A hyper luminous starburst at z = 4.72 magnified by a lensing galaxy pair at z = 1.48

¹ Aix Marseille Univ., CNRS, CNES, LAM, Marseille, France
e-mail: laure.ciesla@lam.fr
² Laboratoire AIM-Paris-Saclay, CEA/DSM/Irfu – CNRS – Université Paris Diderot, CEA-Saclay, 91191 Gif-sur-Yvette, France
³ Univ Lyon, Univ Lyon1, ENS de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon UMR5574, 69230 Saint-Genis-Laval, France
⁴ Núcleo de Astronomía, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Ejército Libertador 441, Santiago 8320000, Chile
⁵ Astronomy Centre, Department of Physics and Astronomy, University of Sussex, Brighton BN1 9QH, UK
⁶ Centro de Astronomía (CITEVA), Universidad de Antofagasta, Avenida Angamos 601, Antofagasta, Chile
⁷ Institute of Astronomy, Graduate School of Science, The University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan
⁸ Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
⁹ European Southern Observatory, Alonso de Córdova 3107, Casilla, 19001 Vitacura, Santiago, Chile
¹⁰ School of Physics, O’Brien Centre for Science North, University College Dublin, Belfield, Dublin 4, Ireland
¹¹ Department of Physics and Astronomy, Rutgers, The State University of New Jersey, 136 Frelinghuysen Road, Piscataway, NJ 08854-8019, USA
¹² Department of Physics, University of Crete, 71003 Heraklion, Greece
¹³ Institute of Astrophysics, Foundation for Research and Technology-Hellas, 71110 Heraklion, Greece
¹⁴ National Research Council, 5071 W. Saanich Road, Victoria, BC V9E 2E7, Canada
¹⁵ Instituto de Física, Pontificia Universidad Católica de Valparaíso, Casilla 4059, Valparaíso, Chile

Received: 18 September 2019
Accepted: 9 January 2020

Abstract

We serendipitously discovered in the Herschel Reference Survey an extremely bright infrared source with S₅₀₀ ∼ 120 mJy in the line of sight of the Virgo cluster which we name Red Virgo 4 (RV4). Based on IRAM/EMIR and IRAM/NOEMA detections of the CO(5−4), CO(4−3), and [CI] lines, RV4 is located at a redshift of 4.724, yielding a total observed infrared luminosity of 1.1 ± 0.6 × 10¹⁴ L_⊙. At the position of the Herschel emission, three blobs are detected with the VLA at 10 cm. The CO(5−4) line detection of each blob confirms that they are at the same redshift with the same line width, indicating that they are multiple images of the same source. In Spitzer and deep optical observations, two sources, High-z Lens 1 (HL1) West and HL1 East, are detected at the center of the three VLA/NOEMA blobs. These two sources are placed at z = 1.48 with X-shooter spectra, suggesting that they could be merging and gravitationally lensing the emission of RV4. HL1 is the second most distant lens known to date in strong lensing systems. Constrained by the position of the three VLA/NOEMA blobs, the Einstein radius of the lensing system is 2.2″ ± 0.2 (20 kpc). The high redshift of HL1 and the large Einstein radius are highly unusual for a strong lensing system. In this paper, we present the insterstellar medium properties of the background source RV4. Different estimates of the gas depletion time yield low values suggesting that RV4 is a starburst galaxy. Among all high-z submillimeter galaxies, this source exhibits one of the lowest L_[CI] to L_IR ratios, 3.2 ± 0.9 × 10⁻⁶, suggesting an extremely short gas depletion time of only 14 ± 5 Myr. It also shows a relatively high L_[CI] to L_CO(4−3) ratio (0.7 ± 0.2) and low L_CO(5−4) to L_IR ratio (only ∼50% of the value expected for normal galaxies) hinting at low density of gas. Finally, we discuss the short depletion time of RV4. It can be explained by either a very high star formation efficiency, which is difficult to reconcile with major mergers simulations of high-z galaxies, or a rapid decrease of star formation, which would bias the estimate of the depletion time toward an artificially low value.