Redshift of the lensing galaxy in three doubly imaged quasars

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Abstract
Recently, the TDCOSMO Collaboration presented a sample of eight doubly imaged quasars (doubles) that will be used to measure the Hubble constant from time-delay cosmography. While multi-band observations of this sample with the Hubble Space Telescope enabled the TDCOSMO Collaboration to make an initial reconstruction of the Fermat potential difference between the two images of each double (assuming an isothermal mass distribution of the lensing galaxy plus external shear), three lens redshifts remain unmeasured or without a direct spectroscopic confirmation, and these are crucial ingredients for time-delay cosmography and measuring the Hubble constant using this sample. From archive spectroscopic observations, we extracted the spectra of the two lensing galaxies with an as-yet-unknown redshift, and obtained z l = 0.595 ± 0.001 for DES J0407−5006 and z l = 0.255 ± 0.001 for DES J0602−4335 . Additionally, archive spectroscopic data of SDSS J1515+1511 allowed us to confirm the previously adopted value for the lens redshift and estimate its uncertainty, z l = 0.742 ± 0.001. We also discussed the role that deviations from isothermality and external convergences can play in a time-delay cosmography from the sample of doubles.

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