UWCScholar :: Browsing by Author "Santos, Mario"

Browsing by Author "Santos, Mario"

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Classifying non-redundancy in the HERA array
(University of the Western Cape, 2022) Malapane, Kabelo; Santos, Mario
HERA is a highly redundant radio interferometer array, where pairs of receivers with the same position vector between them should see exactly the same signal from the sky. We can use this fact to do a really good job of calibrating them. Unfortunately, the receivers are not perfectly identical, and so they don’t see exactly the same signal. This is called "non-redundancy". This project classifies the level of redundancy using a clustering machine learning technique. The aim is to see if any particular clustering algorithm can group different segments of the array into very similar blocks, so we can at least do a good job of redundantly calibrating within those blocks. We call this new calibration method, logi_cal, while the standard calibration method used in HERA is called redcal.
Cosmology with HI intensity mapping: effect of higher order corrections
(University of the Western Cape, 2020) Randrianjanahary, Liantsoa Finaritra; Santos, Mario
One of the main challenges of cosmology is to unveil the nature of dark energy and dark matter. They can be constrained with baryonic acoustic oscillations (BAO) and redshift space distortions, amongst others. Both have characteristic signatures in the dark matter power spectrum. Biased tracers of dark matter, such as neutral hydrogen, are used to quantify the underlying dark matter density field. It is generally assumed that on large scales the bias of the tracer is linear. However, there is a coupling between small and large scales of the biased tracer which gives rise to a significant non-linear contribution on linear scales in the power spectrum of the biased tracer. The Hydrogen Intensity and Real-time eXperiment (HIRAX) will map the brightness temperature of neutral hydrogen (HI) over BAO scales thanks to the intensity mapping technique. We forecasted cosmological parameters for HIRAX taking into account non-linear corrections to the HI power spectrum and compared them to the linear case. We used methods based on Fisher matrices. We found values for the bias to error ratio of the cosmological parameters as high as 1 or 7, depending on the noise level. We also investigated the change in peaks location on the baryonic acoustic oscillations signal. The value of the shift goes up to Δk = 10-2h/Mpc with a reduction of amplitude of the BAO features from 16:33% to 0:33%, depending on the scales.
Cosmology with next generation radio telescopes
(University of the Western Cape, 2019) Witzemann, Amadeus; Santos, Mario; Weltman, Amanda; Clarkson, Chris
The next generation of radio telescopes will revolutionize cosmology by providing large three-dimensional surveys of the universe. This work presents forecasts using the technique 21cm intensity mapping (IM) combined with results from the cosmic microwave background, or mock data of galaxy surveys. First, we discuss prospects of constraining curvature independently of the dark energy (DE) model, finding that the radio instrument HIRAX will reach percent-level accuracy even when an arbitrary DE equation of state is assumed. This is followed by a study of the potential of the multi-tracer technique to surpass the cosmic variance limit, a crucial method to probe primordial non-Gaussianity and large scale general relativistic e↵ects. Using full sky simulations for the Square Kilometre Array phase 1 (SKA 1 MID) and the Large Synoptic Survey Telescope (LSST), including foregrounds, we demonstrate that the cosmic variance contaminated scenario can be beaten even in the noise free case. Finally, we derive the signal to noise ratio for the cosmic magnification signal from foreground HI intensity maps combined with background galaxy count maps. Instruments like SKA1 MID and HIRAX are highly complementary and well suited for this measurement. Thanks to the powerful design of the planned radio instruments, all results confirm their potential and promise an exciting future for cosmology.
Detecting Baryon Acoustic Oscillations with HI Intensity Mapping using MeerKAT
(University of the Western Cape, 2019) Engelbrecht, Brandon; Santos, Mario; Fonseca, José
Future radio surveys as the Square Kilometer Array (SKA) and its precursor, the "Meer" Karoo Array Telescope (MeerKAT), will map the Neutral Hydrogen (HI) in large areas of the sky using the intensity mapping (IM). HI IM is currently one of the most promising ways of accessing the Large-Scale Structure of the Universe. The distribution of matter in the Universe not only encodes its composition but also how it evolves and its initial conditions. An effect on the matter distribution that will be detected by the SKA on the post re-ionization Universe are the Baryonic Acoustic Oscillations (BAO). While it has been shown that in single dish mode the SKA can measure the BAO peak in the radial 21cm power spectrum at low redshifts, this possibility has not yet been studied in detail for the MeerKAT. In this thesis we construct a set of full sky simulations to test how well MeerKAT will be able to extract the BAO wiggles along the line of sight. These simulations are done for the frequencies corresponding to MeerKAT L-band. The maps combine the cosmological HI signal, systematic noise, cosmological foregrounds and the instrumental telescope beam. A model-independent estimator is used to extract the BAO wiggles by subtracting a smooth polynomial component from the 21cm radial power spectrum. We test with simulations if this estimator is biased and the signal to noise of the extraction. We conclude that we are able to remove contaminants and recover the cosmological HI signal while not risking the recovery of the BAO signal. We investigate the effects of varying the sky area and the observational hours on the signal to noise ratio for the BAO wiggles. We found that for a HI IM experiment using MeerKAT, the optimal sky area to detect the BAO along the line of sight is 50% of the sky. With a signal-to-noise ratio of 3.37. This can be achieved with 2000 hours of exposure time
