The Composite Spectrum of the Little Red Dots from an Inner Standard Disk and an Outer Gravitationally Unstable Disk

Abstract: One of the most mysterious results from observations of the James Webb Space Telescope (JWST) is the detection of numerous, high-redshift, very red, extremely compact, broad-line sources termed ``little red dots'' (LRDs). It is unclear whether the LRDs belong to an active galactic nucleus (AGN) or simply a collection of very compact star clusters. We build spectral energy distributions (SEDs) for 29 LRDs at $z \approx 3-8.5$ based on JWST photometric and spectroscopic observations. We find that the V-shaped SEDs of these LRDs exhibit a roughly similar break frequency at $\nu_{\rm b}=10^{14.96\pm0.06}$ Hz, which corresponds to $\lambda_{\rm b}=3287_{-424}^{+487} \text{\AA}$ in the rest frame. We propose that this unique SED can be explained by the combination of an inner standard disk and an outer gravitationally unstable accretion disk with Toomre parameter $Q\sim1$. The outer disk has a temperature of $\sim2000-4000$ K for typical AGN parameters, which can well reproduce the near-infrared to optical bump as observed in LRDs. This model can naturally explain the strong infrared to optical emission and the V-shaped SED with a similar break frequency $\simeq 10^{15}$ Hz for LRDs without invoking strong dust extinction or unusual stellar contribution from a host galaxy. Most LRDs stay in sub-Eddington state based on the SED modeling, which are intrinsically weak in X-rays.