Abstract
Timedelay cosmography of lensed quasars has achieved 2.4% precision on the measurement of the Hubble constant, H_{0}. As part of an ongoing effort to uncover and control systematic uncertainties, we investigate three potential sources: 1 stellar kinematics, 2 lineofsight effects, and 3 the deflector mass model. To meet this goal in a quantitative way, we reproduced the H0LiCOW/SHARP/STRIDES (hereafter TDCOSMO) procedures on a set of real and simulated data, and we find the following. First, stellar kinematics cannot be a dominant source of error or bias since we find that a systematic change of 10% of measured velocity dispersion leads to only a 0.7% shift on H_{0} from the seven lenses analyzed by TDCOSMO. Second, we find no bias to arise from incorrect estimation of the lineofsight effects. Third, we show that elliptical composite (stars + dark matter halo), powerlaw, and cored powerlaw mass profiles have the flexibility to yield a broad range in H_{0} values. However, the TDCOSMO procedures that model the data with both composite and powerlaw mass profiles are informative. If the models agree, as we observe in real systems owing to the “bulgehalo” conspiracy, H_{0} is recovered precisely and accurately by both models. If the two models disagree, as in the case of some pathological models illustrated here, the TDCOSMO procedure either discriminates between them through the goodness of fit, or it accounts for the discrepancy in the final error bars provided by the analysis. This conclusion is consistent with a reanalysis of six of the TDCOSMO (real) lenses: the composite model yields H_{0} = 74.0_{−1.8}^{+1.7} km s^{−1} Mpc^{−1}, while the powerlaw model yields 74.2_{−1.6}^{+1.6} km s^{−1} Mpc^{−1}. In conclusion, we find no evidence of bias or errors larger than the current statistical uncertainties reported by TDCOSMO.
Original language  English 

Article number  A101 
Number of pages  19 
Journal  Astronomy and Astrophysics 
Volume  639 
DOIs  
Publication status  Published  16 Jul 2020 
Keywords
 astroph.CO
 gravitational lensing: strong
 methods: data analysis
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Data availability statement for 'TDCOSMO. I. An exploration of systematic uncertainties in the inference of H0 from timedelay cosmography'.
Collett, T. (Creator), EDP Sciences, 16 Jul 2020
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