HI intensity mapping with the MIGHTEE survey: power spectrum estimates
(Monthly Notices of the Royal Astronomical Society, 2021) Sourabh, Paul; Santos, Mario; Townsend, Junaid
Intensity mapping (IM) with neutral hydrogen is a promising avenue to probe the large scale structure of the Universe. In this paper, we demonstrate that using the 64-dish MeerKAT radio telescope as a connected interferometer, it is possible to make a statistical detection of HI in the post-reionization Universe. With the MIGHTEE (MeerKAT International GHz Tiered Extragalactic Exploration) survey project ob- serving in the L-band (856 < < 1712 MHz, z < 0:66), we can achieve the required sensitivity to measure the HI IM power spectrum on quasi-linear scales, which will provide an important complementarity to the single-dish IM MeerKAT observations. We present a purpose-built simulation pipeline that emulates the MIGHTEE obser- vations and forecast the constraints that can be achieved on the HI power spectrum at z = 0:27 for k > 0:3 Mpc􀀀1 using the foreground avoidance method. We present the power spectrum estimates with the current simulation on the COSMOS eld that includes contributions from HI, noise and point source models constructed from the observed MIGHTEE data. The results from our visibility based pipeline are in qualitative agreement to the already available MIGHTEE data. This paper demonstrates that MeerKAT can achieve very high sensitivity to detect HI with the full MIGHTEE survey on quasi-linear scales (signal-to-noise ratio > 7 at k = 0:49 Mpc􀀀1) which are instrumental in probing cosmological quantities such as the spectral index of uctuation, constraints on warm dark matter, the quasi-linear redshift space distortions and the measurement of the HI content of the Universe up to z 0:5.
Measuring the quasar luminosity function below the detection threshold
(University of the Western Cape, 2016) Malefahlo, Eliab; Santos, Mario; Zwart, Jonathan; Jarvis, Matt; Maartens, Roy
The radio emission of radio-quiet active galactic nuclei (AGN) is thought to be from star formation and AGN related emission. I investigate these sources using 1.4 GHz radio data from FIRST and three optical quasars samples from the SDSS: (i) a volume-limited sample in the redshift range 0:2 < z < 0:4 defined by Mi < -23 (ii) magnitude-limited sample in the redshift range 1:8 < z < 2:5 defined by mr ≤ 18:5 and (iii) a uniform sample in the redshift range 0:2 < z < 3:5 (divided into 12 redshift bins). I constructed radio source counts and radio luminosity functions (RLFs) using the optical quasars detected in FIRST, which are consistent with literature results obtained using SDSS and NVSS quasars. There are differences at the low uxs end because of the different resolutions of FIRST and NVSS. I applied a median stack method to the 12 redshift bins of the uniform sample and found that the median ux decreases from 182 µJy in the lowest redshift bin to 39 µJy and the highest redshift bin. This is because the high redshift quasars although more luminous than their low redshift counterparts, they are much further away so they have lower uxes. I probed the quasar radio source counts to lower levels using reconstructed source counts obtained by applying the Bayesian stack technique. The reconstructed radio source counts were then used to constructed the quasar RLF to lower levels, where I found: (i) for z < 1 the constructed quasar RLF has the same slope as the detected quasars, suggesting that like the detect quasars their radio emission is dominated by AGN related emission (ii) above z = 1 the constructed RLF steepens with redshift, which suggests the strong link between accretion rate and radio jet power is gradually breaking down towards faint optical luminosities at high redshift.
Neutral hydrogen intensity mapping on small scales using MeerKAT
(University of the Western Cape, 2021) Townsend, Mogamad-Junaid; Santos, Mario
In the post-reionisation universe, intensity mapping (IM) with the 21 cm line of neutral hydrogen (HI) provides a potential means of probing the large-scale structure of the universe. With such a probe, a wide variety of interesting phenomena such as the Baryon Acoustic Oscillations (BAO) and Redshift Space Distortions (RSD) can be studied. The MeerKAT telescope has the potential to make full use of this technique, especially in the single-dish mode, which will probe the scales relevant to BAO and RSD. A useful complementary of this is HI IM with MeerKAT in interferometer-mode, which will enable the extraction of cosmological information on semi-linear and small scales. In this study, full end-to-end simulations of interferometric observations with MeerKAT for HI IM were developed. With this, the power spectrum extraction was analysed using the foreground avoidance technique. This took into account the foreground wedge from point source contamination extracted from real MIGHTEE COSMOS data, as well as RFI flagging. The errors on the power spectrum estimator were then calculated through a Monte Carlo process using 1000s of realisations of both the thermal noise and HI signal. In doing so, precision constraints on the HI power spectrum are found at z = 0:27 on scales 0:4 < k < 10 Mpc-1 for mock visibility data sets which contain the HI signal contaminated by noise, mimicking the MIGHTEE COSMOS field for total observation times & 20 hours. These results illustrate the potential of doing precision cosmology with MeerKAT’s MIGHTEE survey and interferometer-mode HI IM.
