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Structure of brightest cluster galaxies and intracluster light
Structure of brightest cluster galaxies and intracluster light
The first part of this thesis presents the results of a photometric survey of Brightest Cluster Galaxies (BCGs), specifically targeting their low-surface-brightness Intracluster Light (ICL) outskirts. It is the deepest and largest sample of galaxy clusters to this date, consisting of 170 local (z<0.08) clusters in the northern hemisphere. The observations have been obtained with the Wendelstein Telescope Wide Field Imager (WWFI). Surface brightness (SB) profiles are measured down to ~2000 times fainter levels than the sky brightness in a dark, moonless night. Precise calibration is crucial here to exclude systematics. Background inhomogeneities induced by scattered light are reduced down to ∆SB > 31 g' mag arcsec^(-2) by large dithering and subtraction of night-sky flats. The extended wings of bright foreground stars are subtracted, relative bias offsets between the readout ports of the CCDs are corrected, and charge persistence is masked. For the first time, the broadening effect of the point-spread function is determined and corrected down to the faintest SBs. Residual background inhomogeneities brighter than SB_σ < 27.6 g' mag arcsec^(-2) caused by galactic cirrus are detected in front of 23% of the clusters. However, the large field of view allows discrimination between accretion signatures and galactic cirrus. I detect accretion signatures in form of tidal streams in 22%, shells in 9.4% and multiple nuclei in 47% of the brightest cluster galaxies (BCGs) and find two BCGs in 7% of the clusters. Semimajor-axis SB profiles of the BCGs and their surrounding ICL are measured down to a limiting SB of SB = 30 g' mag arcsec^(-2). The spatial resolution in the inner regions is increased by combining the WWFI light profiles with those that we measured from archival Hubble Space Telescope images or deconvolved WWFI images. I find that 71% of the BCG+ICL systems have SB profiles that are well described by a single Sérsic (SS) function, whereas 29% require a double Sérsic (DS) function to obtain a good fit. SS BCGs, having more symmetric isophotal shapes and fewer detected accretion signatures than DS BCGs, appear to have slightly more relaxed morphology than their DS counterparts. Members of the latter type encompass S2 = 52±21% of their total light in the outer Sérsic component. There is a wide scatter in transition radii r_× between the two Sérsic components and SB at the transition radii SB(r_×). The integrated brightnesses of the BCG+ICL systems correlate only weakly with S2, r_× and SB(r_×). That indicates that the outer Sérsic component is unlikely to trace the dynamically hot ICL since BCG+ICL systems grow at present epoch predominantly in their outskirts. I find that BCGs have scaling relations that differ markedly from those of normal ellipticals, likely due to their indistinguishable embedding in the ICL. The most extended BCG+ICL systems have luminosities and radii comparable to whole clusters. I use different plausible estimates for the ICL component (based on an integrated brightness threshold, SB thresholds and profile decompositions), and find that they do not affect the conclusions about the ICL properties. On average, the ICL seems to be better aligned than the BCG with the host cluster in terms of position angle and centering. That makes it a potential Dark Matter tracer. I find positive correlations between BCG+ICL brightness and cluster mass, cluster radius, cluster richness and integrated satellite brightness, confirming that BCG/ICL growth is indeed coupled with cluster growth. The second part of this thesis documents the WWFI data reduction pipeline, which I have developed during the course of this PhD thesis project and applied to process the raw observational data. It is optimized for low-SB photometry of extended objects. However, its utility is not limited to that specific science case. It can be used as a standard pipeline to reduce any WWFI images. This documentation provides detailed technical descriptions about all basic and advanced features. It is meant to serve as both a reference and a manual for the pipeline.
Photometry, BCG, ICL, Galaxies Clusters
Kluge, Matthias
2020
English
Universitätsbibliothek der Ludwig-Maximilians-Universität München
Kluge, Matthias (2020): Structure of brightest cluster galaxies and intracluster light. Dissertation, LMU München: Faculty of Physics
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Abstract

The first part of this thesis presents the results of a photometric survey of Brightest Cluster Galaxies (BCGs), specifically targeting their low-surface-brightness Intracluster Light (ICL) outskirts. It is the deepest and largest sample of galaxy clusters to this date, consisting of 170 local (z<0.08) clusters in the northern hemisphere. The observations have been obtained with the Wendelstein Telescope Wide Field Imager (WWFI). Surface brightness (SB) profiles are measured down to ~2000 times fainter levels than the sky brightness in a dark, moonless night. Precise calibration is crucial here to exclude systematics. Background inhomogeneities induced by scattered light are reduced down to ∆SB > 31 g' mag arcsec^(-2) by large dithering and subtraction of night-sky flats. The extended wings of bright foreground stars are subtracted, relative bias offsets between the readout ports of the CCDs are corrected, and charge persistence is masked. For the first time, the broadening effect of the point-spread function is determined and corrected down to the faintest SBs. Residual background inhomogeneities brighter than SB_σ < 27.6 g' mag arcsec^(-2) caused by galactic cirrus are detected in front of 23% of the clusters. However, the large field of view allows discrimination between accretion signatures and galactic cirrus. I detect accretion signatures in form of tidal streams in 22%, shells in 9.4% and multiple nuclei in 47% of the brightest cluster galaxies (BCGs) and find two BCGs in 7% of the clusters. Semimajor-axis SB profiles of the BCGs and their surrounding ICL are measured down to a limiting SB of SB = 30 g' mag arcsec^(-2). The spatial resolution in the inner regions is increased by combining the WWFI light profiles with those that we measured from archival Hubble Space Telescope images or deconvolved WWFI images. I find that 71% of the BCG+ICL systems have SB profiles that are well described by a single Sérsic (SS) function, whereas 29% require a double Sérsic (DS) function to obtain a good fit. SS BCGs, having more symmetric isophotal shapes and fewer detected accretion signatures than DS BCGs, appear to have slightly more relaxed morphology than their DS counterparts. Members of the latter type encompass S2 = 52±21% of their total light in the outer Sérsic component. There is a wide scatter in transition radii r_× between the two Sérsic components and SB at the transition radii SB(r_×). The integrated brightnesses of the BCG+ICL systems correlate only weakly with S2, r_× and SB(r_×). That indicates that the outer Sérsic component is unlikely to trace the dynamically hot ICL since BCG+ICL systems grow at present epoch predominantly in their outskirts. I find that BCGs have scaling relations that differ markedly from those of normal ellipticals, likely due to their indistinguishable embedding in the ICL. The most extended BCG+ICL systems have luminosities and radii comparable to whole clusters. I use different plausible estimates for the ICL component (based on an integrated brightness threshold, SB thresholds and profile decompositions), and find that they do not affect the conclusions about the ICL properties. On average, the ICL seems to be better aligned than the BCG with the host cluster in terms of position angle and centering. That makes it a potential Dark Matter tracer. I find positive correlations between BCG+ICL brightness and cluster mass, cluster radius, cluster richness and integrated satellite brightness, confirming that BCG/ICL growth is indeed coupled with cluster growth. The second part of this thesis documents the WWFI data reduction pipeline, which I have developed during the course of this PhD thesis project and applied to process the raw observational data. It is optimized for low-SB photometry of extended objects. However, its utility is not limited to that specific science case. It can be used as a standard pipeline to reduce any WWFI images. This documentation provides detailed technical descriptions about all basic and advanced features. It is meant to serve as both a reference and a manual for the pipeline.