Local Analogs of Little Red Dots: Optical Variability and Evidence for an AGN Origin

Ruqiu Lin, Zhen-Ya Zheng, Junxian Wang, Luis C. Ho, Jorge A. Zavala, Zijian Zhang, Chunyan Jiang, Jiaqi Lin, Fang-Ting Yuan, Linhua Jiang, Tinggui Wang, Xiaer Zhang

First listed 2026-03-02 | Last updated 2026-03-16

Abstract

Little red dots (LRDs) draw extensive attention because of their unique observational characteristics and apparent overabundance in the early Universe, raising new insights into early black hole formation and growth. Early studies show that LRDs exhibit weak variability in broad-band photometry and emission-line fluxes, suggesting a preference for super-Eddington accretion or disfavouring an AGN origin. However, the cadence of the current data, and therefore, the resulting light curves for LRDs, is limited, preventing us from placing strong constraints on their variability. Based on Zwicky Transient Facility (ZTF) light curves with a baseline of $\sim6$ years, we here study the optical variability of seven previously reported local analogs of LRDs at $z \sim 0.3$, offering an insight into LRDs from a low-redshift sample. Three out of seven local analogs show excess variances on all three bands of their light curves, and two of them can be fitted with the damping random walk model, supporting their AGN origins for the variability. The remaining sources show weak variance in at least one band, but no detectable variability at the current sensitivity level, exhibiting $\rm SF_\infty$ upper limits consistent with estimates from high-redshift (high-$z$) LRDs. Their non-detection of variability is likely due to the large photometric uncertainty. As an implication, by simulating long baseline light curves with the variability amplitude of local analogs and adopting JWST observation cadence, we investigate the limitation of the variability amplitude estimate for LRDs. Our mock observations imply that the current constraints on LRDs' variability are probably underestimated. This underestimation might be induced by the short temporal baseline of observations, as well as the intrinsic scatter of the empirical $M_{\rm BH}-τ$ relation.

Short digest

Six-year ZTF g/r/i light curves for seven z≈0.3 local analogs of little red dots reveal optical variability consistent with AGN activity. Three objects show significant excess variance across all bands, and two sources (J092834+292136, J115438+065025) are well fit by a damped random-walk, strengthening the AGN interpretation. The others show weak or undetected variability, with SF_∞ upper limits consistent with high‑z LRD estimates and likely masked by photometric noise. Mock JWST‑cadence sampling of long baselines indicates current high‑z variability amplitudes are probably underestimated due to short baselines and scatter in the empirical M_BH–τ relation.

Key figures to inspect

• Figure 1: Compare the SED of J115438+065025 to the high‑z LRD UDS‑47509 to see how closely the V‑shaped UV–optical SEDs and broad Hα profiles overlap after matching spectral resolution—this motivates the ‘local analog’ premise.
• Figure 2: Scan the rebinned ZTF g/r/i light curves to identify which three sources show band‑by‑band excess variance; note seasonal gaps and photometric scatter to judge why several objects fall below variability detection thresholds.
• Figure 3: Inspect DRW fits for J092834+292136 and J115438+065025—check the predictive envelopes against data and read the posterior τ and SF_∞ to gauge characteristic timescales and amplitudes driving the AGN verdict.
• Figure 4: Place the two DRW‑modeled local LRDs on τ versus M_BH alongside dwarf‑AGN and quasar references to evaluate consistency with the empirical M_BH–τ trend and the implied scatter relevant for high‑z LRD interpretations.