Neutral hydrogen intensity mapping on small scales using MeerKAT
(University of the Western Cape, 2021) Townsend, Mogamad-Junaid; Santos, Mario
In the post-reionisation universe, intensity mapping (IM) with the 21 cm line of neutral hydrogen (HI) provides a potential means of probing the large-scale structure of the universe. With such a probe, a wide variety of interesting phenomena such as the Baryon Acoustic Oscillations (BAO) and Redshift Space Distortions (RSD) can be studied. The MeerKAT telescope has the potential to make full use of this technique, especially in the single-dish mode, which will probe the scales relevant to BAO and RSD. A useful complementary of this is HI IM with MeerKAT in interferometer-mode, which will enable the extraction of cosmological information on semi-linear and small scales. In this study, full end-to-end simulations of interferometric observations with MeerKAT for HI IM were developed. With this, the power spectrum extraction was analysed using the foreground avoidance technique. This took into account the foreground wedge from point source contamination extracted from real MIGHTEE COSMOS data, as well as RFI flagging. The errors on the power spectrum estimator were then calculated through a Monte Carlo process using 1000s of realisations of both the thermal noise and HI signal. In doing so, precision constraints on the HI power spectrum are found at z = 0:27 on scales 0:4 < k < 10 Mpc-1 for mock visibility data sets which contain the HI signal contaminated by noise, mimicking the MIGHTEE COSMOS field for total observation times & 20 hours. These results illustrate the potential of doing precision cosmology with MeerKAT’s MIGHTEE survey and interferometer-mode HI IM.
Point source simulations and Foreground cleaning techniques for HI intensity mapping
(University of the Western Cape, 2018) Ngobese, Sibonelo; Santos, Mario
We created a full sky point source catalogue at frequencies around 1 GHz, including the intensity, polarisation, rotation measure and spectral index of the point source. These values were matched to data where available and simulations were used otherwise. With this, we were able to generate full sky intensity and polarisation maps from point sources at the frequencies required for intensity mapping. These are crucial for end to end simulations that take into account the telescope beam effects. We showed that the polarisation intensity at these frequencies is reasonably smooth in frequency, making it easier to clean.
Probing galaxy evolution below the noise threshold with radio observations
(University of the Western Cape, 2020) Malefahlo, Eliab D; Santos, Mario
The faint radio population consisting of star forming galaxies (SFG) and radio-quiet active galactic nuclei (AGN) is important in the study of galaxy evolution. However, the bulk of the faint population is below the detection threshold of the current radio surveys. I study this population through a Bayesian-stacking technique that I have adapted to probe the radio luminosity function (RLF) below the typical 5σ detection threshold. The technique works by fitting RLF models to radio flux densities extracted at the position of galaxies selected from an auxiliary catalogue. I test the technique by adding Gaussian noise (σ) to simulated data and the RLF models are in agreement with the simulated data for up to three orders of magnitude (3 dex) below the detection threshold (5σ). The source of radio emission from radio quiet quasars (subset of AGN) is widely debated. I apply the technique to 1.4-GHz flux densities from the Faint Images of the Radio Sky at Twenty-cm survey (FIRST) at the positions of the optical quasars from the Sloan Digital Sky Survey (SDSS). The RLF models are constrained to 2 dex below the FIRST detection threshold. I found that the radio luminosity where radio-quiet quasars emerge coincides with the luminosity where SFGs are expected to start to dominate the RLF. This Implies that the radio emission of radio-quiet quasars and radio-quiet AGN, in general, could have a significant contribution from star formation in the host galaxies.
Reconstruction of the ionization history from 21cm maps with deep learning
(University of the Western Cape, 2020) Mangena; Santos, Mario; Hassan, Sultan
Upcoming and ongoing 21cm surveys, such as the Square Kilometre Array (SKA), Hydrogen Epoch of Reionization Array (HERA) and Low Frequency Array (LOFAR), will enable imaging of the neutral hydrogen distribution on cosmological scales in the early Universe. These experiments 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 astrophysical and cosmological parameters, which might break the degeneracies in the power spectral analysis. The global history of reionization remains fairly unconstrained. In this thesis, we explore the viability of directly using the 21cm images to reconstruct and constrain the reionization history. Using Convolutional Neural Networks (CNN), we create a fast estimator of the global ionization fraction from the 21cm images as produced by our Large Semi-numerical Simulation (SimFast21). Our estimator is able to efficiently recover the ionization fraction (xHII) at several redshifts, z = 7; 8; 9; 10 with an accuracy of 99% as quantified by the coefficient of determination R2 without being given any additional information about the 21cm maps. This approach, contrary to estimations based on the power spectrum, is model independent. When adding the thermal noise and instrumental effects from these 21cm arrays, the results are sensitive to the foreground removal level, affecting the recovery of high neutral fractions. We also observe similar trend when combining all redshifts but with an improved accuracy. Our analysis can be easily extended to place additional constraints on other astrophysical parameters such as the photon escape fraction. This work represents a step forward to extract the astrophysical and cosmological information from upcoming 21cm surveys.

Browsing by Author "Santos, Mario"

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