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1. The Pan-STARRS1 z > 5.6 Quasar Survey. II. Discovery of 55 Quasars at 5.6 < z < 6.5

   https://doi.org/10.3847/1538-4365/acb3c7  

   Bañados, Eduardo; Schindler, Jan-Torge; Venemans, Bram P.; Connor, Thomas; Decarli, Roberto; Farina, Emanuele Paolo; Mazzucchelli, Chiara; Meyer, Romain A.; Stern, Daniel; Walter, Fabian; et al (March 2023, The Astrophysical Journal Supplement Series)

   Abstract The identification of bright quasars atz≳ 6 enables detailed studies of supermassive black holes, massive galaxies, structure formation, and the state of the intergalactic medium within the first billion years after the Big Bang. We present the spectroscopic confirmation of 55 quasars at redshifts 5.6 <z< 6.5 and UV magnitudes −24.5 <M₁₄₅₀< −28.5 identified in the optical Pan-STARRS1 and near-IR VIKING surveys (48 and 7, respectively). Five of these quasars have independently been discovered in other studies. The quasar sample shows an extensive range of physical properties, including 17 objects with weak emission lines, 10 broad absorption line quasars, and 5 objects with strong radio emission (radio-loud quasars). There are also a few notable sources in the sample, including a blazar candidate atz= 6.23, a likely gravitationally lensed quasar atz= 6.41, and az= 5.84 quasar in the outskirts of the nearby (D∼ 3 Mpc) spiral galaxy M81. The blazar candidate remains undetected in NOEMA observations of the [Cii]and underlying emission, implying a star formation rate <30–70M_⊙yr⁻¹. A significant fraction of the quasars presented here lies at the foundation of the first measurement of thez∼ 6 quasar luminosity function from Pan-STARRS1 (introduced in a companion paper). These quasars will enable further studies of the high-redshift quasar population with current and future facilities. 

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2. The STRong lensing Insights into the Dark Energy Survey (STRIDES) 2017/2018 follow-up campaign: discovery of 10 lensed quasars and 10 quasar pairs

   https://doi.org/10.1093/mnras/staa652  

   Lemon, C; Auger, M W; McMahon, R; Anguita, T; Apostolovski, Y; Chen, G C-F; Fassnacht, C D; Melo, A D; Motta, V; Shajib, A; et al (May 2020, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We report the results of the STRong lensing Insights into the Dark Energy Survey (STRIDES) follow-up campaign of the late 2017/early 2018 season. We obtained spectra of 65 lensed quasar candidates with ESO Faint Object Spectrograph and Camera 2 on the NTT and Echellette Spectrograph and Imager on Keck, confirming 10 new lensed quasars and 10 quasar pairs. Eight lensed quasars are doubly imaged with source redshifts between 0.99 and 2.90, one is triply imaged (DESJ0345−2545, z = 1.68), and one is quadruply imaged (quad: DESJ0053−2012, z = 3.8). Singular isothermal ellipsoid models for the doubles, based on high-resolution imaging from SAMI on Southern Astrophysical Research Telescope or Near InfraRed Camera 2 on Keck, give total magnifications between 3.2 and 5.6, and Einstein radii between 0.49 and 1.97 arcsec. After spectroscopic follow-up, we extract multi-epoch grizY photometry of confirmed lensed quasars and contaminant quasar + star pairs from DES data using parametric multiband modelling, and compare variability in each system’s components. By measuring the reduced χ2 associated with fitting all epochs to the same magnitude, we find a simple cut on the less variable component that retains all confirmed lensed quasars, while removing 94 per cent of contaminant systems. Based on our spectroscopic follow-up, this variability information improves selection of lensed quasars and quasar pairs from 34-45 per cent to 51–70 per cent, with most remaining contaminants being star-forming galaxies. Using mock lensed quasar light curves we demonstrate that selection based only on variability will over-represent the quad fraction by 10 per cent over a complete DES magnitude-limited sample, explained by the magnification bias and hence lower luminosity/more variable sources in quads. 

