In order to precisely measure the cosmological parameters and answer the fundamental questions in cosmology, it is necessary to develop new, powerful cosmological probes, in addition to the proposed next-generation optical survey projects. The neutral hydrogen 21 cm sky surveys will provide a promising tool to study the late-universe evolution, helping shed light on the nature of dark energy. The Square Kilometre Array is the largest radio telescope in the world to be constructed in the near future, and it will push the 21 cm cosmology into a new era and greatly promote the development of cosmology in the forthcoming decades.
This is a design study into the capabilities of the Australian Square Kilometre Array Pathfinder in performing a full-sky low redshift neutral hydrogen survey, termed WALLABY, and the potential cosmological constraints one can attain from measurement of the galaxy power spectrum. We find that the full sky survey will likely attain 0.6 million redshifts which, when combined with expected Planck CMB data, will constrain the Dark Energy equation of state to 20%, representing a coming of age for radio observations in creating cosmological constraints.
Unveiling the Microhertz Gravitational-Wave Sky with the Square Kilometre Array Observatory
Kavli Institute for Cosmology, University of Cambridge（剑桥大学卡弗里宇宙学研究所）；DAMTP, University of Cambridge（剑桥大学应用数学与理论物理系）；Institut de Fisica d'Altes Energies (IFAE), The Barcelona Institute of Science and Technology（巴塞罗那科学与技术研究院高能物理研究所）；Department of Physics, University of Wisconsin-Madison（威斯康星大学麦迪逊分校物理系）
One of the major science goals of the Square Kilometre Array (SKA) is to understand the role played by atomic hydrogen (HI) gas in the evolution of galaxies throughout cosmic time. The hyperfine transition line of the hydrogen atom at 21-cm is one of the best tools to detect and study the properties of HI gas associated with galaxies. In this article, we review our current understanding of HI gas and its relationship with galaxies through observations of the 21-cm line both in emission and absorption. In addition, we provide an overview of the HI science that will be possible with SKA and its …
The Square Kilometre Array (SKA) will be both the largest radio telescope ever constructed and the largest Big Data project in the known Universe. The first phase of the project will generate on the order of 5 zettabytes of data per year. A critical task for the SKA will be its ability to process data for science, which will need to be conducted by science pipelines. Together with polarization data from the LOFAR Multifrequency Snapshot Sky Survey (MSSS), we have been developing a realistic SKA-like science pipeline that can handle the large data volumes generated by LOFAR at 150 MHz. The pipe…
We review how the Square Kilometre Array (SKA) will address fundamental questions in cosmology, focussing on its use for neutral Hydrogen (HI) surveys. A key enabler of its unique capabilities will be large (but smart) receptors in the form of aperture arrays. We outline the likely contributions of Phase-1 of the SKA (SKA1), Phase-2 SKA (SKA2) and pathfinding activities (SKA0). We emphasise the important role of cross-correlation between SKA HI results and those at other wavebands such as: surveys for objects in the EoR with VISTA and the SKA itself; and huge optical and near-infrared redshift…
Tests of general relativity (GR) are still in their infancy on cosmological scales, but forthcoming experiments promise to greatly improve their precision over a wide range of distance scales and redshifts. One such experiment, the Square Kilometre Array (SKA), will carry out several wide and deep surveys of resolved and unresolved neutral hydrogen (HI) 21cm line-emitting galaxies, mapping a significant fraction of the sky from $0 \le z \lesssim 6$. I present forecasts for the ability of a suite of possible SKA HI surveys to detect deviations from GR by reconstructing the cosmic expansion and …
Cosmic reionization is known to be a major phase transition of the gas in the Universe. Since astronomical objects formed in the early Universe, such as the first stars, galaxies and black holes, are expected to have caused cosmic reionization, the formation history and properties of such objects are closely related to the reionization process. In spite of the importance of exploring reionization, our understandings regarding reionization is not sufficient yet. Square Kilometre Array (SKA) is a next-generation large telescope that will be operated in the next decade. Although several programs …
