arXiv subjects

Andrei Mesinger

Publications and source records attributed to Andrei Mesinger.

At least 145 records · Page 8Linked to original sources

arXiv2016

Upper Limits on the 21 cm Power Spectrum at z = 5.9 from Quasar Absorption Line Spectroscopy

We present upper limits on the 21 cm power spectrum at $z = 5.9$ calculated from the model-independent limit on the neutral fraction of the intergalactic medium of $x_{\rm H{\small I }} < 0.06 + 0.05\ (1σ)$ derived from dark pixel statistics of quasar absorption spectra. Using 21CMMC, a Markov chain Monte Carlo Epoch of Reionization analysis code, we explore the probability distribution of 21 cm power spectra consistent with this constraint on the neutral fraction. We present 99 per cent confidence upper limits of $Δ^2(k) < 10$ to $20\ {\rm mK}^2$ over a range of $k$ from 0.5 to $2.0\ h{\rm Mpc}^{-1}$, with the exact limit dependent on the sampled $k$ mode. This limit can be used as a null test for 21 cm experiments: a detection of power at $z=5.9$ in excess of this value is highly suggestive of residual foreground contamination or other systematic errors affecting the analysis.

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arXiv2016

Dark-ages reionization & galaxy formation simulation IV: UV luminosity functions of high-redshift galaxies

In this paper we present calculations of the UV luminosity function from the Dark-ages Reionization And Galaxy-formation Observables from Numerical Simulations (DRAGONS) project, which combines N-body, semi-analytic and semi-numerical modelling designed to study galaxy formation during the Epoch of Reionization. Using galaxy formation physics including supernova feedback, the model naturally reproduces the UV LFs for high-redshift star-forming galaxies from $z{\sim}5$ through to $z{\sim}10$. We investigate the luminosity--star formation rate (SFR) relation, finding that variable SFR histories of galaxies result in a scatter around the median relation of $0.1$--$0.3$ dex depending on UV luminosity. We find close agreement between the model and observationally derived SFR functions. We use our calculated luminosities to investigate the luminosity function below current detection limits, and the ionizing photon budget for reionization. We predict that the slope of the UV LF remains steep below current detection limits and becomes flat at $M_\mathrm{UV}{\gtrsim}{-14}$. We find that $48$ ($17$) per cent of the total UV flux at $z{\sim}6$ ($10$) has been detected above an observational limit of $M_\mathrm{UV}{\sim}{-17}$, and that galaxies fainter than $M_\mathrm{UV}{\sim}{-17}$ are the main source of ionizing photons for reionization. We investigate the luminosity--stellar mass relation, and find a correlation for galaxies with $M_\mathrm{UV}{<}{-14}$ that has the form $M_*{\propto}10^{-0.47M_\mathrm{UV}}$, in good agreement with observations, but which flattens for fainter galaxies. We determine the luminosity--halo mass relation to be $M_\mathrm{vir}{\propto}10^{-0.35M_\mathrm{UV}}$, finding that galaxies with $M_\mathrm{UV}{=}{-20}$ reside in host dark matter haloes of $10^{11.0\pm 0.1}\mathrm{M_\odot}$ at $z{\sim}6$, and that this mass decreases towards high redshift.

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arXiv2016

Constraining High Redshift X-ray Sources with Next Generation 21 cm Power Spectrum Measurements

We use the Fisher matrix formalism and semi-numerical simulations to derive quantitative predictions of the constraints that power spectrum measurements on next-generation interferometers, such as the Hydrogen Epoch of Reionization Array (HERA) and the Square Kilometre Array (SKA), will place on the characteristics of the X-ray sources that heated the high redshift intergalactic medium. Incorporating observations between $z=5$ and $z=25$, we find that the proposed 331 element HERA and SKA phase 1 will be capable of placing $\lesssim 10\%$ constraints on the spectral properties of these first X-ray sources, even if one is unable to perform measurements within the foreground contaminated "wedge" or the FM band. When accounting for the enhancement in power spectrum amplitude from spin temperature fluctuations, we find that the observable signatures of reionization extend well beyond the peak in the power spectrum usually associated with it. We also find that lower redshift degeneracies between the signatures of heating and reionization physics lead to errors on reionization parameters that are significantly greater than previously predicted. Observations over the heating epoch are able to break these degeneracies and improve our constraints considerably. For these two reasons, 21\,cm observations during the heating epoch significantly enhance our understanding of reionization as well.

