Little Red and Blue Dots: simply stratified Broad Line Regions

Abstract

It has been claimed that a fraction of the so-called Little Red Dots (LRDs) are characterised by exponential broad line profiles, which have been ascribed to broadening from electron scattering by an ionised cocoon. In this work, we investigate the H$α$ broad line profiles of 32 AGN, including Little Red Dots (LRDs), Little Blue Dots (LBDs), and X-ray detected sources, using high SNR and resolution spectroscopy. We find that while single Gaussian models are statistically rejected, the exponential model is not universally preferred. Lorentzian and multi-Gaussian profiles provide equally good or superior fits for the majority of the sample, with no statistical preference for exponential profiles in $\sim$60% of cases across all AGN subtypes. There are indications that exponential profiles are preferred more frequently among LBDs, indicating that exponential profiles are not a prerogative of LRDs, which actually seem to more often favour Lorentzian profiles. Furthermore, we demonstrate that exponential wings can emerge naturally from the stratification of BLR clouds in virial motion, without invoking any scattering process. More generally, we also show that stacking multiple broad lines (either from multiple objects, as done in previous works, or from different BLR components within the same object) generally yields an exponential profile, even if none of the individual profiles are exponential. Explaining the exponential profiles in terms of BLR stratification solves various observational tensions with the electron scattering interpretation. While electron scattering may play a role, there is no evidence that it dominates the line profiles and that it significantly affects the inferred black hole masses.

Figures (15)

• Figure 1: Overview of the sample used in this work. Left: JWST/NIRCam RGB image (R -- F444W, G -- F277W, B -- F090W); the stamp size is $1\times1$ arcseconds. For XID2028 we used an HST I-band image as no NIRCam imaging is available, while for 1244, we used HST I, J and H-band images. Middle panel: Higher- spectral-resolution spectrum of H. The best fit is shown as a red dashed line, the BLR model as an orange dashed line, and the narrow component (H or [N][ii][][][6550,][85]) as a blue dashed line. Right panel: The PRISM spectrum when available.
• Figure 2: Plot of $\beta_{\rm opt}$ vs $\beta_{\rm UV}$ for selecting LRDs in the sample. The dashed lines show the different regions for selecting LRDs and LBDs. We highlight the sources selected as LRDs, LBDs and X-ray AGN as red, blue and orange circles, respectively. We also show the best fits for each object with different symbols.
• Figure 3: Examples of the model fitting procedure used in this work. The top and bottom panels show examples of targets with and without and absorption in the Balmer line. From left to right: Electron scattering model, double Gaussian model, Lorentzian model and single Gaussian model. The data are shown as a black solid line with the best-fit model shown as a red dashed line. The BLR model is shown as an orange line and the narrow H component as a blue dashed line. We show the $\chi$ residuals in the bottom panel for each model. For 42046, the best-fit model is the Lorentzian profile.
• Figure 4: Comparison of the fitted models in this work for each source. The horizontal dashed line shows $\delta$BIC of 10 from the best model. The colour of the labels shows the type of the target: LRD (red), LBD (blue) and X-ray AGN (orange).
• Figure 5: Overview of the best fit profile, with error bars (1$\sigma$) considering all of the broad H$\alpha$ profiles with $\delta$BIC$<$10. We see no evidence that LRDs or LBDs statistically prefer the electron scattering model. The X-ray AGN prefer the 2 Gaussian model.