The Epoch of Reionization (EoR) is the transitional period in the universe's evolution which starts when the first luminous sources begin to ionize the intergalactic medium for the first time since recombination, and ends when the most of the hydrogen is ionized by about a redshift of 6. Observations of the 21cm emission from hyperfine splitting of the hydrogen atom can carry a wealth of cosmological information from this epoch since the redshifted line can probe the entire volume. The GMRT-EoR experiment is an ongoing effort to make a statistical detection of the power spectrum of 21cm neutral hydrogen emission due to the patchwork of neutral and ionized regions present during the transition. In this work we detail approximately five years of observations at the GMRT, comprising over 900 hours, and an in-depth analysis of about 50 hours which have lead to the first upper limits on the 21cm power spectrum in the range z=8.1 to 9.2. This includes a concentrated radio frequency interference (RFI) mitigation campaign around the GMRT area, a novel method for removing broadband RFI with a singular value decomposition, and calibration with a pulsar as both a phase and polarization calibrator. Preliminary results from 2011 showed a 2-sigma upper limit to the power spectrum of (70 mK)^2. However, we find that foreground removal strategies tend to reduce the cosmological signal significantly, and modeling this signal loss is crucial for interpretation of power spectrum measurements. Using a simulated signal to estimate the transfer function of the real 21cm signal through the foreground removal procedure, we are able to find the optimal level of foreground removal and correct for the signal loss. Using this correction, we report a 2-sigma upper limit of (248 mK)^2 at k=0.5 h/Mpc.

The Epoch of Reionization (EoR) is the transitional period in the universe's evolution which starts when the first luminous sources begin to ionize the intergalactic medium for the first time since recombination, and ends when the most of the hydrogen is ionized by about a redshift of 6. Observations of the 21cm emission from hyperfine splitting of the hydrogen atom can carry a wealth of cosmological information from this epoch since the redshifted line can probe the entire volume. The GMRT-EoR experiment is an ongoing effort to make a statistical detection of the power spectrum of 21cm neutral hydrogen emission due to the patchwork of neutral and ionized regions present during the transition. In this work we detail approximately five years of observations at the GMRT, comprising over 900 hours, and an in-depth analysis of about 50 hours which have lead to the first upper limits on the 21cm power spectrum in the range z=8.1 to 9.2. This includes a concentrated radio frequency interference (RFI) mitigation campaign around the GMRT area, a novel method for removing broadband RFI with a singular value decomposition, and calibration with a pulsar as both a phase and polarization calibrator. Preliminary results from 2011 showed a 2-sigma upper limit to the power spectrum of (70 mK)^2. However, we find that foreground removal strategies tend to reduce the cosmological signal significantly, and modeling this signal loss is crucial for interpretation of power spectrum measurements. Using a simulated signal to estimate the transfer function of the real 21cm signal through the foreground removal procedure, we are able to find the optimal level of foreground removal and correct for the signal loss. Using this correction, we report a 2-sigma upper limit of (248 mK)^2 at k=0.5 h/Mpc.

abstract: The Universe transitioned from a state of neutral hydrogen (HI) shortly after recombination to its present day ionized state, but this transition, the Epoch of Reionization (EoR), has been poorly constrained by observational data. Estimates place the EoR between redshifts 6 < z <13 (330-770 Myr). The interaction of the 21 cm hyperfine ground state emission/absorption-line of HI with the cosmic microwave background (CMB) and the radiation from the first luminous sources in the universe can be used to extract cosmological information about the EoR. Theorists have created global redshifted 21 cm EoR models of this interaction that predict the temperature perturbations to the CMB in the form of a sky-averaged difference temperature, Tb. The difficulty in measuring Tb is that it is predicted to be on the order of 20 to 100 mK, while the sky foreground is dominated by synchrotron radiation that is 105 times brighter. The challenge is to subtract the much brighter foreground radiation without subtracting the Tb signal and can only be done when the data has small error levels. The Experiment to Detect the Global EoR Signature (EDGES) is an effort to measure Tb with a single wide field-of-view well-calibrated antenna. This dissertation focuses on reducing systematic errors by quantifying the impact of the chromatic nature of the antenna’s beam directivity and by measuring the variability of the spectral index of the radio sky foreground. The chromatic beam study quantified the superior qualities of the rectangular blade-shaped antenna and led to its adoption over the previously used fourpoint-shaped antenna and determined that a 5 term polynomial was optimum for removing the foreground. The spectral index, β, of the sky was measured, using 211 nights of data, to be −2.60 > β > −2.62 in lower LST regions, increasing to −2.50 near the Galactic plane. This matched simulated results using the Guzm´an et al. (2011) sky map (∆β < 0.05) and demonstrated the exceptional stability of the EDGES instrument. Lastly, an EoR model by Kaurov & Gnedin (2016) was shown to be inconsistent with measured EDGES data at a significance level of 1.9. / Dissertation/Thesis / Doctoral Dissertation Astrophysics and Astronomy 2017

