Planck’s dusty GEMS

III. A massive lensing galaxy with a bottom-heavy stellar initial mass function at z = 1.5^⋆

¹ Institut d’Astrophysique Spatiale, CNRS, Univ. Paris-Sud, Université Paris-Saday, Bât. 121, 91405 Orsay Cedex, France
e-mail: raoul.canameras@ias.u-psud.fr
² European Southern Observatory, ESO Vitacura, Alonso de Cordova 3107, Vitacura, Casilla 19001, Santiago, Chile
³ Atacama Large Millimeter/submillimeter Array, ALMA Santiago Central Offices, Alonso de Cordova 3107, Vitacura, Casilla 763-0355, Santiago, Chile
⁴ Aix-Marseille Université, CNRS, LAM, Laboratoire d’Astrophysique de Marseille, Marseille, France
⁵ Institut d’Astrophysique de Paris, 75014 Paris, UPMC Univ. Paris 6, UMR 7095, France
⁶ Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, 6658 British Columbia, Canada
⁷ Steward Observatory, University of Arizona, Tucson, AZ 85721, USA
⁸ Chalmers University of Technology, Onsala Space Observatory, Onsala, Sweden
⁹ Laboratoire AIM, CEA/DSM/IRFU, CNRS, Université Paris-Diderot, Bât. 709, 91191 Gif-sur-Yvette, France
¹⁰ Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK
¹¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany

Received: 26 December 2016
Accepted: 15 February 2017

Abstract

We study the properties of the foreground galaxy of the Ruby, the brightest gravitationally lensed high-redshift galaxy on the sub-millimeter sky as probed by the Planck satellite, and part of our sample of Planck’s dusty GEMS. The Ruby consists of an Einstein ring of 1.4′′ diameter at z = 3.005 observed with ALMA at 0.1′′ resolution, centered on a faint, red, massive lensing galaxy seen with HST/WFC3, which itself has an exceptionally high redshift, z = 1.525 ± 0.001, as confirmed with VLT/X-shooter spectroscopy. Here we focus on the properties of the lens and the lensing model obtained with LENSTOOL. The rest-frame optical morphology of this system is strongly dominated by the lens, while the Ruby itself is highly obscured, and contributes less than 10% to the photometry out to the K band. The foreground galaxy has a lensing mass of (3.70 ± 0.35) × 10¹¹M_⊙. Magnification factors are between 7 and 38 for individual clumps forming two image families along the Einstein ring. We present a decomposition of the foreground and background sources in the WFC3 images, and stellar population synthesis modeling with a range of star-formation histories for Chabrier and Salpeter initial mass functions (IMFs). Only the stellar mass range obtained with the latter agrees well with the lensing mass. This is consistent with the bottom-heavy IMFs of massive high-redshift galaxies expected from detailed studies of the stellar masses and mass profiles of their low-redshift descendants, and from models of turbulent gas fragmentation. This may be the first direct constraint on the IMF in a lens at z = 1.5, which is not a cluster central galaxy.