A Detection of Cosmological 21 cm Emission from CHIME in Cross-correlation with eBOSS Measurements of the Lyman-α\alpha Forest

We report the detection of 21 cm emission at an average redshift $\bar{z} = 2.3$ in the cross-correlation of data from the Canadian Hydrogen Intensity Mapping Experiment (CHIME) with measurements of the Lyman-$\alpha$ forest from eBOSS. Data collected by CHIME over 88 days in the $400-500$~MHz frequency band ($1.8 < z < 2.5$) are formed into maps of the sky and high-pass delay filtered to suppress the foreground power, corresponding to removing cosmological scales with $k_\parallel \lesssim 0.13\ \text{Mpc}^{-1}$ at the average redshift. Line-of-sight spectra to the eBOSS background quasar locations are extracted from the CHIME maps and combined with the Lyman-$\alpha$ forest flux transmission spectra to estimate the 21 cm-Lyman-$\alpha$ cross-correlation function. Fitting a simulation-derived template function to this measurement results in a $9\sigma$ detection significance. The coherent accumulation of the signal through cross-correlation is sufficient to enable a detection despite excess variance from foreground residuals $\sim6-10$ times brighter than the expected thermal noise level in the correlation function. These results are the highest-redshift measurement of \tcm emission to date, and set the stage for future 21 cm intensity mapping analyses at $z>1.8$

Hunting Elusive Excess Variance in Big LOFAR Data

The Epoch of Reionisation (EoR) is a watershed period of the universe when the predominantly neutral intergalactic medium was ionised and the first luminous sources formed. LOFAR (Low Frequency Array) is a radio interferometer which can detect the 21-cm signal from the EoR. The detection is challenging due to the strong astrophysical foregrounds, radio frequency interference, ionospheric effects and instrumental effects. Even after calibration, the remaining residuals are above the estimated thermal noise, known as the "excess variance". My thesis is dedicated to studying complex correlations between excess variance and its sources. In Chapter 2, I found that the excess variance has a Local Sidereal Time dependence related to distant and bright sources in the sky such as Cassiopeia A and Cygnus A. In Chapter 3, I compared the performance of a new direction-dependent calibration method, DDECAL, to our current method, SAGECAL on an unexplored field around our target field, the North Celestial Pole. Similar imprints from Cassiopeia A and Cygnus A are shown in this analysis as well. To further identify the contribution of bright sources in sky images more efficiently, I introduce a new data analysis tool, Self-Organising Attribute Maps. This method explores clusters in vector attributes of a component tree, the max-tree, with an unsupervised machine learning technique, self-organising maps (SOMs). The applications on medical and LOFAR sky images show that this method is promising for exploring morphological features in images without manually thresholding vector attributes

Assessing the impact of two independent direction-dependent calibration algorithms on the LOFAR 21-cm signal power spectrum:And applications to an observation of a field flanking the North Celestial Pole

Detecting the 21-cm signal from the Epoch of Reionisation (EoR) is challenging due to the strong astrophysical foregrounds, ionospheric effects, radio frequency interference and instrumental effects. Understanding and calibrating these effects are crucial for the detection. In this work, we introduce a newly developed direction-dependent (DD) calibration algorithm DDECAL and compare its performance with an existing algorithm, SAGECAL, in the context of the LOFAR-EoR 21-cm power spectrum experiment. In our data set, the North Celestial Pole (NCP) and its flanking fields were observed simultaneously. We analyse the NCP and one of its flanking fields. The NCP field is calibrated by the standard pipeline, using SAGECAL with an extensive sky model and 122 directions, and the flanking field is calibrated by DDECAL and SAGECAL with a simpler sky model and 22 directions. Additionally, two strategies are used for subtracting Cassiopeia A and Cygnus A. The results show that DDECAL performs better at subtracting sources in the primary beam region due to the application of a beam model, while SAGECAL performs better at subtracting Cassiopeia A and Cygnus A. This indicates that including a beam model during DD calibration significantly improves the performance. The benefit is obvious in the primary beam region. We also compare the 21-cm power spectra on two different fields. The results show that the flanking field produces better upper limits compared to the NCP in this particular observation. Despite the minor differences between DDECAL and SAGECAL due to the beam application, we find that the two algorithms yield comparable 21-cm power spectra on the LOFAR-EoR data after foreground removal. Hence, the current LOFAR-EoR 21-cm power spectrum limits are not likely to depend on the DD calibration method

A Detection of Cosmological 21 cm Emission from CHIME in Cross-correlation with eBOSS Measurements of the Lyα Forest

We report the detection of 21 cm emission at an average redshift $\bar{z}=2.3$ in the cross-correlation of data from the Canadian Hydrogen Intensity Mapping Experiment (CHIME) with measurements of the Ly α forest from eBOSS. Data collected by CHIME over 88 days in the 400–500 MHz frequency band (1.8 1.8