Recent observations have suggested that the C III] lambda 1907/1909 emission lines could be alternative diagnostic lines for galaxies in the reionization epoch. We use the F128N narrowband filter on the Hubble Space Telescope's (HST) Wide Field Camera 3 (WFC3) to search for C III] emission in a sample of five galaxies at z = 5.7 in the Subaru Deep Field and the Subaru/XMM-Newton Deep Field. Using the F128N narrowband imaging, together with the broadband imaging, we do not detect C III] emission for the five galaxies with JAB ranging from 24.10 to 27.00 in our sample. For the brightest galaxy J132416.13+274411.6 in our sample (z = 5.70, J(AB) = 24.10), which has a significantly higher signal to noise, we report a C III] flux of 3.34 +/- 1.81 x 10(-18) erg s(-1)cm(-2), which places a stringent 3 sigma upper limit of 5.43 x 10(-18) erg s(-1)cm(-2) on C III] flux and 6.57 angstrom on the C III] equivalent width. Using the stacked image, we put a 3 sigma upper limit on the mean C III] flux of 2.55 x 10(-18) erg s(-1) cm(-2) and a 3 sigma upper limit on the mean C III] equivalent width of 4.20 angstrom for this sample of galaxies at z = 5.70. Combined with strong C III] detection reported among high-z galaxies in the literature, our observations suggest that the equivalent widths of C III] from galaxies at z > 5.70 exhibit a wide range of distribution. Our strong limits on C III] emission could be used as a guide for future observations in the reionization epoch.

Luminous high-redshift quasars can be used to probe of the intergalactic medium in the early universe because their UV light is absorbed by the neutral hydrogen along the line of sight. They help us to measure the neutral hydrogen fraction of the high-z universe, shedding light on the end of reionization epoch. In this paper, we present a discovery of a new quasar (PSO J006.1240+39.2219) at redshift z = 6.61 +/- 0.02 from Panoramic Survey Telescope & Rapid Response System 1.Including this quasar, there are nine quasars above z > 6.5 up to date. The estimated continuum brightness is M-1450 = -25.96 +/- 0.08. PSO J006.1240+39.2219 has a strong Ly alpha emission compared with typical low-redshift quasars, but the measured near-zone region size is R-NZ = 3.2 +/- 1.1 proper megaparsecs, which is consistent with other quasars at z similar to 6.

abstract: The universe since its formation 13.7 billion years ago has undergone many changes. It began with expanding and cooling down to a temperature low enough for formation of atoms of neutral Hydrogen and Helium gas. Stronger gravitational pull in certain regions caused some regions to be denser and hotter than others. These regions kept getting denser and hotter until they had centers hot enough to burn the hydrogen and form the first stars, which ended the Dark Ages. These stars did not live long and underwent violent explosions. These explosions and the photons from the stars caused the hydrogen gas around them to ionize. This went on until all the hydrogen gas in the universe was ionized. This period is known as Epoch Of Reionization. Studying the Epoch Of Reionization will help understand the formation of these early stars, the timeline of the reionization and the formation of the stars and galaxies as we know them today. Studying the radiations from the 21cm line in neutral hydrogen, redshifted to below 200MHz can help determine details such as velocity, density and temperature of these early stars and the media around them. The EDGES program is one of the many programs that aim to study the Epoch of Reionization. It is a ground-based project deployed in Murchison Radio-Astronomy Observatory in Western Australia. At ground level the Radio Frequency Interference from the ionosphere and various man-made transmitters in the same frequency range as the EDGES receiver make measurements, receiver design and extraction of useful data from received signals difficult. Putting the receiver in space can help majorly escape the RFI. The EDGES In Space is a proposed project that aims at designing a receiver similar to the EDGES receiver but for a cubesat. This thesis aims at designing a prototype receiver that is similar in architecture to the EDGES low band receiver (50-100MHz) but is significantly smaller in size (small enough to fit on a PCB for a cubesat) while keeping in mind different considerations that affect circuit performance in space. / Dissertation/Thesis / Masters Thesis Electrical Engineering 2019