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arXiv2016

Dark-ages Reionization & Galaxy Formation Simulation I: The dynamical lives of high redshift galaxies

We present the Dark-ages Reionization and Galaxy-formation Observables from Numerical Simulations (DRAGONS) program and Tiamat, the collisionless N-body simulation program upon which DRAGONS is built. The primary trait distinguishing Tiamat from other large simulation programs is its density of outputs at high redshift (100 from z=35 to z=5; roughly one every 10 Myr) enabling the construction of very accurate merger trees at an epoch when galaxy formation is rapid and mergers extremely frequent. We find that the friends-of-friends halo mass function agrees well with the prediction of Watson et al. at high masses, but deviates at low masses, perhaps due to our use of a different halo finder or perhaps indicating a break from "universal" behaviour. We then analyse the dynamical evolution of galaxies during the Epoch of Reionization finding that only a small fraction (~20%) of galactic halos are relaxed. We illustrate this using standard relaxation metrics to establish two dynamical recovery time-scales: i) halos need ~1.5 dynamical times following formation, and ii) ~2 dynamical times following a major (3:1) or minor (10:1) merger to be relaxed. This is remarkably consistent across a wide mass range. Lastly, we use a phase-space halo finder to illustrate that major mergers drive long-lived massive phase-space structures which take many dynamical times to dissipate. This can yield significant differences in the inferred mass build-up of galactic halos and we suggest that care must be taken to ensure a physically meaningful match between the galaxy-formation physics of semi-analytic models and the halo finders supplying their input.

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arXiv2016

The Evolution of 21-cm Structure (EOS): public, large-scale simulations of Cosmic Dawn and Reionization

We introduce the Evolution of 21-cm Structure (EOS) project: providing periodic, public releases of the latest cosmological 21-cm simulations. 21-cm interferometry is set to revolutionize studies of the Cosmic Dawn (CD) and epoch of reionization (EoR), eventually resulting in 3D maps of the first billion years of our Universe. Progress will depend on sophisticated data analysis pipelines, which are in turn tested on large-scale mock observations. Here we present the 2016 EOS data release, consisting of the largest (1.6 Gpc on side with a 1024^3 grid), public 21-cm simulations of the CD and EoR. We include calibrated, sub-grid prescriptions for inhomogeneous recombinations and photo-heating suppression of star formation in small mass galaxies. We present two simulation runs that approximately bracket the contribution from faint unseen galaxies. From these two extremes, we predict that the duration of reionization (defined as a change in the mean neutral fraction from 0.9 to 0.1) should be between 2.7 < Delta z < 5.7. The large-scale 21-cm power during the advanced EoR stages can be different by up to a factor of ~10, depending on the model. This difference has a comparable contribution from: (i) the typical bias of sources; and (ii) a more efficient negative feedback in models with an extended EoR driven by faint galaxies. We also make detectability forecasts. With a 1000h integration, HERA and SKA1-low should achieve a signal-to-noise of ~few-hundreds throughout the EoR/CD, while in the maximally optimistic scenario of perfect foreground cleaning, all instruments should make a statistical detection of the cosmic signal. We also caution that our ability to clean foregrounds determines the relative performance of narrow/deep vs. wide/shallow surveys expected with SKA1. Our 21-cm power spectra, simulation outputs and visualizations are publicly available.

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arXiv2016

Cross-correlation of the cosmic 21-cm signal and Lyman Alpha Emitters during reionization

