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Published January 20, 2023 | Published
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A Highly Magnified Gravitationally Lensed Red QSO at z = 2.5 with a Significant Flux Ratio Anomaly


We present the discovery of a gravitationally lensed dust-reddened QSO at z = 2.517, identified in a survey for QSOs by infrared selection. Hubble Space Telescope imaging reveals a quadruply lensed system in a cusp configuration, with a maximum image separation of ∼1″.8. We find that, compared to the central image of the cusp, the neighboring brightest image is anomalous by a factor of ∼7–10, which is the largest flux anomaly measured to date in a lensed QSO. Incorporating high-resolution Very Large Array radio imaging and submillimeter imaging with the Atacama Large Millimeter/submillimeter Array, we conclude that a low-mass perturber is the most likely explanation for the anomaly. The optical through near-infrared spectrum reveals that the QSO is moderately reddened with E(B − V) ≃ 0.7–0.9. We see an upturn in the ultraviolet spectrum due to ∼1% of the intrinsic emission being leaked back into the line of sight, which suggests that the reddening is intrinsic and not due to the lens. The QSO may have an Eddington ratio as high as L/L_Edd ≈ 0.2. Consistent with previous red QSO samples, this source exhibits outflows in its spectrum, as well as morphological properties suggestive of it being in a merger-driven transitional phase. We find a host galaxy stellar mass of log M⋆/M_⊙ = 11.4, which is higher than the local M_BH versus M⋆ relation but consistent with other high-redshift QSOs. When demagnified, this QSO is at the knee of the luminosity function, allowing for the detailed study of a more typical moderate-luminosity infrared-selected QSO at high redshift.

Additional Information

© 2023. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. We thank the anonymous referee for their careful reading of the manuscript and thoughtful suggestions, which have significantly improved the quality of our final work. E.G. acknowledges the generous support of the Cottrell Scholar Award through the Research Corporation for Science Advancement. E.G. is grateful to the Mittelman Family Foundation for their generous support. This work was performed in part at Aspen Center for Physics, which is supported by National Science Foundation grant PHY-1607611. C.E.R. thanks Paul Schechter for useful discussions. C.S. is grateful for financial support from the National Research Council of Science and Technology, Korea (EU-16-001), and from the Italian Ministry of University and Research—Project Proposal CIR01_00010. S.G.D. and M.J.G. acknowledge partial support from the NSF grants AST-1413600 and AST-1518308, as well as NASA grant 16-ADAP16-0232. This work is based on GO observations made with the NASA/ESA Hubble Space Telescope from the Mikulski Archive for Space Telescopes (MAST), which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. These observations are associated with program No. 14706. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2019.1.00964.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. We thank the staff at the Keck Observatory, where some of the data presented here were obtained. The authors recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. This work makes use of the following Python packages: Astropy (Astropy Collaboration et al. 2013), Numpy (van der Walt et al. 2011), Scipy (Virtanen et al. 2020) and Matplotlib (Hunter 2007). Facilities: HST(WFC3/IR) - Hubble Space Telescope satellite, Keck(LRIS) - , IRTF(SpeX) - , LBT(MODS1B) - , NRAO(VLA) - , ALMA - . Software: Spextool (Cushing et al. 2004), IRAF (Tody 1986, Tody 1993), DrizzlePac (Hack et al. 2012), glafic (Oguri 2010), galfit (Peng et al. 2002).

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August 22, 2023
October 25, 2023