Deep learning approach for identification of H ii regions during reionization in 21-cm observations - II. Foreground contamination

• Published in: Monthly notices of the Royal Astronomical Society Vol. 528; no. 3; pp. 5212 - 5230
• Main Authors: Bianco, Michele, Giri, Sambit K, Prelogović, David, Chen, Tianyue, Mertens, Florent G, Tolley, Emma, Mesinger, Andrei, Kneib, Jean-Paul
• Format: Journal Article
• Language: English
• Published: London Oxford University Press 01.03.2024
• Subjects: Artificial neural networks; Contamination; dark ages; Deep learning; early Universe; first stars; Gaussian process; H II regions; Image contrast; Image enhancement; Infrared telescopes; Ionization; Machine learning; Neural networks; Noise prediction; Polynomials; Principal components analysis; Red shift; reionization; Signal classification; Statistical analysis; Statistical methods; techniques: image processing; techniques: interferometric; early Universe; techniques: image processing; techniques: interferometric; dark ages, reionization, first stars
• DOI: 10.1093/mnras/stae257
• ISSN: 0035-8711; 1365-2966

The upcoming Square Kilometre Array Observatory will produce images of neutral hydrogen distribution during the epoch of reionization by observing the corresponding 21-cm signal. However, the 21-cm signal will be subject to instrumental limitations such as noise and galactic foreground contamination that pose a challenge for accurate detection. In this study, we present the SegU-Net v2 framework, an enhanced version of our convolutional neural network, built to identify neutral and ionized regions in the 21-cm signal contaminated with foreground emission. We trained our neural network on 21-cm image data processed by a foreground removal method based on Principal Component Analysis achieving an average classification accuracy of 71 per cent between redshift z = 7 and 11. We tested SegU-Net v2 against various foreground removal methods, including Gaussian Process Regression, Polynomial Fitting, and Foreground-Wedge Removal. Results show comparable performance, highlighting SegU-Net v2's independence on these pre-processing methods. Statistical analysis shows that a perfect classification score with ${\rm AUC}=95~{{\ \rm per\ cent}}$ is possible for 8 < z < 10. While the network prediction lacks the ability to correctly identify ionized regions at higher redshift and differentiate well the few remaining neutral regions at lower redshift due to low contrast between 21-cm signal, noise, and foreground residual in images. Moreover, as the photon sources driving reionization are expected to be located inside ionized regions, we show that SegU-Net v2 can be used to correctly identify and measure the volume of isolated bubbles with $V_{\rm ion}\gt (10\, {\rm cMpc})^3$ at z > 9, for follow-up studies with infrared/optical telescopes to detect these sources.