Fast radio bursts (FRBs) are mysterious extragalactic radio signals. Revealing their origin is one of the central foci in modern astronomy. Previous studies suggest that occurrence rates of non-repeating and repeating FRBs could be controlled by the cosmic stellar-mass density (CSMD) and star formation-rate density (CSFRD), respectively. The Square Kilometre Array (SKA) is one of the best future instruments to address this subject due to its high sensitivity and high-angular resolution. Here, we predict the number of FRBs to be detected with the SKA. In contrast to previous predictions, we est…
One of the most important parameters in characterizing the Epoch of Reionization, the escape fraction of ionizing photons, $f_\mathrm{esc}$, remains unconstrained both observationally and theoretically. With recent work highlighting the impact of galaxy-scale feedback on the instantaneous value of $f_\mathrm{esc}$, it is important to develop a model in which reionization is self-consistently coupled to galaxy evolution. In this work, we present such a model and explore how physically motivated functional forms of $f_\mathrm{esc}$ affect the evolution of ionized hydrogen within the intergalacti…
Continuum and HI surveys with the Square Kilometre Array (SKA) will allow us to probe some of the most fundamental assumptions of modern cosmology, including the Cosmological Principle. SKA all-sky surveys will map an enormous slice of space-time and reveal cosmology at superhorizon scales and redshifts of order unity. We illustrate the potential of these surveys and discuss the prospects to measure the cosmic radio dipole at high fidelity. We outline several potentially transformational tests of cosmology to be carried out by means of SKA all-sky surveys.
We test the bandpass smoothness performance of two prototype Square Kilometre Array (SKA) SKA1-Low log-periodic dipole antennas, the SKALA2 and SKALA3 (`SKA Log-periodic Antenna'), and the current dipole from the Murchison Widefield Array (MWA) precursor telescope. Throughout this paper, we refer to the output complex-valued voltage response of an antenna when connected to a low noise amplifier (LNA), as the dipole bandpass. In Paper I (de Lera Acedo et al. 2017), the bandpass spectral response of the log-periodic antenna being developed for the SKA1-Low was estimated using numerical electroma…
Upcoming 21cm surveys with the SKA1-LOW telescope will enable imaging of the neutral hydrogen distribution on cosmological scales in the early Universe. These surveys are expected to generate huge imaging datasets that will encode more information than the power spectrum. This provides an alternative unique way to constrain the reionization history, which might break the degeneracy in the power spectral analysis. Using Convolutional Neural Networks (CNN), we create a fast estimator of the neutral fraction from the 21cm maps that are produced by our large semi-numerical simulation. Our estimato…
Numerical simulations offer the unique possibility to forecast the results of surveys and targeted observations that will be performed with next generation instruments like the Square Kilometre Array. In this paper, we investigate for the first time how future radio surveys in polarization will be affected by confusion noise. To do this, we produce 1.4 GHz simulated full-Stokes images of the extra-galactic sky by modelling various discrete radio sources populations. The results of our modelling are compared to data in the literature to check the reliability of our procedure. We also estimate t…
Concerted effort is currently ongoing to open up the Epoch of Reionization (EoR) ($z\sim$15-6) for studies with IR and radio telescopes. Whereas IR detections have been made of sources (Lyman-$α$ emitters, quasars and drop-outs) in this redshift regime in relatively small fields of view, no direct detection of neutral hydrogen, via the redshifted 21-cm line, has yet been established. Such a direct detection is expected in the coming years, with ongoing surveys, and could open up the entire universe from $z\sim$6-200 for astrophysical and cosmological studies, opening not only the EoR, but also…
Detection of the redshifted 21-cm signal of neutral hydrogen from the Cosmic Dawn and the Epoch of Reionization is one of the final frontiers of modern observational cosmology. The inherently faint signal makes it susceptible to contamination by several sources like astrophysical foregrounds and instrumental systematics. Nevertheless, developments achieved in the recent times will combine to make signal detection possible with the upcoming Square Kilometer Array (SKA), both statistically and via tomography. This review describes an indigenously developed end-to-end pipeline that simulates sens…
