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ABSTRACT Core formation and runaway core collapse in models with selfinteracting dark matter (SIDM) significantly alter the central density profiles of collapsed haloes. Using a forward modelling inference framework with simulated datasets, we demonstrate that flux ratios in quadruple image strong gravitational lenses can detect the unique structural properties of SIDM haloes, and statistically constrain the amplitude and velocity dependence of the interaction crosssection in haloes with masses between 106 and 1010 M⊙. Measurements on these scales probe selfinteractions at velocities below $30 \ \rm {km} \ \rm {s^{1}}$, a relatively unexplored regime of parameter space, complimenting constraints at higher velocities from galaxies and clusters. We cast constraints on the amplitude and velocity dependence of the interaction crosssection in terms of σ20, the crosssection amplitude at $20 \ \rm {km} \ \rm {s^{1}}$. With 50 lenses, a sample size available in the near future, and flux ratios measured from spatially compact midIR emission around the background quasar, we forecast $\sigma _{20} \lt 11\rm {\small {}}23 \ \rm {cm^2} \rm {g^{1}}$ at $95 {{\ \rm per\ cent}}$ CI, depending on the amplitude of the subhalo mass function, and assuming cold dark matter (CDM). Alternatively, if $\sigma _{20} = 19.2 \ \rm {cm^2}\rm {g^{1}}$ we can rule out CDM with a likelihood ratio of 20:1, assuming an amplitude of the subhalo mass function that results from doubly efficient tidal disruption in the Milky Way relative to massive elliptical galaxies. These results demonstrate that strong lensing of compact, unresolved sources can constrain SIDM structure on subgalactic scales across cosmological distances, and the evolution of SIDM density profiles over several Gyr of cosmic time.more » « less

Time delay cosmography uses the arrival time delays between images in strong gravitational lenses to measure cosmological parameters, in particular the Hubble constant H 0 . The lens models used in time delay cosmography omit dark matter subhalos and lineofsight halos because their effects are assumed to be negligible. We explicitly quantify this assumption by analyzing mock lens systems that include full populations of dark matter subhalos and lineofsight halos, applying the same modeling assumptions used in the literature to infer H 0 . We base the mock lenses on six quadruply imaged quasars that have delivered measurements of the Hubble constant, and quantify the additional uncertainties and/or bias on a lensbylens basis. We show that omitting dark substructure does not bias inferences of H 0 . However, perturbations from substructure contribute an additional source of random uncertainty in the inferred value of H 0 that scales as the square root of the lensing volume divided by the longest time delay. This additional source of uncertainty, for which we provide a fitting function, ranges from 0.7 − 2.4%. It may need to be incorporated in the error budget as the precision of cosmographic inferences from single lenses improves, and it sets a precision limit on inferences from single lenses.more » « less

Time delay cosmography uses the arrival time delays between images in strong gravitational lenses to measure cosmological parameters, in particular the Hubble constant H0. The lens models used in time delay cosmography omit dark matter subhalos and lineofsight halos because their effects are assumed to be negligible. We explicitly quantify this assumption by analyzing realistic mock lens systems that include full populations of dark matter subhalos and lineofsight halos, applying the same modeling assumptions used in the literature to infer H0. We base the mock lenses on six quadruplyimaged quasars that have delivered measurements of the Hubble constant, and quantify the additional uncertainties and/or bias on a lensbylens basis. We show that omitting dark substructure does not bias inferences of H0. However, perturbations from substructure contribute an additional source of random uncertainty in the inferred value of H0 that scales as the square root of the lensing volume divided by the longest time delay. This additional source of uncertainty, for which we provide a fitting function, ranges from 0.6−2.4%. It may need to be incorporated in the error budget as the precision of cosmographic inferences from single lenses improves, and sets a precision limit on inferences from single lenses.more » « less

ABSTRACT We report the results of the STRong lensing Insights into the Dark Energy Survey (STRIDES) followup campaign of the late 2017/early 2018 season. We obtained spectra of 65 lensed quasar candidates with ESO Faint Object Spectrograph and Camera 2 on the NTT and Echellette Spectrograph and Imager on Keck, confirming 10 new lensed quasars and 10 quasar pairs. Eight lensed quasars are doubly imaged with source redshifts between 0.99 and 2.90, one is triply imaged (DESJ0345−2545, z = 1.68), and one is quadruply imaged (quad: DESJ0053−2012, z = 3.8). Singular isothermal ellipsoid models for the doubles, based on highresolution imaging from SAMI on Southern Astrophysical Research Telescope or Near InfraRed Camera 2 on Keck, give total magnifications between 3.2 and 5.6, and Einstein radii between 0.49 and 1.97 arcsec. After spectroscopic followup, we extract multiepoch grizY photometry of confirmed lensed quasars and contaminant quasar + star pairs from DES data using parametric multiband modelling, and compare variability in each system’s components. By measuring the reduced χ2 associated with fitting all epochs to the same magnitude, we find a simple cut on the less variable component that retains all confirmed lensed quasars, while removing 94 per cent of contaminant systems. Based on our spectroscopic followup, this variability information improves selection of lensed quasars and quasar pairs from 3445 per cent to 51–70 per cent, with most remaining contaminants being starforming galaxies. Using mock lensed quasar light curves we demonstrate that selection based only on variability will overrepresent the quad fraction by 10 per cent over a complete DES magnitudelimited sample, explained by the magnification bias and hence lower luminosity/more variable sources in quads.more » « less