Interferometry of the cosmic 21-cm signal is set to revolutionize our understanding of the Epoch of Reionization (EoR), eventually providing 3D maps of the early Universe. Initial detections however will be low signal-to-noise, limited by systematics. To confirm a putative 21-cm detection, and check the accuracy of 21-cm data analysis pipelines, it would be very useful to cross-correlate against a genuine cosmological signal. The most promising cosmological signals are wide-field maps of Lyman alpha emitting galaxies (LAEs), expected from the Subaru Hyper-Suprime Cam (HSC) Ultra-Deep field. Here we present estimates of the correlation between LAE maps at z~7 and the 21-cm signal observed by both the Low Frequency Array (LOFAR) and the planned Square Kilometer Array Phase 1 (SKA1). We adopt a systematic approach, varying both: (i) the prescription of assigning LAEs to host halos; and (ii) the large-scale structure of neutral and ionized regions (i.e. EoR morphology). We find that the LAE-21cm cross-correlation is insensitive to (i), thus making it a robust probe of the EoR. A 1000h observation with LOFAR would be sufficient to discriminate at >1 standard deviation a fully ionized Universe from one with a mean neutral fraction of xHI~0.50, using the LAE-21cm cross-correlation function on scales of R~3-10 Mpc. Unlike LOFAR, whose detection of the LAE-21cm cross-correlation is limited by noise, SKA1 is mostly limited by ignorance of the EoR morphology. However, the planned 100h wide-field SKA1-Low survey will be sufficient to discriminate an ionized Universe from one with xHI~0.25, even with maximally pessimistic assumptions.

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arXiv2015

Dark-ages Reionization & Galaxy Formation Simulation II: Spin and concentration parameters for dark matter haloes during the Epoch of Reionization

We use high resolution N-Body simulations to study the concentration and spin parameters of dark matter haloes in the mass range $10^8\, {\rm M}_{\odot}\, h^{-1} < {\rm M} < 10^{11}\, {\rm M}_{\odot}\, h^{-1}$ and redshifts $5{<}z{<}10$, corresponding to the haloes of galaxies thought to be responsible for reionization. We build a sub-sample of equilibrium haloes and contrast their properties to the full population that also includes unrelaxed systems. Concentrations are calculated by fitting both NFW and Einasto profiles to the spherically-averaged density profiles of individual haloes. After removing haloes that are out-of-equilibrium, we find a $z{>}5$ concentration$-$mass ($c(M)$) relation that is almost flat and well described by a simple power-law for both NFW and Einasto fits. The intrinsic scatter around the mean relation is $Δc_{\rm{vir}}{\sim1}$ (or 20 per cent) at $z=5$. We also find that the analytic model proposed by Ludlow et al. reproduces the mass and redshift-dependence of halo concentrations. Our best-fit Einasto shape parameter, $α$, depends on peak height, $ν$, in a manner that is accurately described by $α{=}0.0070ν^2{+}0.1839$. The distribution of the spin parameter, $λ$, has a weak dependence on equilibrium state; $λ$ peaks at roughly ${\sim}0.033$ for our relaxed sample, and at ${\sim}0.04$ for the full population. The spin--virial mass relation has a mild negative correlation at high redshift.

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arXiv2015

Constraints on the temperature of the intergalactic medium at z=8.4 with 21-cm observations

We compute robust lower limits on the spin temperature, $T_{\rm S}$, of the $z=8.4$ intergalactic medium (IGM), implied by the upper limits on the 21-cm power spectrum recently measured by PAPER-64. Unlike previous studies which used a single epoch of reionization (EoR) model, our approach samples a large parameter space of EoR models: the dominant uncertainty when estimating constraints on $T_{\rm S}$. Allowing $T_{\rm S}$ to be a free parameter and marginalizing over EoR parameters in our Markov Chain Monte Carlo code 21CMMC, we infer $T_{\rm S}\ge3 {\rm K}$ (corresponding approximately to $1σ$) for a mean IGM neutral fraction of $\bar{x}_{\rm H{\scriptsize I}}\gtrsim0.1$. We further improve on these limits by folding-in additional EoR constraints based on: (i) the dark fraction in QSO spectra, which implies a strict upper limit of $\bar{x}_{\rm H{\scriptsize I}}[z=5.9]\leq 0.06+0.05 \,(1σ)$; and (ii) the electron scattering optical depth, $τ_{\rm e}=0.066\pm0.016\,(1σ)$ measured by the Planck satellite. By restricting the allowed EoR models, these additional observations tighten the approximate $1σ$ lower limits on the spin temperature to $T_{\rm S} \ge 6$ K. Thus, even such preliminary 21-cm observations begin to rule out extreme scenarios such as `cold reionization', implying at least some prior heating of the IGM. The analysis framework developed here can be applied to upcoming 21-cm observations, thereby providing unique insights into the sources which heated and subsequently reionized the very early Universe.

