Examinando por Autor "Galan A."

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    Gravitationally lensed quasars in Gaia – IV. 150 new lenses, quasar pairs, and projected quasars
    (Oxford University Press, 2023-04-01) Lemon C.; Anguita T.; Auger-Williams M.W.; Courbin F.; Galan A.; McMahon R.; Neira F.; Oguri M.; Schechter P.; Shajib A.; Treu T.; Agnello A.; Spiniello C.
    We report the spectroscopic follow-up of 175 lensed quasar candidates selected using Gaia Data Release 2 observations following Paper III of this series. Systems include 86 confirmed lensed quasars and a further 17 likely lensed quasars based on imaging and/or similar spectra. We also confirm 11 projected quasar pairs and 11 physical quasar pairs, while 25 systems are left as unclassified quasar pairs – pairs of quasars at the same redshift, which could be either distinct quasars or potential lensed quasars. Especially interesting objects include eight quadruply imaged quasars of which two have BAL sources, an apparent triple, and a doubly lensed LoBaL quasar. The source redshifts and image separations of these new lenses range between 0.65–3.59 and 0.78–6.23 arcsec, respectively. We compare the known population of lensed quasars to an updated mock catalogue at image separations between 1 and 4 arcsec, showing a very good match at z < 1.5. At z > 1.5, only 47 per cent of the predicted number are known, with 56 per cent of these missing lenses at image separations below 1.5 arcsec. The missing higher redshift, small-separation systems will have fainter lensing galaxies, and are partially explained by the unclassified quasar pairs and likely lenses presented in this work, which require deeper imaging. Of the 11 new reported projected quasar pairs, 5 have impact parameters below 10 kpc, almost tripling the number of such systems, which can probe the innermost regions of quasar host galaxies through absorption studies. We also report four new lensed galaxies discovered through our searches, with source redshifts ranging from 0.62 to 2.79. © 2023 Oxford University Press. All rights reserved.
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    J1721+8842: The first Einstein zigzag len
    (EDP Sciences, 0025-02) Dux F.; Millon M.; Lemon C.; Schmidt T.; Courbin F.; Shajib A.J.; Treu T.; Birrer S.; Wong K.C.; Agnello A.; Andrade A.; Galan A.
    We report the discovery of the first example of an Einstein zigzag lens, an extremely rare lensing configuration. In this system, J1721+8842, six images of the same background quasar are formed by two intervening galaxies, one at redshift z1 = 0.184 and another at z2 = 1.885. Two out of the six multiple images are deflected in opposite directions as they pass the first lens galaxy on one side and the second on the other side – the optical paths forming zigzags between the two deflectors. In this paper we demonstrate that J1721+8842, previously thought to be a lensed dual quasar, is in fact a compound lens, with the more distant lens galaxy also being distorted as an arc by the foreground galaxy. Evidence supporting this unusual lensing scenario includes: (1) identical light curves in all six lensed quasar images obtained from two years of monitoring at the Nordic Optical Telescope; (2) detection of the additional deflector at redshift z2 = 1.885 in JWST/NIRSpec integral field unit data; and (3) a multiple-plane lens model reproducing the observed image positions. This unique configuration offers the opportunity to combine two major lensing cosmological probes, time-delay cosmography and dual source-plane lensing, since J1721+8842 features multiple lensed sources that form two distinct Einstein radii of different sizes, one of which is a variable quasar. We expect to place tight constraints on H0 and w by combining these two probes of the same system. The z2 = 1.885 deflector, a quiescent galaxy, is also the highest-redshift strong galaxy-scale lens with a spectroscopic redshift measurement known to date. © The Authors 2025.