Choudhury, Tirthankar Roy

Ciardi, Benedetta

Conselice, Christopher J.

Cooray, Asantha

Cuby, Jean-Gabriel

Datta, Kanan K.

Dayal, Pratika

Fan, Xiaohui

Finkelstein, Steven

Hirata, Christopher

Hutter, Anne

Iliev, Ilian

Jansen, Rolf

Kakiichi, Koki

Koekemoer, Anton

Maio, Umberto

Majumdar, Suman

Malhotra, Sangeeta

Mellema, Garrelt

Mondal, Rajesh

Papovich, Casey

Rhoads, James

Rhodes, Jason

Sahlén, Martin

Schauer, Anna

Takahashi, Keitaro

Ucci, Graziano

Windhorst, Rogier

Zackrisson, Erik

English

University of Groningen

   University of GroningenA proposal to exploit galaxy-21cm synergies to shed light on the Epoch of ReionizationHutter, Anne; Dayal, Pratika; Malhotra, Sangeeta; Rhoads, James; Choudhury, TirthankarRoy; Ciardi, Benedetta; Conselice, Christopher J.; Cooray, Asantha; Cuby, Jean-Gabriel;Datta, Kanan K.Published in:Bulletin of the American Astronomical SocietyIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2019Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Hutter, A., Dayal, P., Malhotra, S., Rhoads, J., Choudhury, T. R., Ciardi, B., ... Zackrisson, E. (2019). Aproposal to exploit galaxy-21cm synergies to shed light on the Epoch of Reionization. Bulletin of theAmerican Astronomical Society, 51(3), [ID 57 (2019)].CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 19-05-2020Astro2020 Science White PaperA proposal to exploit galaxy-21cm synergiesto shed light on the Epoch of ReionizationThematic Areas:  Planetary Systems  Star and Planet Formation Formation and Evolution of Compact Objects  Cosmology and Fundamental Physics Stars and Stellar Evolution  Resolved Stellar Populations and their Environments Galaxy Evolution Multi-Messenger Astronomy and AstrophysicsPrincipal Author:Name: Anne HutterInstitution: Kapteyn Astronomical Institute, University of GroningenEmail: a.k.hutter@rug.nlPhone: +31 50 3634090Co-authors: (names and institutions)Pratika Dayal (Kapteyn Astronomical Institute, University of Groningen), Sangeeta Malhotra(NASA GSFC), James Rhoads (NASA GSFC), Tirthankar Roy Choudhury (NCRA-TIFR Pune),Benedetta Ciardi (Max Planck Institute for Astrophysics), Christopher J. Conselice (University ofNottingham), Asantha Cooray (University of California, Irvine), Jean-Gabriel Cuby (MarseilleObservatory), Kanan K. Datta (Presidency University Kolkata), Xiaohui Fan (University ofArizona), Steven Finkelstein (The University of Texas at Austin), Christopher Hirata (The OhioState University), Ilian Iliev (University of Sussex), Rolf Jansen (Arizona State University), KokiKakiichi (Department of Physics and Astronomy, University College London), Anton Koekemoer(Space Telescope Science Institute), Umberto Maio (Leibniz-Institute for Astrophysics Potsdam),Suman Majumdar (Indian Institute of Technology Indore), Garrelt Mellema (Department ofAstronomy and Oskar Klein Centre, Stockholm University), Rajesh Mondal (University ofSussex), Casey Papovich (Texas A&M University), Jason Rhodes (NASA JPL), Martin Sahle´n(Department of Physics and Astronomy, Uppsala University), Anna Schauer (The University ofTexas at Austin), Keitaro Takahashi (Faculty of Advanced Science and Technology, KumamotoUniversity), Graziano Ucci (Kapteyn Astronomical Institute, University of Groningen), RogierWindhorst (Arizona State University), Erik Zackrisson (Department of Physics and Astronomy,Uppsala University)1arXiv:1903.03628v1  [astro-ph.CO]  8 Mar 20191 IntroductionThe emergence of the earliest galaxies, a few hundred million years after the Big Bang, ledto a rapid transformation of the homogeneous and largely featureless infant Universe into an in-creasingly complex system. Star formation in these galaxies produced the first photons capable ofionizing the neutral hydrogen (H I ) atoms in the intergalactic medium (IGM) starting the Epochof Reionization (EoR) that marks the last major phase-change of the Universe. The past decadehas witnessed the emergence of a concordance picture in which reionization ended within the firstbillion years of the Universe (Fan et al., 2006; Planck Collaboration et al., 2016). Despite thisenormous progress, the sources, history and topology of reionization (whether reionization pro-ceeded inside-out from the densest to the rarest regions or vice versa) remain key