Survey of Gravitationally lensed Objects in HSC Imaging (SuGOHI)

Strong gravitationally lensed quasars provide a powerful means to study galaxy evolution and cosmology. We use CHITAH, which is an algorithm used to hunt for new lens systems, particularly lensed quasars, in the Hyper Suprime-Cam Subaru Strategic Program (HSC SSP) S16A. We present 46 lens candidates, of which 3 are previously known. We select four high-grade candidates from CHITAH for spectroscopic follow-up observations, and include two additional lenses found by YATTALENS, an algorithm used to classify lensed galaxies. We obtain X-shooter spectra of these six promising candidates for lens confirmation and redshift measurements. We report new spectroscopic redshift measurements for both the lens and source galaxies in four lens systems. We apply the lens modeling software GLEE to model our six X-shooter lenses uniformly. Through our analysis of the HSC images, we find that HSCJ022622−042522, HSCJ115252+004733, and HSCJ141136−010216 have point-like lensed images, and that the lens light distribution is well aligned with the lens mass distribution within 6 deg. We estimate the fluxes of the lensed source emission lines using X-shooter spectra, and use line ratio as a diagnostic on the Baldwin-Phillips-Terlevich (BPT) diagram. As a result, we find that HSCJ022622−042522 has a probable quasar source based on the upper limit of the [N II] flux intensity. We also measure the FWHM of Lyα emission of HSCJ141136−010216 to be ∼233 km s−1, showing that it is a probable Lyman-α emitter.

A dedicated source-position transformation package: pySPT

Modern time-delay cosmography aims to infer the cosmological parameters with a competitive precision from observing a multiply imaged quasar. The success of this technique relies upon a robust modeling of the lens mass distribution. Unfortunately strong degeneracies between density profiles that lead to almost the same lensing observables may bias precise estimates of the Hubble constant. The source position transformation (SPT), which covers the well-known mass-sheet transformation (MST) as a special case, defines a new framework to investigate these degeneracies. In this paper, we present pySPT, a python package dedicated to the SPT. We describe how it can be used to evaluate the impact of the SPT on lensing observables. We review most of its capabilities and elaborate on key features that we used in a companion paper regarding SPT and time delays. The pySPT program also comes with a subpackage dedicated to simple lens modeling. This can be used to generate lensing related quantities for a wide variety of lens models independent of any SPT analysis. As a first practical application, we present a correction to the first estimate of the impact on time delays of the SPT, which has been experimentally found in a previous work between a softened power law and composite (baryons + dark matter) lenses. We find that the large deviations previously predicted have been overestimated because of a minor bug in the public lens modeling code lensmodel (v1.99), which is now fixed. We conclude that the predictions for the Hubble constant deviate by ∼7%, first and foremost as a consequence of an MST. The latest version of pySPT is available on Github, a software development platform, along with some tutorials to describe in detail how making the best use of pySPT.