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3. Predicting the Yields of z > 6.5 Quasar Surveys in the Era of Roman and Rubin

   https://doi.org/10.3847/1538-4357/acf12d  

   Tee, Wei Leong; Fan, Xiaohui; Wang, Feige; Yang, Jinyi; Malhotra, Sangeeta; Rhoads, James E. (October 2023, The Astrophysical Journal)

   Abstract About 70 luminous quasars discovered atz> 6.5 are strongly biased toward the bright end, thus not providing a comprehensive view of quasar abundance beyond the cosmic dawn. We present the predicted results of the Roman/Rubin high-redshift quasar survey, yielding 3 times more, 2–4 mag deeper quasar samples, probing high-redshift quasars across a broad range of luminosities, especially faint quasars atL_bol∼ 10¹⁰L_⊙orM₁₄₅₀∼ −22, which are currently poorly explored. We include high-zquasars, galactic dwarfs, and low-zcompact galaxies with similar colors as quasar candidates. We create mock catalogs based on population models to evaluate selection completeness and efficiency. We utilize the classical color dropout method in thezandYbands to select primary quasar candidates, followed up with the Bayesian selection method to identify quasars. We show that overall selection completeness >80% and efficiency ∼10% at 6.5 <z< 9, with 180 quasars atz> 6.5, 20 atz> 7.5, and 2 atz> 8.5. The quasar yields depend sensitively on the assumed quasar luminosity shape and redshift evolution. Brown dwarf rejection through proper motion up to 50% can be made for stars brighter than 25 mag, low-zgalaxies dominate at fainter magnitude. Our results show that Roman/Rubin are able to discover a statistical sample of the earliest and faintest quasars in the Universe. The new valuable data sets are worth follow-up studies with JWST and Extremely Large Telescopes to determine the quasar luminosity function faint end slope and constraint the supermassive black holes growth in the early Universe. 

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4. Paving the way for Euclid and JWST via probabilistic selection of high-redshift quasars

   https://doi.org/10.1093/mnras/stac1944  

   Nanni, Riccardo; Hennawi, Joseph F; Wang, Feige; Yang, Jinyi; Schindler, Jan-Torge; Fan, Xiaohui (August 2022, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We introduce a probabilistic approach to select 6 ≤ $$z$$ ≤ 8 quasar candidates for spectroscopic follow-up, which is based on density estimation in the high-dimensional space inhabited by the optical and near-infrared photometry. Densities are modelled as Gaussian mixtures with principled accounting of errors using the extreme deconvolution (XD) technique, generalizing an approach successfully used to select lower redshift ($$z$$ ≤ 3) quasars. We train the probability density of contaminants on 1902 071 7-d flux measurements from the 1076 deg2 overlapping area from the Dark Energy Camera Legacy Survey (DECaLS) ($$z$$), VIKING (YJHKs), and unWISE (W1W2) imaging surveys, after requiring they dropout of DECaLS g and r, whereas the distribution of high-$$z$$ quasars are trained on synthetic model photometry. Extensive simulations based on these density distributions and current estimates of the quasar luminosity function indicate that this method achieves a completeness of $$\ge 56{{\ \rm per\ cent}}$$ and an efficiency of $$\ge 5{{\ \rm per\ cent}}$$ for selecting quasars at 6 < $$z$$ < 8 with JAB < 21.5. Among the classified sources are 8 known 6 < $$z$$ < 7 quasars, of which 2/8 are selected suggesting a completeness $$\simeq 25{{\ \rm per\ cent}}$$, whereas classifying the 6 known (JAB < 21.5) quasars at $$z$$ > 7 from the entire sky, we select 5/6 or a completeness of $$\simeq 80{{\ \rm per\ cent}}$$. The failure to select the majority of 6 < $$z$$ < 7 quasars arises because our quasar density model is based on an empirical quasar spectral energy distribution model that underestimates the scatter in the distribution of fluxes. This new approach to quasar selection paves the way for efficient spectroscopic follow-up of Euclid quasar candidates with ground-based telescopes and James Webb Space Telescope. 

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5. Gravitationally lensed quasars in Gaia – IV. 150 new lenses, quasar pairs, and projected quasars

   https://doi.org/10.1093/mnras/stac3721  

   Lemon, C.; Anguita, T.; Auger-Williams, M. W.; Courbin, F.; Galan, A.; McMahon, R.; Neira, F.; Oguri, M.; Schechter, P.; Shajib, A.; et al (December 2022, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT 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. 

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