# Multi-color detection of gravitational arcs: # method and new candidates # # Matteo Maturi(1), Sebastian Mizera(2), and Gregor Seidel(3) # # (1) Zentrum fuer Astronomie der Universitaet Heidelberg, Institut # fuer Theoretische Astrophysik, Philosophenweg 12, 69120 # Heidelberg, Germany # (2) Girton College, University of Cambridge, Huntingdon Road, # Cambridge CB3 0JG, United Kingdom # (3) Max-Planck-Institut fuer Astronomie, Koenigstuhl 17, 69117 # Heidelberg, Germany # # Strong gravitational lensing provides fundamental insights into the # understanding of the dark matter distribution in massive galaxies, # galaxy clusters, and the background cosmology. Despite their # importance, few gravitational arcs have been discovered so far. The # urge for more complete, large samples and unbiased methods of # selecting candidates increases. Several methods for the automatic # detection of arcs have been proposed in the literature, but large # amounts of spurious detections retrieved by these methods force # observers to visually inspect thousands of candidates per square # degree to clean the samples. This approach is largely subjective and # requires a huge amount of checking by eye, especially considering # the actual and upcoming wide field surveys, which will cover # thousands of square degrees. # # In this paper we study the statistical properties of the colours of # gravitational arcs detected in the 37 deg^2 of the # CFHTLS-Archive-Research Survey (CARS). Most of them lie in a # relatively small region of the (g'-r',r'-i') colour-colour # diagram. To explain this property, we provide a model that includes # the lensing optical depth expected in a LCDM cosmology that, in # combination with the sources' redshift distribution of a given # survey, in our case CARS, peaks for sources at redshift z~1. By # furthermore modelling the colours derived from the spectral energy # distribution of the galaxies that dominate the population at that # redshift, the model reproduces the observed colours well. # # By taking advantage of the colour selection suggested by both data # and model, we automatically detected 24 objects out of 90 detected # by eye checking. Compared with the single-band arcfinder, this # multi-band filtering returns a sample complete to 83% and a # contamination reduced by a factor of ~6.5. New gravitational arc # candidates are also proposed. # # (1) ID cat: identification number detection type are: # [a]: satisfy all criteria except for the color selection # [b]: detected by the arcfinder but rejected by our tight geometrical constraints (L>15pix,L/W>4) # [c]: were detected only via eye-ball checking # (2) FIELD: CARS field identifier # (3) RA: arc right ascension (2000.0) # (4) DEC: arc declination (2000.0) # (5) LEN: arc length [arcsec] # (6) L/W: arc length to width ratio # (7) CUR: arc curvature [arcsec^-1] # (8) A: arc area [arcsec^2] # (9) uprime: arc uprime magnitude (aperture photometry based on segmentation) # (10) uprime_err: arc uprime 1-sigma error # (11) gprime: arc gprime magnitude (aperture photometry based on segmentation) # (12) gprime_err: arc gprime 1-sigma error # (13) rprime: arc rprime magnitude (aperture photometry based on segmentation) # (14) rprime_err: arc rprime 1-sigma error # (15) iprime: arc iprime magnitude (aperture photometry based on segmentation) # (16) iprime_err: arc iprime 1-sigma error # (17) z_lens: lens photometric redshift taken from the CARS catalog # (18) z_lens_err_plus: lens photometric redshift 1-sigma error taken from the CARS catalog # (19) z_lens_err_minus: lens photometric redshift 1-sigma error taken from the CARS catalog # (20) RANK: arc ranking # [1=unlikely]: arc structures which seem physically associated to an astrophysical object, or which are # relatively far from a possible lens # [2=unclear]: arc structures with a very promising shape but still being of dubious origin # [3=unclear]: a clear arc structure which curvature is displaced in such a way to ’enclose’ a possible # lens cause of its origin, i.e. large ellipticals or significant concentrations of galaxies. # (21) CAT: catalog number if present, previously known arcs # # http://www.ita.uni-heidelberg.de/~maturi/Public/arcs # Maturi et al. 2013 # arc1a W1m1p1 02:13:17.2 -06:25:58 2.91 4.66 0.21 2.0 25.6 0.6 24.3 0.2 23.7 0.5 23.1 0.2 0.39 -0.09 0.09 2 - 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arc12a W1p1p3 02:21:34.5 -04:24:38 7.44 12.03 0.32 6.0 23.5 0.2 22.6 0.1 22.0 0.1 21.3 0.1 0.70 -0.11 0.11 3 - arc13a W1p2p2 02:25:32.0 -04:50:59 5.49 8.20 0.37 4.4 23.2 0.2 22.6 0.1 21.9 0.1 21.3 0.1 0.62 -0.11 0.11 2 SL2SJ022532-045100 arc14a W1p3p1 02:31:06.7 -05:55:02 3.52 4.50 0.51 2.8 25.1 0.5 24.1 0.2 23.4 0.4 22.8 0.2 0.53 -0.10 0.10 3 SA57 arc15a W3m3m2 14:00:21.2 +52:15:47 7.21 10.00 0.40 6.2 23.1 0.2 22.3 0.1 21.8 0.1 21.1 0.1 0.76 -0.12 0.12 2 - arc16a W3m3m2 14:01:44.6 +53:02:09 3.67 4.20 0.13 3.5 24.0 0.2 23.4 0.1 22.8 0.2 22.0 0.1 0.47 -0.10 0.10 3 SA91 arc17a W4m1m2 22:09:18.3 -00:50:03 3.17 4.16 0.45 2.4 25.3 0.5 23.7 0.2 23.0 0.3 22.4 0.1 0.49 -0.21 0.22 2 - arc18a W4m0m0 22:12:22.0 +01:36:46 4.08 4.61 0.37 4.4 24.5 0.3 23.7 0.1 23.1 0.2 22.4 0.1 0.72 -0.11 0.11 3 - arc19a W4m0m1 22:12:55.3 +00:17:07 6.37 6.34 0.24 6.7 24.0 0.2 23.0 0.1 22.2 0.1 21.4 0.1 0.58 -0.10 0.10 2 - arc20a W4m0m2 22:13:06.9 -00:18:30 5.34 4.26 0.20 5.9 23.8 0.2 22.9 0.1 22.3 0.1 21.7 0.1 0.49 -0.21 0.22 3 - 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arc31b W1p1p1 02:21:14.9 -05:42:42 1.92 2.05 0.27 2.1 24.7 0.4 23.6 0.1 22.9 0.3 22.3 0.1 0.28 -0.08 0.09 2 - arc32b W1p1p1 02:23:15.3 -06:29:03 2.33 3.39 0.26 2.0 24.9 0.4 23.6 0.1 22.9 0.1 22.2 0.1 0.57 -0.10 0.10 3 SA40 arc33b W1p2p2 02:24:27.3 -04:55:43 1.34 2.01 0.21 1.0 25.2 0.6 24.4 0.2 23.9 0.5 23.3 0.2 0.56 -0.10 0.10 3 - arc34b W1p4p3 02:33:07.2 -04:38:38 2.23 3.98 0.11 1.5 25.5 0.7 24.1 0.2 23.6 0.3 22.9 0.2 0.66 -0.11 0.11 3 SA59 arc35b W3m3m2 13:57:02.4 +52:30:39 2.43 4.71 0.17 1.6 25.7 0.8 24.7 0.4 23.8 0.6 23.0 0.3 0.38 -0.09 0.09 3 - arc36b W3m3m2 13:58:22.4 +52:43:19 2.56 2.93 0.31 2.3 23.9 0.2 23.5 0.1 23.2 0.3 22.5 0.2 0.55 -0.10 0.10 3 - arc37b W3m3m2 13:58:46.6 +52:21:00 1.58 2.25 0.00 1.1 26.2 1.3 24.9 0.4 24.2 0.8 23.4 0.3 0.35 -0.09 0.09 2 - arc38b W4m0m1 22:12:29.9 +00:17:25 2.47 2.80 0.00 2.4 24.6 0.5 23.5 0.1 23.1 0.3 22.3 0.1 0.58 -0.10 0.10 3 - arc39b W4m0m2 22:13:07.1 -00:30:37 3.06 3.75 0.32 2.5 24.9 0.4 24.1 0.1 23.3 0.3 22.4 0.1 0.58 -0.10 0.10 3 SA122 arc40b W4p2m2 22:20:18.6 -00:24:28 2.56 2.80 1.01 2.1 25.2 0.6 24.4 0.3 23.9 0.6 23.0 0.1 0.52 -0.10 0.10 2 - arc41b W4p2m1 22:21:41.8 +00:19:09 2.15 2.65 0.56 1.8 24.6 0.3 23.4 0.1 23.1 0.2 22.7 0.1 0.64 -0.11 0.11 2 - arc42c W1m1p1 02:12:31.6 -06:11:54 7.56 7.33 0.18 8.5 25.4 0.5 23.9 0.1 22.8 0.2 21.8 0.1 0.36 -0.09 0.09 3 - arc43c W1m1p3 02:15:13.5 -03:53:44 3.17 3.52 0.11 3.0 24.8 0.5 24.1 0.2 23.0 0.2 22.1 0.1 0.51 -0.11 0.10 2 - arc44c W1m1p3 02:15:57.1 -04:10:27 2.19 2.20 1.10 1.9 24.7 0.5 24.1 0.2 23.1 0.2 22.4 0.1 0.82 -0.12 0.12 3 - arc45c W1m0p1 02:19:32.7 -06:28:44 3.71 3.54 0.32 4.5 25.2 0.5 24.5 0.3 23.4 0.2 22.4 0.2 0.65 -0.11 0.11 2 - arc46c W1p3p1 02:29:32.1 -06:15:32 2.22 2.02 0.00 2.4 24.9 0.4 24.2 0.2 23.7 0.3 22.8 0.1 0.58 -0.10 0.10 2 - arc47c W3m3m3 13:57:23.9 +51:21:30 2.70 3.04 0.61 2.6 25.6 0.7 24.3 0.2 23.4 0.3 22.3 0.1 0.60 -0.10 0.11 2 - arc48c W3m3m3 14:00:09.5 +52:06:24 4.97 5.67 0.67 7.3 24.4 0.3 23.2 0.1 22.4 0.1 21.5 0.1 0.54 -0.10 0.10 2 - arc49c W3m3m2 14:00:28.6 +52:55:26 2.16 2.51 0.47 1.9 25.3 0.6 24.3 0.2 23.3 0.3 22.6 0.1 0.53 -0.12 0.10 2 - arc50c W3m3m2 14:01:02.7 +52:37:23 8.06 7.55 0.30 11.0 24.8 0.4 23.3 0.1 22.5 0.2 21.6 0.1 0.50 -0.11 0.10 3 - 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arc90c W4p2m1 22:22:17.6 +00:12:02 2.19 2.57 0.45 2.1 24.9 0.4 23.5 0.1 22.5 0.1 21.7 0.1 0.51 -0.10 0.10 3 -