outstandingquestions in the field of physical cosmology (Dayal & Ferrara, 2018). Over the next decade anumber of facilities, most notably the Square Kilometre Array (SKA), that aim to detect H I in theEoR through its 21cm (spin-flip) transition, will be crucial in shedding light on the propagationof ionized regions. However, establishing the veracity of the 21cm signal and understanding theglobal sources and topology of reionization will require combining 21cm data with that from theunderlying galaxy population. One such ideal data set is provided by a class of galaxies knownas Lyman-α Emitters (LAEs), detected by means of their Lyman-α (Lyα) emission line at 1216A˚in the galaxy rest-frame. Lyman-α Emitters are particularly appropriate both to infer the globalIGM ionization state and to study the reionization topology. This is due to (i) the precise red-shifts yielded by the Lyα line that significantly reduce smearing of the signal when correlatinggalaxy positions and 21cm data; and (ii) the preferential location of LAEs in highly ionized re-gions that result in a clear negative correlation with the 21cm signal (e.g. Wyithe & Loeb, 2007;Vrbanec et al., 2016; Hutter et al., 2017; Heneka et al., 2017; Kubota et al., 2018). Deep slitlessspectroscopy with WFIRST, deep narrow-band imaging surveys with the Subaru Hyper Suprime-Cam and CTIO Dark Energy Camera, and wide-field spectroscopy with >25m-class telescopes,such as the European Extremely Large Telescope (E-ELT), Giant Magellan Telescope (GMT), andThirty-Meter Telescope (TMT), will together afford an unprecedented combination of sensitivity,volume, and redshift coverage for studying LAEs in the epoch of reionization. Different facilitiesoffer complementary strengths for this science. The >25m telescopes offer the most sensitive linespectroscopy, but are strongest for z < 7 followup of already-identified sources, where wide fieldoptical spectrographs are most effective. SKA and WFIRST offer continuous redshift coverage,allowing the techniques we discuss to be extended well beyond z = 7. While a number of workshave focused on Subaru-SKA synergies (Hutter et al., 2018; Kubota et al., 2018), here we focuson optimising the synergy between SKA and WFIRST observations to allow a complementary andcompetitive approach to shed light on both the reionization history and topology. Throughout thispaper we assume the standard ΛCDM cosmology with parameter values of ΩΛ = 0.73, Ωm = 0.27,Ωb = 0.047, H0 = 100h = 70km s−1Mpc−1 and σ8 = 0.82.2 Theoretical modelWe use a state-of-the-art model that couples a cosmological smoothed particle hydrodynamic(SPH) simulations run using GADGET-2, an interstellar medium (ISM) dust model (Dayal et al.,2010) and the PCRASH radiative transfer code (Partl et al., 2011) to jointly track the evolutionof reionization (and hence the 21cm emission) and the corresponding LAE population (Hutteret al., 2014). This unique framework can be used, both, to explore the power of combining SKA2and WFIRST observations and to delineate the best survey strategies to maximise such synergies.Focusing on z ∼ 6.6, deep in the reionization era, the hydrodynamical GADGET-2 (Springel, 2005)simulation has a box size of 80h−1 comoving Mpc (cMpc) and contains 2×10243 dark matter and,initially, the same number of gas particles; these correspond to a dark matter (baryonic) massresolution of 3.6 × 107h−1M (6.3 × 106h−1M). The simulation includes physical descriptionsfor star formation, metal production and feedback and uses a Salpeter (Salpeter, 1955) initial massfunction (IMF) between 0.1 − 100M. For each “resolved” galaxy, corresponding to a halo massMh > 109.2M the intrinsic spectrum is derived by summing over all the spectra of its star particlesusing the stellar population synthesis code STARBURST99 (Leitherer et al., 1999). The observedLyα luminosity is computed accounting for attenuation by both ISM dust and IGM H I . Startingfrom a fully neutral IGM (i.e. with a global H I fraction 〈χHI〉= 1), we then run the radiativetransfer code PCRASH on the z ∼ 6.6 snapshot until it is fully reionized. This model yields 21cmbrightness temperatures for all intermediate ionization states (〈χHI〉∼ 1− 10−4). For brevity, herewe focus on results using an ionizing photon escape fraction value of fesc = 0.05 in reasonableagreement with observations (Finkelstein et al., 2019). To approximate potential WFIRST sampleproperties, galaxies with a Lyα equivalent width EWα = Lobsα /Lobsc ≥ 20 A˚ and a Lyα luminosityLα >∼ 1042.5erg s−1 are identified as LAEs for different 〈χHI〉values and we derive the differential21cm brightness temperature fields from the respective ionization field following Iliev et al. (2012).