Finding and Characterizing the Giant Arcs

Bingxiao XuJohns Hopkins University

Outlines

• Science motivation• Automate arcfinder• Test the arcfinder by simulations• Priliminary results• Future prospects

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

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

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

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)

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

Shift the pixels to local maxima

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

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

Removal of the star spikes

Test the arcfinder(Furlanetto et al. 2013)

Detection rate test

Contamination rate test

Preliminary Results

Preliminary Results ( 176 arcs )

Future Prospects

• Arc statistics • Distribution of the Einstein radius• De-lensing the giant arcs to study galaxy

formation and evolution at high redshift

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