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arXiv2015

PAPER-64 Constraints On Reionization II: The Temperature Of The z=8.4 Intergalactic Medium

We present constraints on both the kinetic temperature of the intergalactic medium (IGM) at z=8.4, and on models for heating the IGM at high-redshift with X-ray emission from the first collapsed objects. These constraints are derived using a semi-analytic method to explore the new measurements of the 21 cm power spectrum from the Donald C. Backer Precision Array for Probing the Epoch of Reionization (PAPER), which were presented in a companion paper, Ali et al. (2015). Twenty-one cm power spectra with amplitudes of hundreds of mK^2 can be generically produced if the kinetic temperature of the IGM is significantly below the temperature of the Cosmic Microwave Background (CMB); as such, the new results from PAPER place lower limits on the IGM temperature at z=8.4. Allowing for the unknown ionization state of the IGM, our measurements find the IGM temperature to be above ~5 K for neutral fractions between 10% and 85%, above ~7 K for neutral fractions between 15% and 80%, or above ~10 K for neutral fractions between 30% and 70%. We also calculate the heating of the IGM that would be provided by the observed high redshift galaxy population, and find that for most models, these galaxies are sufficient to bring the IGM temperature above our lower limits. However, there are significant ranges of parameter space that could produce a signal ruled out by the PAPER measurements; models with a steep drop-off in the star formation rate density at high redshifts or with relatively low values for the X-ray to star formation rate efficiency of high redshift galaxies are generally disfavored. The PAPER measurements are consistent with (but do not constrain) a hydrogen spin temperature above the CMB temperature, a situation which we find to be generally predicted if galaxies fainter than the current detection limits of optical/NIR surveys are included in calculations of X-ray heating.

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arXiv2015

Optimal core baseline design and observing strategy for probing the astrophysics of reionization with the SKA

With the first phase of the Square Kilometre Array (SKA1) entering into its final pre-construction phase, we investigate how best to maximise its scientific return. Specifically, we focus on the statistical measurement of the 21 cm power spectrum (PS) from the epoch of reionization (EoR) using the low frequency array, SKA1-low. To facilitate this investigation we use the recently developed MCMC based EoR analysis tool 21CMMC (Greig & Mesinger). In light of the recent 50 per cent cost reduction, we consider several different SKA core baseline designs, changing: (i) the number of antenna stations; (ii) the number of dipoles per station; and also (iii) the distribution of baseline lengths. We find that a design with a reduced number of dipoles per core station (increased field of view and total number of core stations), together with shortened baselines, maximises the recovered EoR signal. With this optimal baseline design, we investigate three observing strategies, analysing the trade-off between lowering the instrumental thermal noise against increasing the field of view. SKA1-low intends to perform a three tiered observing approach, including a deep 100 deg$^{2}$ at 1000 h, a medium-deep 1000 deg$^{2}$ at 100 h and a shallow 10,000 deg$^{2}$ at 10 h survey. We find that the three observing strategies result in comparable ($\lesssim$ per cent) constraints on our EoR astrophysical parameters. This is contrary to naive predictions based purely on the total signal-to-noise, thus highlighting the need to use EoR parameter constraints as a figure of merit, in order to maximise scientific returns with next generation interferometers.

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arXiv2015

The clustering of Lyman alpha emitters at z=7: implications for reionization and host halo masses

The Ly$α$ line of high-redshift galaxies has emerged as a powerful probe of both early galaxy evolution and the epoch of reionization (EoR). Motivated by the upcoming wide-field survey with the Subaru Hyper Suprime-Cam (HSC), we study the angular correlation function (ACF) of narrow-band selected, $z\approx7$ Ly$α$ emitting galaxies (LAEs). The clustering of LAEs is determined by both: (i) their typical host halo masses, $\bar{M}_{\rm h}$; and (ii) the absorption due to a patchy EoR, characterized by an average neutral fraction of the intergalactic medium, $\bar{x}_{\rm HI}$. We bracket the allowed LAE ACFs by exploring extreme scenarios for both the intrinsic Ly$α$ emission and the large-scale pattern (i.e. morphology) of cosmic ionized patches in physical EoR models. Independent of the EoR morphology, current $z\approx 7$ ACF measurements constrain $\bar{x}_{\rm HI}\lesssim 0.5$ (1-$σ$). We also find that the low values of the currently-observed ACF imply that LAEs are hosted by relatively small dark matter halos: $\bar{M}_{\rm h} \lesssim 10^{10} M_{\odot}$, with corresponding duty cycles of $\lesssim$ few per cent. These values are over an order of magnitude lower than the analogous ones for color-selected, Lyman break galaxies, suggesting that $z\approx 7$ narrow-band LAEs searches are preferentially selecting young, star-burst galaxies, residing in less massive halos. The upcoming Ultra Deep campaign with the HSC will significantly improve constraints on both the EoR and LAE host halos.

