Improved dark matter measurements with flexible modeling of resolved strongly-lensed quasar narrow-line emission

Abstract

The relative brightnesses of strongly lensed quasar images, called flux ratios, respond to perturbations from low-mass dark matter halos, enabling tests of dark matter models. The quasar narrow-line region (NLR) is ideal for flux-ratio studies: large enough to be insensitive to stellar microlensing, yet compact enough to remain sensitive to dark matter halo substructure. While nuclear emission dominates NLR flux, many quasars show low surface brightness extended emission spanning kiloparsec scales that could bias measurements. To test this potential bias, we generated mock Keck OSIRIS AO observations of seven $z<1$, $L_\mathrm{bol}\sim10^{46}$ erg s$^{-1}$ quasars characteristic of sources. Only one system shows detectable extended emission after lensing. We introduce a new pipeline for simultaneously fitting point sources (nuclear) + Sérsic elliptical profiles (extended [O\,III]). We show that we recover the true flux-ratios to $<5\%$ even when the extended emission is boosted to 100 times its original flux. We also demonstrate that visual inspection of lenses reliably determines whether to use point-source-only or include extended emission modeling in the pipeline; both achieve $<5\%$ accuracy -- which is below the typical spectral fitting precision. The new pipeline and fitting procedure ensures reliable flux-ratio measurements can be made of narrow-line flux ratios for the thousands of lenses which will be discovered by Euclid, Rubin and Roman Space Telescopes.

Figures (13)

• Figure 1: Visualization of the unlensed [O III] emission reveals sources with complex morphological features, including elongation, skewness, and clumpiness. The detailed properties of these sources are summarized in Table \ref{['tab:sourceTable']}.
• Figure 2: Mock lenses created from the 7 original sources in the cross (left), fold (center), and cusp (right) configurations. Out of these only 3C 273 (top row) has detectable extended [O III] emission and thus requires our new two-component (PS + Extended source) fit. The rest can be fit with a point source only model.
• Figure 3: Two-component PS + extended source fit results using our newly developed fitting model for 3C 273, showing one example for the cross configuration (Sec. \ref{['sec:fittingObs']}). Left: mock OSIRIS image. Middle: best-fit reconstruction. Right: normalized residuals. Low residual levels indicate an excellent model fit.
• Figure 4: Recovered vs. input point-source spectra for 3C 273 images A--D (brightest: A). Blue lines show input spectra (unresolved emission from \ref{['subsec:pointSourcesLensing']}); orange points show recovered spectra, visually overlapping almost perfectly. Purple regions show residuals (recovered $-$ input). Flux ratios are calculated by integrating flux over [O III] $\lambda5007$ wavelength range for each image, then computing ratios relative to brightest image A (B/A, C/A, D/A). This yields "true" values (input spectra) and "recovered" values (extracted spectra) for comparing pipeline accuracy.
• Figure 5: Mock lenses for PG 0026+129 and PG 1426+015 with extended [O III] emission boosted 50$\times$ in cross (left), fold (center), and cusp (right) configurations. Both show detectable extended emission by-eye inspection, requiring our PS + Extended Source model.