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

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.

Figures (9)

• Figure 1: An example of the spectral energy distribution of the local analog to LRDs. Green line and dots are the SDSS spectrum and UV-IR photometries of J115438+065025. Red line and dots are JWST/NIRSpec prism spectrum (RUBIES; PID 4233, PI: de Graaff) and NIRCam and MIRI photometry (PRIMER; PID 1837, PI: Dunlop) of an LRD UDS-47509 Setton2025ApJ...995..118S, retrieved from the Dawn JWST Archive (DJA, Valentino2023ApJ...947...20Vde_Graaff2025AA...697A.189DHeintz2024Sci...384..890H). The SDSS spectrum is rebinned to meet the spectral resolution of UDS-47509's spectrum. The zoom-in window shows the broad-line H$\alpha$ of these two sources.
• Figure 2: ZTF light curves of local LRDs. Gray dots are measurements after removing data with catflags$\neq 0$ and airmass$>2$, and colored dots are rebinned by 7 days, while colors blue, green, and red correspond to $g$, $r$, and $i$ bands, respectively. The weighted excess variance $\sigma_{\rm rms}$ and $\sigma_{\rm rms}^2$/err($\sigma_{\rm rms}^2$) for each band are indicated in the upper right corner.
• Figure 3: Light-curve modeling of J092834+292136 ( top) and J115438+065025 ( bottom). The left panels show the observed light curves (black dots) together with the best-fitting DRW model (orange curves) obtained from the best-fitting parameters. The shaded regions indicate the $1\sigma$ predictive uncertainties of the model. The right panels present the posterior probability distributions of the DRW decorrelation timescale, $\tau_{\rm DRW}$, and the asymptotic structure-function amplitude, $\rm SF_\infty$. The blue lines mark the median (50th percentile) values of the posterior distributions.
• Figure 4: Decorrelation timescale $\tau_{\rm DRW}$vs. BH mass $M_{\rm BH}$. The green stars represent J092834+292136 and J115438+065025. The pink dot is the dwarf AGNs selected based on ZTF light curves Wang2023MNRAS.521...99W. The blue square represents the quasar sample Burke2021Sci...373..789B.
• Figure 5: $\rm SF_\infty$ versus $M_{\rm BH}$. Light-green stars indicate the $\rm SF_\infty$ upper limits of the seven local LRDs in our sample. Dark-green stars mark the two sources (J092834+292136 and J115438+065025) for which $\rm SF_\infty$ is derived from DRW light-curve modeling. Purple and red symbols denote high-redshift LRD samples reported by Kokubo2025ApJ...995...24K and Zhang2025ApJ...985..119Z, respectively, while the dark-red triangle represents the average upper limit of the LRD samples. Black dots in the background show SDSS quasars taken from Burke2023MNRAS.518.1880B. Blue curves represent the expected $\rm SF_\infty$ from AGN variability alone for different $M_{\rm BH}$ and Eddington ratios ($\lambda_{\rm Edd} \equiv L_{\rm bol}/L_{\rm Edd} = 1, 0.1, 0.01, 0.001$). Black curves show the expected $\rm SF_\infty$ when including host-galaxy dilution, assuming a color index of $g - r = 0.5$ and a covering factor of 10% Burke2023MNRAS.518.1880B. The gray shaded region indicates the $1\sigma$ uncertainty.