Analysis of Multi-epoch JWST Images of $\sim 300$ Little Red Dots: Tentative Detection of Variability in a Minority of Sources

TL;DR

The paper investigates whether the faint, red, compact high-redshift LRDs exhibit photometric variability using all public multi-epoch JWST NIRCam (and some MIRI) data across five deep fields. It develops a rigorous photometric pipeline with systematic zero-point and uncertainty calibration to measure $SNR_{\rm var}$ for $\Delta m$ across visits, finding the LRD population, on average, does not show strong variability; eight sources show significant variability, with COS-584 and COS-593 standing out as robust cases. The authors interpret the generally weak variability as compatible with super-Eddington accretion in massive black holes or substantial host-galaxy dilution, supported by DRW-based variability limits and simulated $SNR_{\rm var}$ distributions that favor high Eddington ratios. They also demonstrate how variability constraints can help decompose AGN and host contributions in SED fitting, and discuss PSF-related systematics and future JWST campaigns to enhance temporal coverage. Overall, the study provides a foundational variability census for LRDs and highlights a path to distinguishing AGN activity from galaxy-dominated emission in the early universe.

Abstract

James Webb Space Telescope (JWST) has revealed a population of red and compact sources at $z \gtrsim 5$ known as ``Little Red Dots'' (LRDs) that are likely active galactic nuclei (AGNs). Here we present a comprehensive study of the variability of 314 LRDs with multi-epoch JWST observations in five deep fields: UDS, GOODS-S, GOODS-N, Abell 2744, and COSMOS. Our analyses use all publicly available JWST NIRCam imaging data in these fields, together with multi-epoch JWST MIRI images available. We measure the significance (signal-to-noise ratio or ${\rm SNR}_{\rm var}$) of the variabilities for all LRDs and statistically evaluate their variabilities using the ${\rm SNR}_{\rm var}$ distributions. We pay particular attention to the systematic offsets of photometric zero points among different epochs that seem to commonly exist. The derived ${\rm SNR}_{\rm var}$ distributions of the LRDs, including those with broad H$α$/H$β$ emission lines, follow the standard Gaussian distribution, and are generally consistent with those of the comparison samples of objects detected in the same images. This finding suggests that the LRD population on average does not show strong variability, which can be explained by super-Eddington accretion of the black holes in AGNs. Alternatively, many of them may be dominated by galaxies. We also find eight strongly variable LRD candidates with variability amplitudes of 0.24 -- 0.82 mag. The rest-frame optical SEDs of these variable LRDs should have significant AGN contribution. Future JWST observations will provide more variability information of LRDs.

Figures (19)

• Figure 1: (a) Color-magnitude diagram of F444W vs. F150W$-$F444W and (b) color-color diagram of F277W$-$F356W vs. F277W$-$F444W for the 314 LRDs that have multi-epoch detections. The LRDs from 2024arXiv240610341A, 2024arXiv240403576K, and 2024ApJ...968...38K are denoted as the large solid yellow dots, blue open circles, and small solid red dots, respectively. LRDs from other works are denoted as the black crosses. The eight variable LRD candidates in Section \ref{['subsec:individual_LRD_variability']} are enclosed in the black squares. For comparison, all sources identified in these five fields are shown in gray.
• Figure 2: JWST/NIRCam coverage of the five fields (north is up and east to the left) and the regions of multi-epoch NIRCam observations for UDS, A2744, GOODS-S, GOODS-N, and COSMOS fields. The background images are the co-added mosaic images of all bands in each field. Regions that overlap between visits in the same band are highlighted in the orange shades. A darker color indicates more visits with all bands combined, as shown in the color bars. The blue stars denote the LRDs with multi-epoch detections at least in one band. The green open circles denote the LRDs that do not have multi-epoch detections in any band.
• Figure 3: Density distribution of systematic photometric zero point offsets of all NIRCam visit pairs that have more than 50 sources before (blue solid line histogram) and after correction (orange shaded histogram).
• Figure 4: Magnitude difference versus the mean magnitude of the F277W band for the GOODS-S field. The small gray dots are the $3\sigma$ clipped sample used to calculate the mean and standard deviation of $\Delta m$ in the magnitude bins, while the small red dots are clipped $3\sigma$ outliers. The green and black dots are the mean and standard deviation of $\Delta m$ in each bin, respectively. The brown dots are the median values of the measured errors of $\Delta m$ propagated from the errors of $m_{\rm visit1}$ and $m_{\rm visit2}$. The yellow dashed curves are the $1\sigma$, $2\sigma$, and $3\sigma$ deviation from the mean value. The black dashed curves represent $\Delta m = 0$.
• Figure 5: The $\Delta m$ error correction curves of different bands for the GOODS-S field.