For purposes of stellar population analysis, emission corrections for Balmer series indices on the Lick index system in Sloan Digital Sky Survey (SDSS) stacked quiescent galaxy spectra are derived, along with corrections for continuum shape and gross stellar content, as a function of the Mg $b$ Lick index strength. These corrections are obtained by comparing the observed Lick index measurements of the SDSS with new observed measurements of 13 Virgo Cluster galaxies, and checked with model grids. From the H$α$ Mg $b$ diagram a linear correction for the observed measurement is constructed using …
Recent analyses of 21-cm neutral hydrogen (HI) emission have demonstrated that HI gas is organized into linear filamentary structures that are preferentially aligned with the local magnetic field, and that the coherence of these structures in velocity space traces line-of-sight magnetic field tangling. On this basis, we introduce a paradigm for modeling the properties of magnetized, dusty regions of the interstellar medium, using the orientation of HI structure at different velocities to map "magnetically coherent" regions of space. We construct three-dimensional (position-position-velocity) S…
The distribution of cosmological neutral hydrogen will provide a new window into the large-scale structure of the Universe with the next generation of radio telescopes and surveys. The observation of this material, through 21cm line emission, will be confused by foreground emission in the same frequencies. Even after these foregrounds are removed, the reconstructed map may not exactly match the original cosmological signal, which will introduce systematic errors and offset into the measured correlations. In this paper, we simulate future surveys of neutral hydrogen using the Horizon Run 4 (HR4…
We investigate the potential of the Square Kilometre Array (SKA) for measuring the magnetic fields in clusters of galaxies via Faraday rotation of background polarised sources. [...] We find that about 10 per cent of the sky is covered by a significant extragalactic Faraday screen. Most of it has rotation measures between 10 and 100 rad/m/m. We argue that the cluster centres should have up to about 5000 rad/m/m. We show that the proposed mid frequency aperture array of the SKA as well as the lowest band of the SKA dish array are well suited to make measurements for most of these rotation measu…
We use the Galaxies-Intergalactic Medium Interaction Calculation (GIMIC) cosmological hydrodynamic simulation at z=0 to study the distribution and environmental dependence of neutral hydrogen (HI) gas in the outskirts of simulated galaxies. This gas can currently be probed directly in, for example, Ly$α$ absorption via the observation of background quasars. Radio facilities, such as the Square Kilometre Array, will provide a complementary probe of the diffuse HI in emission and will constrain the physics underpinning the complex interplay between accretion and feedback mechanisms which affect …
As the largest radio telescope in the world, the Square Kilometre Array (SKA) will lead the next generation of radio astronomy. The feats of engineering required to construct the telescope array will be matched only by the techniques developed to exploit the rich scientific value of the data. To drive forward the development of efficient and accurate analysis methods, we are designing a series of data challenges that will provide the scientific community with high-quality datasets for testing and evaluating new techniques. In this paper we present a description and results from the first such …
Theoretical uncertainties on non-linear scales are among the main obstacles to exploit the sensitivity of forthcoming galaxy and hydrogen surveys like Euclid or the Square Kilometre Array (SKA). Here, we devise a new method to model the theoretical error that goes beyond the usual cut-off on small scales. The advantage of this more efficient implementation of the non-linear uncertainties is tested through a Markov-Chain-Monte-Carlo (MCMC) forecast of the sensitivity of Euclid and SKA to the parameters of the standard $Λ$CDM model, including massive neutrinos with total mass $M_ν$, and to 3 ext…
We investigate the contribution that a local over- or under-density can have on linear cosmic dipole estimations. We focus here on radio surveys, such as the NRAO VLA Sky Survey (NVSS), and forthcoming surveys such as those with the LOw Frequency ARray (LOFAR), the Australian Square Kilometre Array Pathfinder (ASKAP) and the Square Kilometre Array (SKA). The NVSS has already been used to estimate the cosmic radio dipole; it was shown recently that this radio dipole amplitude is larger than expected from a purely kinematic effect, assuming the velocity inferred from the dipole of the cosmic mic…
Precise instrument calibration is critical to the success of 21 cm Cosmology experiments. Unmitigated errors in calibration contaminate the Epoch of Reionization (EoR) signal, precluding a detection. Barry et al. 2016 characterizes one class of inherent errors that emerge from calibrating to an incomplete sky model, however it has been unclear if errors in the sky model affect the calibration of redundant arrays. In this paper, we show that redundant calibration is vulnerable to errors from sky model incompleteness even in the limit of perfect antenna positioning and identical beams. These err…
