Examinando por Autor "Shajib A.J."

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    Discovery of three strongly lensed quasars in the Sloan Digital Sky Survey
    (Oxford University Press, 2018-06) Williams P.R.; Agnello A.; Treu T.; Apostolovski Y.; Chen G.C.; Fassnacht C.D.; Hsueh J.W.; Lemaux B.C.; Motta V.; Oldham L.; Rojas K.; Rusu C.E.; Shajib A.J.; Wang X.; Abramson L.E.
    We present the discovery of three quasar lenses in the Sloan Digital Sky Survey, selected using two novel photometry-based selection techniques. The J0941+0518 system, with two point sources separated by 5.46 arcsec on either side of a galaxy, has source and lens redshifts 1.54 and 0.343. Images of J2257+2349 show two point sources separated by 1.67 arcsec on either side of an E/S0 galaxy. The extracted spectra show two images of the same quasar at zs = 2.10. SDSS J1640+1045 has two quasar spectra at zs = 1.70 and fits to the SDSS and Pan-STARRS images confirm the presence of a galaxy between the two point sources. We observed 56 photometrically selected lens candidates in this follow-up campaign, confirming three new lenses, re-discovering one known lens, and ruling out 36 candidates, with 16 still inconclusive. This initial campaign demonstrates the power of purely photometric selection techniques in finding lensed quasars. © 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society.
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    STRIDES: Automated uniform models for 30 quadruply imaged quasars
    (Oxford University Press, 2023-01) Schmidt T.; Treu T.; Birrer S.; Shajib A.J.; Lemon C.; Millon M.; Sluse D.; Agnello A.; Anguita T.; Auger-Williams M.W.; McMahon R.G.; Motta V.; Spiniello C.; Kayo I.; Courbin F.; Ertl S.; Fassnacht C.D.; Frieman J.A.; More A.; Schuldt S.; Suyu S.H.; Aguena M.; Andrade-Oliveira F.; Annis J.; Bacon D.; Bertin E.; Brooks D.; Burke D.L.; Carnero Rosell A.; Carrasco Kind M.; Carretero J.; Conselice C.; Costanzi M.; Da Costa L.N.; Pereira M.E.S.; De Vicente J.; Desai S.; Doel P.; Everett S.; Ferrero I.; Friedel D.; García-Bellido J.; Gaztanaga E.; Gruen D.; Gruendl R.A.; Gschwend J.; Gutierrez G.; Hinton S.R.; Hollowood D.L.; Honscheid K.; James D.J.; Kuehn K.; Lahav O.; Menanteau F.; Miquel R.; Palmese A.; Paz-Chinchón F.; Pieres A.; Plazas Malagón A.A.; Prat J.; Rodriguez-Monroy M.; Romer A.K.; Sanchez E.; Scarpine V.; Sevilla-Noarbe I.; Smith M.; Suchyta E.; Tarle G.; To C.; Varga T.N.
    Gravitational time delays provide a powerful one-step measurement of H0, independent of all other probes. One key ingredient in time-delay cosmography are high-accuracy lens models. Those are currently expensive to obtain, both, in terms of computing and investigator time (105-106 CPU hours and ∼0.5-1 yr, respectively). Major improvements in modelling speed are therefore necessary to exploit the large number of lenses that are forecast to be discovered over the current decade. In order to bypass this roadblock, we develop an automated modelling pipeline and apply it to a sample of 31 lens systems, observed by the Hubble Space Telescope in multiple bands. Our automated pipeline can derive models for 30/31 lenses with few hours of human time and <100 CPU hours of computing time for a typical system. For each lens, we provide measurements of key parameters and predictions of magnification as well as time delays for the multiple images. We characterize the cosmography-readiness of our models using the stability of differences in the Fermat potential (proportional to time delay) with respect to modelling choices. We find that for 10/30 lenses, our models are cosmography or nearly cosmography grade (<3 per cent and 3-5 per cent variations). For 6/30 lenses, the models are close to cosmography grade (5-10 per cent). These results utilize informative priors and will need to be confirmed by further analysis. However, they are also likely to improve by extending the pipeline modelling sequence and options. In conclusion, we show that uniform cosmography grade modelling of large strong lens samples is within reach. © 2022 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society.
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    STRIDES: Spectroscopic and photometric characterization of the environment and effects of mass along the line of sight to the gravitational lenses des J0408-5354 and WGD 2038-4008
    (Oxford University Press, 2020-11-01) Buckley-Geer E.J.; Lin H.; Rusu C.E.; Poh J.; Palmese A.; Agnello A.; Christensen L.; Frieman J.; Shajib A.J.; Treu T.; Collett T.; Birrer S.; Anguita T.; Fassnacht C.D.; Meylan G.; Mukherjee S.; Wong K.C.; Aguena M.; Allam S.; Avila S.; Bertin E.; Bhargava S.; Brooks D.; Carnero Rosell A.; Carrasco Kind M.; Carretero J.; Castander F.J.; Costanzi M.; Da Costa L.N.; De Vicente J.; Desai S.; Diehl H.T.; Doel P.; Eifler T.F.; Everett S.; Flaugher B.; Fosalba P.; Garciá-Bellido J.; Gaztanaga E.; Gruen D.; Gruendl R.A.; Gschwend J.; Gutierrez G.; Hinton S.R.; Honscheid K.; James D.J.; Kuehn K.; Kuropatkin N.; Maia M.A.G.; Marshall J.L.; Melchior P.; Menanteau F.; Miquel R.; Ogando R.L.C.; Paz-Chinchón F.; Plazas A.A.; Sanchez E.; Scarpine V.; Schubnell M.
    In time-delay cosmography, three of the key ingredients are (1) determining the velocity dispersion of the lensing galaxy, (2) identifying galaxies and groups along the line of sight with sufficient proximity and mass to be included in the mass model, and (3) estimating the external convergence κext from less massive structures that are not included in the mass model. We present results on all three of these ingredients for two time-delay lensed quad quasar systems, DES J0408-5354 and WGD 2038-4008. We use the Gemini, Magellan, and VLT telescopes to obtain spectra to both measure the stellar velocity dispersions of the main lensing galaxies and to identify the line-of-sight galaxies in these systems. Next, we identify 10 groups in DES J0408-5354 and two groups in WGD 2038-4008 using a group-finding algorithm. We then identify the most significant galaxy and galaxy-group perturbers using the 'flexion shift' criterion. We determine the probability distribution function of the external convergence κext for both of these systems based on our spectroscopy and on the DES-only multiband wide-field observations. Using weighted galaxy counts, calibrated based on the Millennium Simulation, we find that DES J0408-5354 is located in a significantly underdense environment, leading to a tight (width ∼ 3%), negative-value κext distribution. On the other hand, WGD 2038-4008 is located in an environment of close to unit density, and its low source redshift results in a much tighter κext of ~1%, as long as no external shear constraints are imposed.