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arXiv2015

Early galaxy formation in warm dark matter cosmologies

We present a framework for high-redshift ($z \geq 7$) galaxy formation that traces their dark matter (DM) and baryonic assembly in four cosmologies: Cold Dark Matter (CDM) and Warm Dark Matter (WDM) with particle masses of $m_x =$ 1.5, 3 and 5 ${\rm keV}$. We use the same astrophysical parameters regulating star formation and feedback, chosen to match current observations of the evolving ultra violet luminosity function (UV LF). We find that the assembly of observable (with current and upcoming instruments) galaxies in CDM and $m_x \geq 3 {\rm keV}$ WDM results in similar halo mass to light ratios (M/L), stellar mass densities (SMDs) and UV LFs. However the suppression of small-scale structure leads to a notably delayed and subsequently more rapid stellar assembly in the $1.5 {\rm keV}$ WDM model. Thus galaxy assembly in $m_x \leq 2 {\rm keV}$ WDM cosmologies is characterized by: (i) a dearth of small-mass halos hosting faint galaxies; and (ii) a younger, more UV bright stellar population, for a given stellar mass. The higher M/L ratio (effect ii) partially compensates for the dearth of small-mass halos (effect i), making the resulting UV LFs closer to CDM than expected from simple estimates of halo abundances. We find that the redshift evolution of the SMD is a powerful probe of the nature of DM. Integrating down to a limit of $M_{UV} =-16.5$ for the James Webb Space Telescope (JWST), the SMD evolves as $\log$(SMD)$\propto -0.63 (1+z)$ in $m_x = 1.5 {\rm keV}$ WDM, as compared to $\log$(SMD)$\propto -0.44 (1+z)$ in CDM. Thus high-redshift stellar assembly provides a powerful testbed for WDM models, accessible with the upcoming JWST.

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arXiv2015

21CMMC: an MCMC analysis tool enabling astrophysical parameter studies of the cosmic 21 cm signal

We introduce 21CMMC: a parallelized, Monte Carlo Markov Chain analysis tool, incorporating the epoch of reionization (EoR) seminumerical simulation 21CMFAST. 21CMMC estimates astrophysical parameter constraints from 21 cm EoR experiments, accommodating a variety of EoR models, as well as priors on model parameters and the reionization history. To illustrate its utility, we consider two different EoR scenarios, one with a single population of galaxies (with a mass-independent ionizing efficiency) and a second, more general model with two different, feedback-regulated populations (each with mass-dependent ionizing efficiencies). As an example, combining three observations (z=8, 9 and 10) of the 21 cm power spectrum with a conservative noise estimate and uniform model priors, we find that interferometers with specifications like the Low Frequency Array/Hydrogen Epoch of Reionization Array (HERA)/Square Kilometre Array 1 (SKA1) can constrain common reionization parameters: the ionizing efficiency (or similarly the escape fraction), the mean free path of ionizing photons and the log of the minimum virial temperature of star-forming haloes to within 45.3/22.0/16.7, 33.5/18.4/17.8 and 6.3/3.3/2.4 per cent, ~$1σ$ fractional uncertainty, respectively. Instead, if we optimistically assume that we can perfectly characterize the EoR modelling uncertainties, we can improve on these constraints by up to a factor of ~few. Similarly, the fractional uncertainty on the average neutral fraction can be constrained to within $\lesssim10$ per cent for HERA and SKA1. By studying the resulting impact on astrophysical constraints, 21CMMC can be used to optimize (i) interferometer designs; (ii) foreground cleaning algorithms; (iii) observing strategies; (iv) alternative statistics characterizing the 21 cm signal; and (v) synergies with other observational programs.

