Too Quiet for Comfort: Local Little Red Dots Lack Variability over Decades

Colin J. Burke, Zachary Stone, Yue Shen, Yan-Fei Jiang

First listed 2025-11-20 | Last updated 2025-11-20

Abstract

Several local ($z\lesssim 0.2$) metal-poor dwarf AGNs have remarkably similar properties to those of high-redshift Little Red Dots (LRDs), and are recently proposed to be local analogs of LRDs. We use long-term photometric and spectroscopic observations of three local LRDs spanning $\sim 20$ years to measure variability in their rest-frame optical continuum and broad H$α$ emission lines. Using ZTF light curves over a rest-frame $\sim 5$ yr baseline, the $r$-band intrinsic rms variability is $(9\times 10^{-5})_{\rm -9E-5}^{+0.014}$ mag (J1022), $0.025\pm0.004$ mag (J1025) and $0.020\pm0.005$ mag (J1047), indicating low intrinsic variability ($<3-4\%$ at 3$σ$). These rms variability amplitudes are much lower than those for dwarf AGNs and more massive quasars. There is little structure in the optical variability structure functions for the three local LRDs, in contrast to normal AGN variability. Using available multi-epoch spectra, we constrain the broad H$α$ line flux variability to be less than a few percent, without significant profile changes, over a rest-frame baseline of $\sim 15$ yrs in J1025 and J1047, respectively. The three LRDs stand out in the Balmer line properties compared with normal broad-line AGNs, with exceptionally large H$α$ equivalent widths and H$α$/H$β$ ratios far exceeding the Case B recombination value. In the context of recent theoretical models of LRDs as dense gas-enshrouded massive black holes with super-Eddington accretion, our results suggest that the photosphere emission is long-term stable and the broad Balmer lines are primarily collisonally excited. This scenario is consistent with the lack of variability, large H$α$/H$β$ ratios and little dust extinction, as well as the expected high gas density. Virial black hole mass estimates using broad H$α$ assuming photoionization are therefore highly questionable for LRDs.

Short digest

Long-term light curves and spectra of three proposed local LRD analogs (J1022, J1025, J1047) show that both the optical continuum and broad Hα are strikingly steady. ZTF r-band monitoring over a ~5 yr rest-frame baseline yields intrinsic rms amplitudes of ~0–0.025 mag (≤3–4% at 3σ) with featureless structure functions, while multi-epoch spectra constrain broad Hα flux and profiles to vary by only a few percent over ~15 yr. The sources also exhibit extreme Balmer properties—very large Hα equivalent widths and Hα/Hβ far above Case B—unlike normal dwarf AGNs and quasars. The authors favor dense, gas-enshrouded, super‑Eddington photospheres with collisionally excited Balmer lines, implying standard virial M_BH from Hα is unreliable for LRDs.

Key figures to inspect

• Figure 2: Inspect the ZTF r-band light curves to see how flat J1022 is and how J1025/J1047 hover at ~0.02 mag rms; note the Gemini epochs (dashed lines) to confirm no contemporaneous jumps.
• Figure 3: Check WISE W1 light curves for corresponding mid-IR steadiness (weak hot-dust variability) and align with the Keck/Gemini dates to verify multi-band stability.
• Figure 4: Compare Zubercal versus difference-image light curves; the near-identical trends and tiny best-fit sinusoidal amplitudes support the very low intrinsic variability and robust error modeling.
• Figure 1: Locate J1022/J1025/J1047 on L_Hα–z relative to the 573-object dwarf-AGN comparison; use the Eddington-ratio coloring to see how these LRD analogs sit within the broader parameter space.