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We study the properties of the foreground galaxy of the Ruby, the brightestgravitationally lensed high-redshift galaxy on the sub-millimeter sky as probedby the Planck satellite, and part of our sample of Planck's Dusty GEMS. TheRuby consists of an Einstein ring of 1.4" diameter at z = 3.005 observed withALMA at 0.1" resolution, centered on a faint, red, massive lensing galaxy seenwith HST/WFC3, which itself has an exceptionally high redshift, z = 1.525 $\pm$0.001, as confirmed with VLT/X-Shooter spectroscopy. Here we focus on theproperties of the lens and the lensing model obtained with LENSTOOL. Therest-frame optical morphology of this system is strongly dominated by the lens,while the Ruby itself is highly obscured, and contributes less than 10% to thephotometry out to the K band. The foreground galaxy has a lensing mass of (3.70$\pm$ 0.35) $\times$ 10$^{11}$ M$_{\odot}$. Magnification factors are between 7and 38 for individual clumps forming two image families along the Einsteinring. We present a decomposition of the foreground and background sources inthe WFC3 images, and stellar population synthesis modeling with a range ofstar-formation histories for Chabrier and Salpeter initial mass functions(IMFs). Only the stellar mass range obtained with the latter agrees well withthe lensing mass. This is consistent with the bottom-heavy IMFs of massivehigh-redshift galaxies expected from detailed studies of the stellar masses andmass profiles of their low-redshift descendants, and from models of turbulentgas fragmentation. This may be the first direct constraint on the IMF in a lensat z = 1.5, which is not a cluster central galaxy.

Planck's Dusty GEMS. III. A massive lensing galaxy with a bottom-heavy stellar initial mass function at z=1.5