Browsing by Author "Ford, H. C."

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    Discovery of a very bright strongly lensed galaxy candidate at z ≈ 7.6
    (2008) Bradley, L. D.; Bouwens, R. J.; Ford, H. C.; Illingworth, G. D.; Jee, M. J.; Benitez, N.; Broadhurst, T. J.; Franx, M.; Frye, B. L.; Infante, L.; Motta, V.; Rosati, P.; White, R. L.; Zheng, W.
    Using Hubble Space Telescope (HST) and Spitzer IRAC imaging, we report the discovery of a very bright strongly lensed Lyman break galaxy (LBG) candidate at z similar to 7.6 in the field of the massive galaxy clusterAbell 1689 (z = 0.18). The galaxy candidate, which we refer to as A1689-zD1, shows a strong z(850) - J(110) break of at least 2.2 mag and is completely undetected (< 1 sigma) in HST Advanced Camera for Surveys (ACS) g(475), r(625), i(775), and z(850) data. These properties, combined with the very blue J(110) - H(160) and H(160) - [4.5 mu m] colors, are exactly the properties of an z similar to 7.6 LBG, and can only be reasonably fit by a star- forming galaxy at z = 7.6 +/- 0.4 (chi(2)(nu) = 1.1). Attempts to reproduce these properties with a model galaxy at z < 4 yield particularly poor fits (chi(2)(nu) >= 25). . A1689- zD1 has an observed (lensed) magnitude of 24.7 AB (8 sigma) in the NICMOS H(160) band and is similar to 1.3 mag brighter than the brightest known z(850)-dropout galaxy. When corrected for the cluster magnification of similar to 9.3 at z similar to 7.6, the candidate has an intrinsic magnitude of H(160) = 27.1 AB, or about an L(*) galaxy at z similar to 7: 6. The source- plane deprojection shows that the star formation is occurring in compact knots of size less than or similar to 300 pc. The best- fit stellar population synthesis models yield a median redshift of 7.6, stellar masses (1.6-3.9) x 10(9) M(circle dot), stellar ages 45-320 Myr, star formation rates less than or similar to 7.6M(circle dot) yr(-1), and low reddening with A(V) <= 0.3. These properties are generally similar to those of LBGs found at z similar to 5-6. The inferred stellar ages suggest a formation redshift of z similar to 8-10 (t less than or similar to 0.63 Gyr). A1689-zD1 is the brightest observed, highly reliable z > 7.0 galaxy candidate found to date.
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    Hubble Space Telescope ACS Multiband Coronagraphic Imaging of the Debris Disk around β Pictoris
    (2006) Golimowski, D. A.; Ardila, D. R.; Krist, J. E.; Clampin, M.; Ford, H. C.; Illingworth, G. D.; Bartko, F.; Benitez, N.; Blakeslee, J. P.; Bouwens, R. J.; Bradley, L. D.; Broadhurst, T. J.; Brown, R. A.; Burrows, C. J.; Cheng, E. S.; Cross, N. J. G.; Demarco, R.; Feldman, P. D.; Franx, M.; Goto, T.; Gronwall, C.; Hartig, G. F.; Holden, B. P.; Homeier, N. L.; Infante, L.; Jee, M. J.; Kimble, R. A.; Lesser, M. P.; Martel, A. R.; Mei, S.; Menanteau, F.; Meurer, G. R.; Miley, G. K.; Motta, V.; Postman, M.; Rosati, P.; Sirianni, M.; Sparks, W. B.; Tran, H. D.; Tsvetanov, Z. I.; White, R. L.; Zheng, W.; Zirm, A. W.