ABSTRACT The magnifications of compactsource lenses are extremely sensitive to the presence of lowmass dark matter haloes along the entire sightline from the source to the observer. Traditionally, the study of dark matter structure in compactsource strong gravitational lenses has been limited to radioloud systems, as the radio emission is extended and thus unaffected by microlensing which can mimic the signal of dark matter structure. An alternate approach is to measure quasar nuclearnarrowline emission, which is free from microlensing and present in virtually all quasar lenses. In this paper, we double the number of systems which can be used for gravitational lensing analyses by presenting measurements of narrowline emission from a sample of eight quadruply imaged quasar lens systems, WGD J0405−3308, HS 0810+2554, RX J0911+0551, SDSS J1330+1810, PS J1606−2333, WFI 2026−4536, WFI 2033−4723, and WGD J2038−4008. We describe our updated grism spectral modelling pipeline, which we use to measure narrowline fluxes with uncertainties of 2–10 per cent, presented here. We fit the lensed image positions with smooth mass models and demonstrate that these models fail to produce the observed distribution of image fluxes over the entire sample of lenses. Furthermore, typical deviations are larger than those expected from macromodel uncertainties. This discrepancy indicates the presence of perturbations caused by smallscale dark matter structure. The interpretation of this result in terms of dark matter models is presented in a companion paper.more » « less

Abstract The massconcentration relation of dark matter halos reflects the assembly history of objects in hierarchical structure formation scenarios, and depends on fundamental quantities in cosmology such as the slope of the primordial matter powerspectrum. This relation is unconstrained by observations on subgalactic scales. We derive the first measurement of the massconcentration relation using the image positions and flux ratios from eleven quadrupleimage strong gravitational lenses (quads) in the mass range 106 − 1010M⊙, assuming cold dark matter. We model both subhalos and line of sight halos, finitesize background sources, and marginalize over nuisance parameters describing the lens macromodel. We also marginalize over the the logarithmic slope and redshift evolution of the massconcentration relation, using flat priors that encompass the range of theoretical uncertainty in the literature. At z = 0, we constrain the concentration of 108M⊙ halos $c=12_{5}^{+6}$ at $68 \%$ CI, and $c=12_{9}^{+15}$ at $95 \%$ CI. For a 107M⊙ halo, we obtain $68 \%$ ($95 \%$) constraints $c=15_{8}^{+9}$ ($c=15_{11}^{+18}$), while for 109M⊙ halos $c=10_{4}^{+7}$ ($c=10_{7}^{+14}$). These results are consistent with the theoretical predictions from massconcentration relations in the literature, and establish strong lensing by galaxies as a powerful probe of halo concentrations on subgalactic scales across cosmological distance.more » « less

ABSTRACT The freestreaming length of dark matter depends on fundamental dark matter physics, and determines the abundance and concentration of dark matter haloes on subgalactic scales. Using the image positions and flux ratios from eight quadruply imaged quasars, we constrain the freestreaming length of dark matter and the amplitude of the subhalo mass function (SHMF). We model both main deflector subhaloes and haloes along the line of sight, and account for warm dark matter freestreaming effects on the mass function and mass–concentration relation. By calibrating the scaling of the SHMF with host halo mass and redshift using a suite of simulated haloes, we infer a global normalization for the SHMF. We account for finitesize background sources, and marginalize over the mass profile of the main deflector. Parametrizing dark matter freestreaming through the halfmode mass mhm, we constrain the thermal relic particle mass mDM corresponding to mhm. At $95 \, {\rm per\, cent}$ CI: mhm < 107.8 M⊙ ($m_{\rm {DM}} \gt 5.2 \ \rm {keV}$). We disfavour $m_{\rm {DM}} = 4.0 \,\rm {keV}$ and $m_{\rm {DM}} = 3.0 \,\rm {keV}$ with likelihood ratios of 7:1 and 30:1, respectively, relative to the peak of the posterior distribution. Assuming cold dark matter, we constrain the projected mass in substructure between 106 and 109 M⊙ near lensed images. At $68 \, {\rm per\, cent}$ CI, we infer $2.0{}6.1 \times 10^{7}\, {{\rm M}_{\odot }}\,\rm {kpc^{2}}$, corresponding to mean projected mass fraction $\bar{f}_{\rm {sub}} = 0.035_{0.017}^{+0.021}$. At $95 \, {\rm per\, cent}$ CI, we obtain a lower bound on the projected mass of $0.6 \times 10^{7} \,{{\rm M}_{\odot }}\,\rm {kpc^{2}}$, corresponding to $\bar{f}_{\rm {sub}} \gt 0.005$. These results agree with the predictions of cold dark matter.more » « less