmic microwave background (CMB) radiation (Komatsu et al. duration and timing of reionization, the properties of major ioniz- 2011; Planck Collaboration XIII 2015) and the absorption spectra ing sources, the relative contribution to the ionizing photon budget of high-redshift quasars (Becker et al. 2001, 2015; Fan et al. 2003; from various kinds of sources, and the typical size and distribution of ionized bubbles. Observations of the redshifted 21 cm line, originating from spin E-mail: [email protected], [email protected] (SM); Hannes. flip transitions in neutral hydrogen atoms, are expected to be the [email protected] (HJ) key to resolving many of these long-standing issues. The brightness C 2015 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society EoR sources & 21cm redshift space distortio 2081 temperature of the redshifted 21 cm line directly probes the H I of quantifying this anisotropy, and study the prospects of distin- distribution at the epoch where the radiation originated. Observing guishing between the source models in observations with current this line enables us, in principle, to track the entire reionization and upcoming interferometers. history as it proceeds with redshift. The structure of this paper is as follows. In Section 2, we describe Motivated by this, a huge effort is underway to detect the red- the simulations and source models that we have used to generate shifted 21 cm signal from the EoR using low-frequency radio inter- a collection of reionization scenarios. Here, we also describe the ferometers, such as the GMRT (Paciga et al. 2013), LOFAR (van method that we use to include the redshift-space distortions in the Haarlem et al. 2013; Yatawatta et al. 2013;Jelicetal.´ 2014), MWA simulated signal. In Section 3, we evaluate different methods of (Bowman et al. 2013;Tingayetal.2013), PAPER (Parsons et al. quantifying the power spectrum anisotropy and discuss the inter- 2014) and 21CMA (Wang et al. 2013). These observations are com- pretation of the anisotropy using the quasi-linear model of Mao plicated to a large degree by foreground emissions, which can be et al. (2012). In Section 4, we describe the observed large-scale ∼4–5 orders of magnitude stronger than the expected signal (e.g. Di features of the redshift-space anisotropy for the different reioniza- Matteo et al. 2002; Ali, Bharadwaj & Chengalur 2008;Jelicetal.´ tion scenarios that we have considered. In Section 5, we discuss the 2008), and system noise (Morales 2005;McQuinnetal.2006). So feasibility of observing these anisotropic signatures in present and far only weak upper limits on the 21 cm signal have been obtained future radio interferometric surveys, and what we can learn from (Paciga et al. 2013; Dillon et al. 2014; Parsons et al. 2014; Ali et al. the redshift-space distortions about the reionization history. Finally, 2015). in Section 6 we summarize our findings. Owing to the low sensitivity of the first generation interferome- Throughout the paper we present our results for the cosmological ters, they will probably not be capable of directly imaging the H I parameters from WMAP five-year data release: h = 0.7, m = 0.27, 2 distribution. This will have to wait for the arrival of the extremely = 0.73, bh = 0.0226 (Komatsu et al. 2009). sensitive next generation of telescopes such as the SKA (Mellema et al. 2013, 2015; Koopmans et al. 2015). The first-generation tele- 2 REIONIZATION SIMULATIONS FOR scopes are instead expected to detect and characterize the signal DIFFERENT SOURCE MODELS through statistical estimators such as the variance (e.g. Patil et al. 2014) and the power spectrum (e.g. Pober et al. 2014). To study the effects of the sources of reionization on the redshift- Many studies to date have focused on the spherically averaged space anisotropy of the 21 cm signal, we simulate a number of dif- 21-cm power spectrum (e.g. McQuinn et al. 2007;Lidzetal.2008; ferent reionization scenarios, assuming different source properties. Barkana 2009; Iliev et al. 2012). By averaging in spherical shells All of these simulations are based on a single N-body simulation of in Fourier space, one can obtain good signal-to-noise for the power the evolving dark matter density field. spectrum, while still preserving many important features of the sig- nal. However, the spherically averaged power spectrum does not 2.1 N-body simulations contain all information about the underlying 21 cm field.