δTb(~x) = T0 〈χHI〉 [1 + δ(~x)] [1 + δHI(~x)] (1)Here, 1 + δ(~x) = ρ(~x)/〈ρ〉 and 1 + δHI(~x) = χHI(~x)/〈χHI〉 refer to the local gas density andH I fraction compared to their corresponding average global values, respectively. The fact that the21cm-LAE correlations are derived for an un-evolving underlying galaxy population then allowsus to disentangle the effects of reionization from galaxy evolution on LAE visibility.3 Strategies to synergise WFIRST-SKA observationsThe distribution of LAEs and 21cm emission: Given the large over-densities required for earlygalaxy formation, most galaxies lie in neutral regions in the initial stages of reionization (〈χHI〉'0.9), as shown in Fig. 1. By the time the IGM is half ionized, many galaxies (possibly thosein clustered regions) are embedded in a fully ionised IGM with χHI ' 10−4, though some stilloccupy neutral regions (χHI ' 1). Galaxies first become visible as LAEs when the IGM becomes(roughly) half ionised, since the Lyα flux gets attenuated by H I in the IGM for higher 〈χHI〉 values.We find z ' 6.6 LAEs to have halo masses >∼ 109.5M (Dayal & Ferrara, 2012). Observablybright LAEs at z > 6 are among the high-mass end of the galaxy population, and lie in the mostover-dense and highly ionized regions (Castellano et al., 2016). LAE hosting regions therefore havea much lower 21cm brightness temperature (compared to regions without LAEs) at any redshiftwhere significant neutral gas remains.Hints on the topology of reionization: Given that observable LAEs occupy the largest halos inthe most ionized regions, the brightness temperature in regions without LAEs (TnoLAE) is generallyhigher than in regions hosting LAEs (Tnogal), and both show a steady decrease as the IGM becomesincreasingly ionized. In terms of SKA observations (Fig. 2), the brightness temperature differencebetween regions with and without LAEs (∆T (LAE)) can be used to robustly differentiate betweenan IGM that is 10% neutral to one that is 50% ionized at these scales, irrespective of the fesc3-1 0 1Log (1 + )-4-202Log T b (mK)HI = 0.9-1 0 1Log (1 + )HI = 0.5-1 0 1Log (1 + )HI = 10 4f esc=0.0510 5 10 4 10 3 10 2 10 1 100probabilityFigure 1: The probability density distribution of the IGM gas as a function of gas over-density(1 + δ) and the 21cm differential brightness temperature (δTb) as the IGM progresses from being90% (left panel) to 50% neutral (central panel) to fully reionized (right panel). The dark and lightred contours show the regions occupied by 90% and 50% of all galaxies, respectively; the darkblue contours show the regions occupied by LAEs. The thick solid orange line shows the meanvalue of Log(δTb) for all cells; the cyan line shows the much lower mean Log(δTb) value in cellshosting LAEs.values used (Hutter et al., 2017). The WFIRST-SKA synergy can be used to verify our predictionof an inside-out reionization scenario, where ionized regions percolate from over- to under-denseregions in the IGM, characterised by a lower differential 21cm brightness temperature in regionsaround LAEs compared to regions not containing LAEs. It also provides a unique tool to trace thetypical size of ionized regions around LAEs by tracking the scale at which the 21cm-LAE crosspower spectrum turns over (Lidz et al., 2009).Hints on the reionization state: To determine the best survey design for detecting the 21cm-LAEcross correlation ξ21,LAE with SKA1-Low, we assume an SKA integration time of 1000h and thearray configuration V4A1. We find that observational uncertainties will be critical in detecting the21cm-LAE cross correlation signal and constraining 〈χHI〉. The key issue is that while uncertain-ties in the 21cm signal detection are reduced by larger survey volumes, the shot noise arising froma finite number of LAEs decreases as we probe to deeper Lyα luminosities (Hutter et al., 2018).As shown in Fig. 3, LAE surveys