Connecting the little dots in polarized light

Piero Madau, Roberto Maiolino, Francesco D'Eugenio

First listed 2026-09-08 | Last updated 2026-09-05

Abstract

Recent observations of the local Little Red Dot (LRD) analog SDSS~J1025+1402 have revealed that the optical continuum is polarized at the ~1.5% level, while broad Halpha has lower polarization (~0.7%) and a different polarization angle. We explain these properties in a scenario in which LRDs are dust-obscured Little Blue Dots (LBDs) viewed at high inclination and powered by super-Eddington accretion. In this framework, electron scattering in the geometrically thick inner accretion flow produces intrinsic continuum polarization, while unpolarized emission from the outer thin disk dilutes the signal, particularly at optical wavelengths. A circumnuclear dust screen contributes dichroic polarization to both the continuum and broad-line emission. Broad Halpha lacks the intrinsic disk component and therefore has lower polarization and a different position angle. We show that this model quantitatively reproduces the observed polarization properties, although degeneracies between accretion rate and inclination remain. These results illustrate the potential of spectropolarimetry as a probe of accretion-flow geometry and orientation in the little-dot population, complementary to spectral-energy-distribution fitting and line diagnostics.

Short digest

Madau, Maiolino, and D’Eugenio model the polarized continuum and Balmer-line signal of the local LRD analog SDSS J1025+1402 in an orientation-based picture where little red dots are dust-obscured, high-inclination little blue dots powered by super-Eddington accretion. Electron scattering in a geometrically thick inner flow produces intrinsically polarized continuum emission, while unpolarized outer thin-disk light increasingly dilutes that signal toward optical wavelengths; a circumnuclear dust screen adds dichroic polarization to both continuum and broad Hα. Their combined Stokes-vector model quantitatively reproduces the roughly 1.5% continuum polarization, the lower roughly 0.7% broad-Hα polarization, and the position-angle offset because the line lacks the intrinsic disk-polarized component. The result makes spectropolarimetry a direct test of accretion-flow geometry and orientation in the little-dot population, though accretion rate and inclination remain degenerate.

Key figures to inspect

• Figure 1. This figure establishes the central orientation diagnostic: continuum polarization rises with inclination, while the unpolarized thin disk suppresses the signal most strongly at optical wavelengths. It directly motivates why an inclined, obscured little-dot system can show modest but nonzero continuum polarization.
• Figure 3. The polarization spectra show the predicted inclination dependence and the UV-to-optical decline caused by thin-disk dilution. This is the clearest population-facing prediction for testing the thick-plus-thin super-Eddington flow picture with future LRD spectropolarimetry.
• Figure 4. This is the core observational comparison for SDSS J1025+1402, fitting intensity, normalized Stokes parameters, polarization fraction, and position angle across the continuum and broad Hα. It demonstrates how dilution of disk polarization relative to dichroic line polarization produces the observed lower line polarization and position-angle rotation.
• Figure 5. The Stokes Q–U construction provides the most compact physical synthesis of the fit: intrinsically disk-polarized continuum and dichroic dust polarization combine vectorially, whereas broad Hα retains only the dichroic component. It makes the origin of the continuum-to-line polarization offset especially transparent.