This thesis considers various topics and open questions in galaxy formation during the epoch of reionization and presents multiple new computational techniques developed specifically to study this era. This work naturally divides into two main sections: 1) The formation of the first massive black holes and 2) Interpreting ALMA observations of galaxy formation during the epoch of reionization. The first topic addresses the existence of super massive black holes (SMBHs) with $M_{\rm BH} > 10^9$M$_{\odot}$ at $z > 6$. It is well established that stellar mass black holes are very unlikely to be able to accrete matter efficiently enough to grow to this mass at this redshift. For this reason, many alternative channels have been proposed for black hole formation that produce objects with significantly larger initial masses. In this thesis, I consider a mechanism whereby runaway stellar collisions in dense primordial star clusters form a very massive star that is likely to collapse to an intermediate mass black hole (IMBH) with $M_{\rm BH} > 10^3$M$_{\odot}$. In order to test this scenario, I added 12 species non-equilibrium chemistry to the massively parallel adaptive mesh refinement code RAMSES, and simulated, at sub-pc resolution, the collapse of the first metal-enriched halo which is likely to host a Population II star cluster. The properties of the central gas cloud in the collapsing halo were then extracted from the simulation and used to create initial conditions for the direct N-body integrator, NBODY6. These star clusters were simulated for 3.5Myr (until the first supernova is expected to occur) and it was determined that the properties of the gas clouds that form in cosmological simulations were indeed suitable to form a very massive star by collisional runaway. This suggests that this mechanism is a promising channel for forming the seeds of SMBHs at high redshift. The second topic of this thesis aims to help interpret the plethora of recent and upcoming ALMA observations of star forming galaxies during the epoch of reionization. These observations target far-infrared lines such as [CII] and [OIII] which directly probe the interstellar medium (ISM) of these $z > 6$ galaxies. In order to study this epoch, I employ the RAMSES-RT code, which allows for the computation of multifrequency radiative transfer on-the-fly. I modified this code in a number of ways so that it can handle radiation-coupled H$_2$ non-equilibrium chemistry (including Lyman-Werner band radiation) and I developed the variable speed of light approximation which changes the speed of light in the simulation depending on the density of gas so that ionisation fronts propagate at the correct speed in all gas phases. Cosmological boxes were initialised to include galaxies with masses comparable to the observations of Maiolino et al. (2015) and run at various resolutions to test convergence properties. One of the major goals of this study was to identify the physical mechanism responsible for the spatial offset observed between [CII] and UV/Lyα in many high-redshift galaxies.

No description available.

Masters of Science / 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.

The fraction of ionizing photons escaping from high-redshift star-forming galaxies is a key obstacle in evaluating whether galaxies were the primary agents of cosmic reionization. We previously proposed using the covering fraction of low-ionization gas, measured via deep absorption-line spectroscopy, as a proxy. We now present a significant update, sampling seven gravitationally lensed sources at 4 < z < 5. We show that the absorbing gas in our sources is spatially inhomogeneous, with a median covering fraction of 66%. Correcting for reddening according to a dust-in-cloud model, this implies an estimated absolute escape fraction of similar or equal to 19% +/- 6%. With possible biases and uncertainties, collectively we find that the average escape fraction could be reduced to no less than 11%, excluding the effect of spatial variations. For one of our lensed sources, we have sufficient signal-tonoise ratio to demonstrate the presence of such spatial variations and scatter in its dependence on the Ly alpha equivalent width, consistent with recent simulations. If this source is typical, our lower limit to the escape fraction could be reduced by a further factor similar or equal to 2. Across our sample, we find a modest anticorrelation between the inferred escape fraction and the local star formation rate, consistent with a time delay between a burst and leaking Lyman continuum photons. Our analysis demonstrates considerable variations in the escape fraction, consistent with being governed by the small-scale behavior of star-forming regions, whose activities fluctuate over short timescales. This supports the suggestion that the escape fraction may increase toward the reionization era when star formation becomes more energetic and burst-like.