Composite spectrum of Little Red Dot from a standard inner disk and an unstable outer disk

Abstract

James Webb Space Telescope (JWST) has revealed a new class of high-redshift, very red, compact broad-line sources, termed as "little red dots" (LRDs). The physical mechanism driving these properties remains elusive. We construct spectral energy distributions (SEDs) with spectroscopic redshift for 28 LRDs and find they exhibit V-shaped SEDs with a common break frequency of $ν_{\rm b}\simeq10^{14.96\pm0.06}$ Hz. We propose that the unique SEDs can be well explained by the combination of an inner standard disk and an outer gravitationally unstable accretion disk with Toomre parameter $Q\sim1$, where the outer disk has a temperature of $\sim2000-4000 K$ and mainly radiates in near-infrared to optical wavebands. The composite spectrum from this model naturally explains the V-shaped continuum and reproduces intrinsically luminous infrared-optical emission without requiring extreme dust extinction or unusual stellar populations. Even considering possible dense gas around the disk to account for pronounced Balmer breaks in some LRDs, the intrinsic optical-UV emission is only suppressed by factors of $\lesssim2-3$, which suggests that most LRDs are sub-Eddington and intrinsically weak. These results provide new insights into early-phase black hole growth and galaxy evolution.

Figures (8)

• Figure 1: Broken-power-law fits for two typical LRDs. The blue points with 1 sigma error bars indicate the photometric data used for fitting. The black points are MIRI measurements excluded from the fit, and the grey-outlined white point in the left panel marks the Ly$\alpha$-blueward flux that is also masked. The red line with the shaded region shows the best fits with $1\sigma$ uncertainty.
• Figure 1: Photometric SEDs and broken-power-law fits of LRDs. The blue circles denote the photometric measurements used in the fitting with $1\sigma$ error bars. The V-shaped SEDs are fitted with a broken-power-law function (red line), where the shaded region represents $1\sigma$ uncertainty in the fitting. Points with low SNR or affected by emission line are shown in grey ponit.
• Figure 2: Distribution of Break wavelengths and normalized SEDs of LRDs.The left panel shows the histogram of the break wavelength in V-shaped LRD SEDs. The right panel shows the normalized SEDs at $\lambda_{\rm b} = 3287\,$Å, corresponding to the break frequency $\nu_{\rm b}=10^{14.96}$ Hz, where the binned value with standard deviation is shown as black dots.
• Figure 2: Model SEDs and distribution of effective disk temperatures. Left panels show model SEDs for different black hole masses at given accretion rates and $R_\mathrm{out}=10R_\mathrm{c}$ based on our standard inner disk and gravitationally unstable outer disk model; right panels show the effective disk temperature distributions. Vertical green lines indicate the NIRCam wavelength coverage at $z=7$.
• Figure 3: Illustration of the two-component accretion-disk model. The top panel shows a schematic diagram of the inner standard disk and the outer gravitationally unstable disk with disk winds. The distribution of the disk temperature and the corresponding SED are shown in the middle and bottom panels for $M_{\rm BH}=10^{7.5}M_{\odot}$ and accretion rates of $\dot{m}=0.03, 0.1, 0.3$. This model predicts V-shaped SEDs, arising from the inner standard thin disk and the outer marginally unstable disk.