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arXiv2015

Probing First Galaxies and Their Impact on the Intergalactic Medium through the 21-cm Observation of the Cosmic Dawn with the SKA

We present an overview of the theory of high-redshift star and X-ray source formation, and how they affect the 21-cm background. Primary focus is given to Lyman alpha pumping and X-ray heating mechanisms at cosmic dawn, opening a new observational window for high-redshift astrophysics by generating sizable fluctuations in the 21-cm background. We describe observational prospects for power spectrum analysis and 3D tomography (imaging) of the signature of these early astrophysical sources by SKA1-LOW and SKA2.

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arXiv2015

Cosmology from the EoR/Cosmic Dawn with the SKA

The SKA will build upon early detections of the EoR by precursor instruments, such as MWA, PAPER, and LOFAR, and planned instruments, such as HERA, to make the first high signal-to-noise measurements of fluctuations in the 21 cm brightness temperature from both reionization and the cosmic dawn. This will allow both imaging and statistical maps of the 21cm signal at redshifts z = 6 - 27 and constrain the underlying cosmology and evolution of the density field. This era includes nearly 60% of the (in principle) observable volume of the Universe and many more linear modes than the CMB, presenting an opportunity for SKA to usher in a new level of precision cosmology. This optimistic picture is complicated by the need to understand and remove the effect of astrophysics, so that systematics rather than statistics will limit constraints. This chapter describes the cosmological, as opposed to astrophysical, information available to SKA. Key areas for discussion include: cosmological parameters constraints using 21cm fluctuations as a tracer of the density field; lensing of the 21cm signal, constraints on heating via exotic physics such as decaying or annihilating dark matter; impact of fundamental physics such as non-Gaussianity or warm dark matter on the source population; and constraints on the bulk flows arising from the decoupling of baryons and photons at z = 1000. The chapter explores the path to separating cosmology from astrophysics, for example via velocity space distortions and separation in redshift. We discuss new opportunities for extracting cosmology made possible by the sensitivity of SKA Phase 1 and explores the advances achievable with SKA2.

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arXiv2015

HI tomographic imaging of the Cosmic Dawn and Epoch of Reionization with SKA

We provide an overview of 21cm tomography of the Cosmic Dawn and Epoch of Reionization as possible with SKA-Low. We show why tomography is essential for studying CD/EoR and present the scales which can be imaged at different frequencies for the different phases of SKA- Low. Next we discuss the different ways in which tomographic data can be analyzed. We end with an overview of science questions which can only be answered by tomography, ranging from the characterization of individual objects to understanding the global processes shaping the Universe during the CD/EoR

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arXiv2015

Epoch of Reionization modelling and simulations for SKA

In this chapter we provide an overview of the current status of the simulations and modelling of the Cosmic Dawn and Epoch of Reionization. We discuss the modelling requirements as dictated by the characteristic scales of the problem and the SKA instrumental properties and the planned survey parameters. Current simulations include most of the relevant physical processes. They can follow the full nonlinear dynamics and are now reaching the required scale and dynamic range, although small-scale physics still needs to be included at sub-grid level. However, despite a significant progress in developing novel numerical methods for efficient utilization of current hardware they remain quite computationally expensive. In response, a number of alternative approaches, particularly semi-analytical/semi-numerical methods, have been developed. While necessarily more approximate, if appropriately constructed and calibrated on simulations they could be used to quickly explore the vast parameter space available. Further work is still required on including some physical processes in both simulations and semi-analytical modelling. This hybrid approach of fast, approximate modelling calibrated on numerical simulations can then be used to construct large libraries of reionization models for reliable interpretation of the observational data.

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arXiv2015

Constraining the Astrophysics of the Cosmic Dawn and the Epoch of Reionization with the SKA

The Square Kilometre Array (SKA) will offer an unprecedented view onto the early Universe, using interferometric observations of the redshifted 21cm line. The 21cm line probes the thermal and ionization state of the cosmic gas, which is governed by the birth and evolution of the first structures in our Universe. Here we show how the evolution of the 21cm signal will allow us to study when the first generations of galaxies appeared, what were their properties, and what was the structure of the intergalactic medium. We highlight qualitative trends which will offer robust insights into the early Universe.

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