Masterpresentasjon: Karl Elwyn Løken Brown

«Identifying quasars and galaxy clusters in ACT’s images of the Cosmic Microwave Background»

Abstract
The Atacama Cosmology Telescope (ACT) is a ground based telescope making smaller scale, high resolution observations of the Cosmic microwave back-ground (CMB), in five broad frequency bands. Due to the high resolution of the signal, we also observe various point sources as foregrounds of the CMB. During this dissertation I will construct a matched filter that detects these point sources and separates them from the background, so that I may analyze them in more detail. I have primarily worked with the f090 band, which observes between 77-112 GHz; the majority of my point sources were detected in this band, so the catalog is expected to be dominated by active galactic nuclei (AGN) and galaxy clusters. I have also included the f150 (124-172GHz) and f220 (182-277 GHz) bands to both compute the spectral index of the detected point sources, and to detect additional point sources, beyond those that are visible in the f090 band. The matched filter constructed seeks to maximize the signal to noise ratio(SNR) of its detections. I applied this filter to all three bands (f090, f150and f220) individually, before combining them into a single catalog, where du-plicate detections were removed. Among other things, the flux value of each source was read from the map. These values were then used as the reference point for the spectral index fitting to each source, using the Python Markov Chain Monte Carlo package emcee [1]. Furthermore, I used these spectral index fittings to more effectively compare my results to The National Aeronautics and Space Administration’s (NASA) Extragalactic Database (NED). 19 example sources, aiming to represent a wide range of observation types, were selected from the catalog in this comparison. The sources with SNR ≥ 5for the fitted flux F0 agreed well with NED, while only one source below this level was determined to be a real detection. As a result I imposed more restrictions on the detections, such as not being located along the edge. This brought the final count of detected sources down to356, including detections of both AGN, galaxy clusters and nearby dusty galaxies with high precision, on average being far above SNR= 5.

Image: ACT Collaboration/Debra Kellner

More information: https://www.mn.uio.no/astro/studier/aktuelt/arrangementer/masterpresentasjoner/2025/Karl%20Elwyn%20Løken%20Brown.html

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