Finding and Characterizing the Giant Arcs
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Finding and Characterizing the Giant Arcs
Bingxiao XuJohns Hopkins University
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Outlines
• Science motivation• Automate arcfinder• Test the arcfinder by simulations• Priliminary results• Future prospects
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Why Giant Arcs?• The abundance of the giant arcs is
sensitive to the inner structure of the clusters and cosmology
• The enhanced signal-to-noise ratio allows us to resolve the substructures within the highly magnified objects
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Advantages of CLASH samples• Less biased selection • Depth: ~20 orbits per cluster Higher limiting mag for arc detection Higher resolution for substructure• Extensive multi-bands imaging Redshift distribution of giant arcs Stellar population within the highly magnified galaxies
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Arc detection algorithm
• Set intensity threshold as positive median value of the difference of Gaussian (DoG) images, to obtain the image segmentation
• Use eccentricity to filter out the less elongated features
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Gradient based algorithm• Calculate the intensity gradient of the each pixel to obtain
an orientation map
• quantize the orientation into 4 directions and assign a digit to each pixel (1,2,3,4)
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Maximum supression
The intensity of the pixels on the arc's rigid line should be larger than that of the adjacent pixel along its gradient direction and opposite direction
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Shift the pixels to local maxima
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Intensity-INDEPENDENT pixel selection
• The orientation of the pixels close to the arc's rigid lines should not change much
• The intensity value of the pixels close to the arc's rigid lines experience in the DoG and original image during the shifting should be positive
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Orientation Criteria
• The tangential arc’s orientation should be perpendicular to the line connecting the arc and the center of cluster
• Turn off the criteria at the very center (r < 100 pix ) to preserve the radial arcs
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Removal of the star spikes
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Test the arcfinder(Furlanetto et al. 2013)
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Detection rate test
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Contamination rate test
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Preliminary Results
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Preliminary Results ( 176 arcs )
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Future Prospects
• Arc statistics • Distribution of the Einstein radius• De-lensing the giant arcs to study galaxy
formation and evolution at high redshift
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Arc Statistics• Order of magnitude discrepancy (Bartelmann M.,
et al 1998) recent works! No longer discrepancy, but tension still exists...
• Possible solutions Central BCGs and substructure (Hennawi et al.
2007; Meneghetti et al 2010) Triaxiality of clusters (Oguri et al. 2003) Major merger (Torri et al. 2004; Fedeli et al.
2006) Distribution of the background sources
(Wambsganss et al. 2004; Dalal et al. 2004) …… there is still a factor of 2 discrepancy out
there