    We present F435W(B), F606W (broad V), and F814W(broad I) coronagraphic images of the debris disk around beta Pictoris obtained with the Hubble Space Telescope's Advanced Camera for Surveys. These images provide the most photometrically accurate and morphologically detailed views of the disk between 30 and 300 AU from the star ever recorded in scattered light. We confirm that the previously reported warp in the inner disk is a distinct secondary disk inclined by similar to 5 degrees from the main disk. The projected spine of the secondary disk coincides with the isophotal inflections, or "butterfly asymmetry,'' previously seen at large distances from the star. We also confirm that the opposing extensions of the main disk have different position angles, but we find that this "wing-tilt asymmetry'' is centered on the star rather than offset from it, as previously reported. The main disk's northeast extension is linear from 80 to 250 AU, but the southwest extension is distinctly bowed with an amplitude of similar to 1 AU over the same region. Both extensions of the secondary disk appear linear, but not collinear, from 80 to 150 AU. Within similar to 120 AU of the star, the main disk is similar to 50% thinner than previously reported. The surface brightness profiles along the spine of the main disk are fitted with four distinct radial power laws between 40 and 250 AU, while those of the secondary disk between 80 and 150 AU are fitted with single power laws. These discrepancies suggest that the two disks have different grain compositions or size distributions. The F606W/F435W and F814W/F435W flux ratios of the composite disk are nonuniform and asymmetric about both projected axes of the disk. The disk's northwest region appears 20%-30% redder than its southeast region, which is inconsistent with the notion that forward scattering from the nearer northwest side of the disk should diminish with increasing wavelength. Within similar to 120 AU, the m(F435W)-m(F606W) and m(F435W)-m(F814W) colors along the spine of the main disk are similar to 10% and similar to 20% redder, respectively, than those of beta Pic. These colors increasingly redden beyond similar to 120 AU, becoming 25% and 40% redder, respectively, than the star at 250 AU. These measurements overrule previous determinations that the disk is composed of neutrally scattering grains. The change in color gradient at similar to 120 AU nearly coincides with the prominent inflection in the surface brightness profile at similar to 115 AU and the expected water-ice sublimation boundary. We compare the observed red colors within similar to 120 AU with the simulated colors of nonicy grains having a radial number density alpha r(-3) and different compositions, porosities, and minimum grain sizes. The observed colors are consistent with those of compact or moderately porous grains of astronomical silicate and/or graphite with sizes greater than or similar to 0.15-0.20 mu m, but the colors are inconsistent with the blue colors expected from grains with porosities greater than or similar to 90%. The increasingly red colors beyond the ice sublimation zone may indicate the condensation of icy mantles on the refractory grains, or they may reflect an increasing minimum grain size caused by the cessation of cometary activity.
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    Scaling Relations and Overabundance of Massive Clusters at z ≳ 1 from Weak-lensing Studies with the Hubble Space Telescope
    (2011) Jee, M. J.; Dawson, K. S.; Hoekstra, H.; Perlmutter, S.; Rosati, P.; Brodwin, M.; Suzuki, N.; Koester, B.; Postman, M.; Lubin, L.; Meyers, J.; Stanford, S. A.; Barbary, K.; Barrientos, F.; Eisenhardt, P.; Ford, H. C.; Gilbank, D. G.; Gladders, M. D.; Gonzalez, A.; Harris, D. W.; Huang, X.; Lidman, C.; Rykoff, E. S.; Rubin, D.; Spadafora, A. L.
    We present weak gravitational lensing analysis of 22 high-redshift (z greater than or similar to 1) clusters based on Hubble Space Telescope images. Most clusters in our sample provide significant lensing signals and are well detected in their reconstructed two-dimensional mass maps. Combining the current results and our previous weak-lensing studies of five other high-z clusters, we compare gravitational lensing masses of these clusters with other observables. We revisit the question whether the presence of the most massive clusters in our sample is in tension with the current. CDM structure formation paradigm. We find that the lensing masses are tightly correlated with the gas temperatures and establish, for the first time, the lensing mass-temperature relation at z greater than or similar to 1. For the power-law slope of the M-T-X relation (M proportional to T-alpha), we obtain alpha = 1.54 +/- 0.23. This is consistent with the theoretical self-similar prediction alpha = 3/2 and with the results previously reported in the literature for much lower redshift samples. However, our normalization is lower than the previous results by 20%-30%, indicating that the normalization in the M-T-X relation might evolve. After correcting for Eddington bias and updating the discovery area with a more conservative choice, we find that the existence of the most massive clusters in our sample still provides a tension with the current. CDM model. The combined probability of finding the four most massive clusters in this sample after the marginalization over cosmological parameters is less than 1%.