with large fields of view (∼ 20 deg2) and detecting LAEs withLα >∼ 1042.5 erg s−1 (corresponding to a flux limit of 6 × 10−18 erg cm−2 s−1 at z ' 6.6), com-parable to anticipated medium-deep WFIRST surveys carried out as parts of the WFIRST HighLatitude Spectroscopic Survey, will be optimal to distinguish between an IGM that is 10%, 25%and 50% neutral. Surveys that focus on LAEs brighter than at Lα ∼ 1043erg s−1 will be unableto exploit 21cm-LAE synergies, since LAE number densities become so low that the mitigation of1https://astronomers.skatelescope.org/wp-content/uploads/2015/11/SKA1-Low-Configuration V4a.pdf4051015200 1 2 3 4 5(a)fesc = 0.05T LAE   [ mK ]θ [arcmin]SKA sensitivity 〈 χHI 〉 = 0.50 〈 χHI 〉 = 0.25 〈 χHI 〉 = 0.10 〈 χHI 〉 = 0.01 〈 χHI 〉 = 10-4051015200 1 2 3 4 5(b)fesc = 0.05T noLAE   [ mK ]θ [arcmin]051015200 1 2 3 4 5(c)fesc = 0.05ΔT (LAE)    [ mK ]θ [arcmin]Figure 2: The differential 21cm brightness temperature in regions containing LAEs (left panel),in regions not containing LAEs (central panel), and their difference ∆T = TnoLAE − TLAE (rightpanel) as a function of the smoothing scale θ. In each panel we show the differential brightnesstemperature at different stages of reionization (〈χHI〉= 0.5 - 10−4), as marked, and the black lineshows the SKA imaging sensitivity limits for a 1000h observation. The difference between thebrightness temperature in regions with and without LAES, easily testable using SKA, can un-equivocally test the “inside-out” topology of reionization predicted by models.-0.5-0.4-0.3-0.2-0.10100 101106 107ξ 21,LAE (r = 3.6 h-1Mpc)FoV [ deg2 ]V [ Mpc3 ] Lα = 1042.5-43.5 erg s-1〈 χHI 〉= 0.50〈 χHI 〉= 0.25〈 χHI 〉= 0.10 WFIRSTξ 21,LAE (r = 3.6 h-1Mpc) WFIRSTFigure 3: 21cm-LAE cross correlation functionat r = 3.6h−1cMpc for a survey Lyα luminos-ity limit of Lα = 1042.5erg s−1 for 1000h ofSKA observations. The orange, green and bluelines represent results for 〈χHI〉 ' 0.1, 0.25and 0.5, respectively. The shaded regions showthe cross correlation function uncertainties as afunction of the survey volume of the SKA andLAE observations. The vertical line shows thesurvey area for WFIRST. Surveying an area of20 deg2 to a depth of Lα = 1042.5erg s−1, acorrelation between WFIRST LAEs and SKA21cm observations will be crucial in sheddinglight on the reionization state of the IGM.5the associated shot noise requires field of views exceeding that of SKA. It is important to note that,quantitatively, our results are quite robust to model assumption including the IMF, fesc and dustcontents, given the degeneracy between these parameters (Dayal et al., 2011; Hutter et al., 2014).Finally, a combination of quasar data, from wide field surveys with, e.g., Euclid or WFIRST, andthe 21cm emission detected by SKA will provide complimentary constraints on the local reioniza-tion state in the most over-dense regions at these early epochs (e.g. Datta et al., 2016).Science Recommendation: Over the next decade, 21cm experiments will aim to map out theprogress of reionization. Synergising such observations with those of the underlying galaxy popu-lation (specially LAEs given their precise redshifts) will be absolutely crucial in shedding lights onthe sources and propagation of reionization (i.e. if the process percolated from over- to under-denseregions or vice versa). SKA and WFIRST offer a unique opportunity to apply these methods overan uninterrupted redshift range spanning the end of the reionization era. We therefore recommendthe utmost effort to maximize the synergy between SKA 21cm and WFIRST LAE observations, tofinally understand the physics of the EoR that remains a crucial frontier in the field of astrophysicsand physical cosmology.Ackowledgements:AH and PD acknowledge support from the European Research Council’s start-ing grant ERC StG-717001 (“DELPHI”). PD also acknowledges support from the European Com-mission’s and University of Groningen’s CO-FUND Rosalind Franklin program. 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A proposal to exploit galaxy-21cm synergies to shed light on the Epoch of Reionization

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A proposal to exploit galaxy-21cm synergies to shed light on the Epoch of Reionization

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