We quantitatively investigate the possibility of detecting baryonic acoustic oscillations (BAO) using single-dish 21cm intensity mapping observations in the post-reionization era. We show that the telescope beam smears out the isotropic BAO signature and, in the case of the Square Kilometer Array (SKA) instrument, makes it undetectable at redshifts $z\gtrsim1$. We however demonstrate that the BAO peak can still be detected in the radial 21cm power spectrum and describe a method to make this type of measurements. By means of numerical simulations, containing the 21cm cosmological signal as well…
Cosmic evolution in the hydrogen content of the Universe through recombination and up to the end of reionization is expected to be revealed as subtle spectral features in the uniform extragalactic cosmic radio background. The redshift evolution in the excitation temperature of the 21-cm spin flip transition of neutral hydrogen appears as redshifted emission and absorption against the cosmic microwave background. The precise signature of the spectral trace from cosmic dawn and the epoch of reionization are dependent on the spectral radiance, abundance and distribution of the first bound systems…
Observations of neutral hydrogen can provide a wealth of information about the distribution and kinematics of galaxies. To detect HI beyond the ionisation edge of galaxy disks, column density sensitivities have to be achieved that probe the regime of Lyman limit systems. Typically HI observations are limited to a brightness sensitivity of NHI~10^19 cm-2 but this has to be improved by at least an order of magnitude. In this paper, reprocessed data is presented that was originally observed for the HI Parkes All Sky Survey (HIPASS). HIPASS provides complete coverage of the region that has been ob…
We investigate the signatures left by massive neutrinos on the spatial distribution of neutral hydrogen (HI) in the post-reionization era by running hydrodynamic simulations that include massive neutrinos as additional collisionless particles. We find that halos in massive/massless neutrino cosmologies host a similar amount of neutral hydrogen, although for a fixed halo mass, on average, the HI mass increases with the sum of the neutrino masses. Our results show that HI is more strongly clustered in cosmologies with massive neutrinos, while its abundance, $Ω_{\rm HI}(z)$, is lower. These effec…
We present a systematic search for molecular hydrogen absorption systems at high redshift in quasar spectra from the Sloan Digital Sky Survey (SDSS) II Data Release 7 and SDSS-III Data Release 9. We have selected candidates using a modified profile fitting technique taking into account that the Ly$α$ forest can effectively mimic H$_2$ absorption systems at the resolution of SDSS data. To estimate the confidence level of the detections, we use two methods: a Monte-Carlo sampling and an analysis of control samples. The analysis of control samples allows us to define regions of the spectral quali…
Aiming to fill a crucial gap in our observational knowledge of the early Universe, experiments around the world continue to attempt to verify the claimed detection of the redshifted 21-cm signal from Cosmic Dawn by the EDGES experiment. This sky-averaged or 'global' signal from neutral hydrogen should be detectable at low radio frequencies (50-200 MHz), but is difficult to measure due to bright foreground emission and difficulties in reaching the required levels of instrumental-calibration precision. In this paper we outline our progress toward using a novel new method to measure the global re…
The low-frequency linearly-polarised radio source population is largely unexplored. However, a renaissance in low-frequency polarimetry has been enabled by pathfinder and precursor instruments for the Square Kilometre Array. In this second paper from the POlarised GaLactic and Extragalactic All-Sky Murchison Widefield Array (MWA) Survey -- the POlarised GLEAM Survey, or POGS -- we present the results from our all-sky MWA Phase I Faraday Rotation Measure survey. Our survey covers nearly the entire Southern sky in the Declination range $-82^{\circ}$ to $+30^{\circ}$ at a resolution between aroun…
Observations of intergalactic neutral hydrogen can provide a wealth of information about structure and galaxy formation, potentially tracing accretion and feedback processes on Mpc scales. Below a column density of NHI ~ 10^19 cm-2, the "edge" or typical observational limit for HI emission from galaxies, simulations predict a cosmic web of extended emission and filamentary structures. We study the distribution of neutral hydrogen and its 21cm emission properties in a cosmological hydrodynamic simulation, to gain more insights into the distribution of HI below NHI ~ 10^19 cm-2. Such Lyman Limit…
