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

TL;DR

This study uses long-term photometric (ZTF) and multi-epoch spectroscopic data to probe the optical continuum and broad H$\alpha$ variability of three local LRD analogs (J1022, J1025, J1047) over rest-frame timescales up to ~15 years. The results show ultra-weak continuum variability ($<3$–$4\%$) on ~5-year rest-frame baselines and broad H$\alpha$ variability at the few-percent level over ~15 years, along with exceptionally large H$\alpha$ equivalent widths and high H$\alpha$/H$\beta$ ratios that point to collisional excitation in a dense, low-dust environment. These findings support dense gas-enshrouded supermassive black hole models with a photospheric emission region at $T_{\rm eff}\sim 5000$ K and challenge the use of broad H$\alpha$ as a virial mass estimator for LRDs. The work thus provides strong empirical constraints on LRD formation scenarios and emphasizes the need for caution when inferring black hole masses in such systems.

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.

Figures (8)

• Figure 1: The H$\alpha$ luminosity against redshift for the dwarf AGN sample and the three local LRDs (labeled). Each dwarf AGN is colored by its Eddington ratio. The dwarf AGNs have H$\alpha$ FWHM $\gtrsim 1000\ {\rm km\,s^{-1}}$.
• Figure 2: Optical ZTF r-band light curves for the three local LRDs in our sample. J1022 and J1025 have been shifted by an amount labeled above their light curves. Each light curve displays the mean (black line) and the mean measurement uncertainty (shaded region around the mean). The vertical dashed line indicates when Gemini spectra were obtained for J1025 and J1047.
• Figure 3: Infrared WISE W1 light curves for the three local LRDs in our sample. J1022 and J1025 have been shifted by an amount labeled above their light curves. Each light curve displays the mean (black line) and the mean measurement uncertainty (shaded region around the mean). The vertical dashed lines indicate when Keck and Gemini spectra were obtained for J1025 and J1047.
• Figure 4: A compairson of the Zubercal and difference image r-band light curves for the local LRDs. For each LRD, the intrinsic variability estimate $\sigma_0$ and mean measurement uncertainty $\overline{\sigma_i}$ are shown for each method. Sinusoidal fits to the difference image light curves are shown in black. The best-fit sinusoidal amplitude and period are shown in the bottom left.
• Figure 5: Intrinsic $r$-band variability measurements for different samples. Top: The normalized $\sigma_0$ distribution in the r-band for the dwarf AGN sample (gray) and a sample of broad-line quasars from Stone_etal_2022. The LRDs are shown as vertical dashed lines. Bottom: The comparison of $\sigma_0$ in the ZTF r-band and WISE W1 bands for the dwarf AGN sample (gray circles) and local LRDs (colored circles).