The Cosmic Radio Dipole and Observer Effects in SKA all-sky surveys
Institute for Fundamental Physics of the Universe（宇宙基础物理研究所）；Royal Observatory Edinburgh（爱丁堡皇家天文台）；University of Oxford（牛津大学）；Yale University（耶鲁大学）；The University of Sydney（悉尼大学）；Guangdong Technion - Israel Institute of Technology（广东以色列理工学院）
This paper presents a search for radio transients at a frequency of 73.8 MHz (4 m wavelength) using the all-sky imaging capabilities of the Long Wavelength Demonstrator Array (LWDA). The LWDA was a 16-dipole phased array telescope, located on the site of the Very Large Array in New Mexico. The field of view of the individual dipoles was essentially the entire sky, and the number of dipoles was sufficiently small that a simple software correlator could be used to make all-sky images. From 2006 October to 2007 February, we conducted an all-sky transient search program, acquiring a total of 106 h…
In the next few years, the 21cm line will enable direct observations of the Dark Ages, Cosmic Dawn, and Reionization, which represent previously unexplored periods in our cosmic history. With a combination of sky-averaged global signal measurements and spatial mapping surveys, the possible science return is enormous. This potentially includes (but is not limited to) constraints on first-generation galaxies (such as their typical masses and luminosities), constraints on cosmological parameters, a measurement of the Hubble parameter at z~15 to 20, the elimination of the optical depth nuisance pa…
The use of continuum emission radio galaxies as cosmological tracers of the large-scale structure will soon move into a new phase. Upcoming surveys from the Australian Square Kilometre Array Pathfinder (ASKAP), MeerKAT, and the Square Kilometre Array project (SKA) will survey the entire available sky down to an ~100uJy flux limit, increasing the number of detected extra-galactic radio sources by several orders of magnitude. External data and machine learning algorithms will also enable some low resolution radial selection (photometric redshift binning) of the sample, increasing the cosmologica…
The Square Kilometre Array (SKA) is a planned radio interferometer of unprecedented scale that will revolutionize low-frequency radio astronomy when completed. In particular, one of its core science drivers is the systematic mapping of the Cosmic Dawn and Epoch of Reionization, which mark the birth of the first stars and galaxies in the Universe and their subsequent ionization of primordial intergalactic hydrogen, respectively. The SKA will offer the most sensitive view of these poorly understood epochs using the redshifted 21 cm hyperfine signal from intergalactic hydrogen. However, significa…
The low-frequency polarisation properties of radio sources are poorly studied, particularly in statistical samples. However, the new generation of low-frequency telescopes, such as the Murchison Widefield Array (MWA; the precursor for the low-frequency component of the Square Kilometre Array) offers an opportunity to probe the physics of radio sources at very low radio frequencies. In this paper, we present a catalogue of linearly-polarised sources detected at 216 MHz, using data from the Galactic and Extragalactic All-sky MWA (GLEAM) survey. Our catalogue covers the Declination range $-17^{\c…
We investigate the impact of feedback - from supernovae (SNe), active galactic nuclei (AGN) and a photo-ionizing background at high redshifts - on the neutral atomic hydrogen (HI) mass function, the $b_{\rm J}$ band luminosity function, and the spatial clustering of these galaxies at $z$=0. We use a version of the semi-analytical galaxy formation model GALFORM that calculates self-consistently the amount of HI in a galaxy as a function of cosmic time and links its star formation rate to its mass of molecular hydrogen (H$_2$). We find that a systematic increase or decrease in the strength of SN…
This paper describes a programme to map large-scale cosmic structures on the largest possible scales by using the Five hundred metre Aperture Spherical Telescope (FAST) to make a 21 cm (red-shifted) intensity map of the sky for the range $0.5 < z < 2.5$. The goal is to map to the angular and spectral resolution of FAST a large swath of the sky by simple drift scans with a transverse set of beams. This approach would be complementary to galaxy surveys and could be completed before the Square Kilometre Array (SKA) could begin a more detailed and precise effort. The science would be to measure th…
Using archival data from the HI Parkes All Sky Survey (HIPASS) we have searched for 21 cm line absorption in 204 nearby radio and star-forming galaxies with continuum flux densities greater than $S_{1.4} \approx 250$ mJy within the redshift range $0 < cz < 12000$ km s$^{-1}$. By applying a detection method based on Bayesian model comparison, we successfully detect and model absorption against the radio-loud nuclei of four galaxies, of which the Seyfert 2 galaxy 2MASX J130804201-2422581 was previously unknown. All four detections were achieved against compact radio sources, which include three …
Line-intensity mapping of the 21cm line is a powerful probe of large scale structure at z<6, tracing large-scale structure via neutral hydrogen content that is found within galaxies. In principle, it enables cost-efficient surveys of the matter distribution up to z~6, unlocking orders of magnitude more modes for observational cosmology. Canada has been a traditional leader in this field, having led the first detections of the cosmological 21cm signal via cross-correlations with optical galaxy surveys and having constructed the Canadian Hydrogen Intensity Mapping Experiment (CHIME). The field i…
Romanian Institute of Science and Technology（罗马尼亚科学技术研究所）
Upcoming experiments such as Hydrogen Epoch of Reionization Array (HERA) and Square Kilometre Array (SKA) are intended to measure the 21cm signal over a wide range of redshifts, representing an incredible opportunity in advancing our understanding about the nature of cosmic Reionization. At the same time these kind of experiments will present new challenges in processing the extensive amount of data generated, calling for the development of automated methods capable of precisely estimating physical parameters and their uncertainties. In this paper we employ Variational Inference, and in partic…
We present a baseline sensitivity analysis of the Hydrogen Epoch of Reionization Array (HERA) and its build-out stages to one-point statistics (variance, skewness, and kurtosis) of redshifted 21 cm intensity fluctuation from the Epoch of Reionization (EoR) based on realistic mock observations. By developing a full-sky 21 cm lightcone model, taking into account the proper field of view and frequency bandwidth, utilising a realistic measurement scheme, and assuming perfect foreground removal, we show that HERA will be able to recover statistics of the sky model with high sensitivity by averaging…
Motivated by the desire for wide-field images with well-defined statistical properties for 21cm cosmology, we implement an optimal mapping pipeline that computes a maximum likelihood estimator for the sky using the interferometric measurement equation. We demonstrate this direct optimal mapping with data from the Hydrogen Epoch of Reionization (HERA) Phase I observations. After validating the pipeline with simulated data, we develop a maximum likelihood figure-of-merit for comparing four sky models at 166MHz with a bandwidth of 100kHz. The HERA data agree with the GLEAM catalogs to <10%. After…
The Rapid ASKAP Continuum Survey (RACS) is the first large-area survey to be conducted with the full 36-antenna Australian Square Kilometre Array Pathfinder (ASKAP) telescope. RACS will provide a shallow model of the ASKAP sky that will aid the calibration of future deep ASKAP surveys. RACS will cover the whole sky visible from the ASKAP site in Western Australia, and will cover the full ASKAP band of $700-1800$ MHz. The RACS images are generally deeper than the existing NRAO VLA Sky Survey (NVSS) and Sydney University Molonglo Sky Survey (SUMSS) radio surveys and have better spatial resolutio…
We investigate the possibility of testing Einstein's general theory of relativity (GR) and the standard cosmological model via the $E_{\rm G}$ statistic using neutral hydrogen (HI) intensity mapping. We generalise the Fourier space estimator for $E_{\rm G}$ to include HI as a biased tracer of matter and forecast statistical errors using HI clustering and lensing surveys that can be performed in the near future, in combination with ongoing and forthcoming optical galaxy and Cosmic Microwave Background (CMB) surveys. We find that fractional errors $< 1\%$ in the $E_{\rm G}$ measurement can be ac…
We discuss absolute calibration strategies for Phase I of the Hydrogen Epoch of Reionization Array (HERA), which aims to measure the cosmological 21 cm signal from the Epoch of Reionization (EoR). HERA is a drift-scan array with a 10 degree wide field of view, meaning bright, well-characterized point source transits are scarce. This, combined with HERA's redundant sampling of the uv plane and the modest angular resolution of the Phase I instrument, make traditional sky-based and self-calibration techniques difficult to implement with high dynamic range. Nonetheless, in this work we demonstrate…
