The Engine and its Flows: Little Red Dot spectra are shaped by the column densities of their gas envelopes

Abstract

JWST data have enabled the abundant identification of compact broad Balmer line sources nicknamed the Little Red Dots. While they share broad lines with active galactic nuclei, they are unusually X-ray and infrared weak. We investigate the origin of the Balmer line profiles based on an empirical analysis of 18 broad H$α$-selected sources with high quality spectra at $z\approx3-7$. The H$α$ line profiles vary systematically with Balmer break strength: sources with blue UV to optical colors show a narrow core profile, redder sources with Balmer breaks a blue shifted absorption (P Cygni shape), and the reddest sources display absorption-dominated cores. All H$α$ lines have symmetric exponential wings, which are more dominant and slightly broader in red sources. Balmer absorption is present in $\sim60$ % of the sample, with H$β$ showing relatively stronger absorption. Drawing upon empirical analogies with stellar phenomena, we interpret these trends as being due to radiative processes that depend on variations in the optical depth, ionisation state and column density of a clumpy, partially ionised envelope. We unveil a correlation between the absorber velocity and Balmer break strength, with the densest absorbers inflowing and bluer sources having faster outflows. This indicates viewing angle or evolutionary effects where optically thick gas is inflowing, as suggested in models of super-Eddington accretion, and the engine can more easily drive outflows in directions with lower column densities. This new understanding of Balmer line profiles as tracing gas properties rather than dynamical broadening helps resolve tensions associated with high inferred black hole masses from standard virial calibrations, and reveals the complex gas environment around the hot central engine.

Figures (13)

• Figure 1: Demographics and comparison of our sample to the literature. The H$\alpha$ luminosity and Balmer break strength of the sample included in this work (red), compared to a recent LRD compilation (deGraaff25b) in grey. Our sample is skewed towards higher H$\alpha$ luminosity as these have preferentially been targeted by deep grating follow up. No strong biases are present in terms of the Balmer break strengths.
• Figure 2: The Balmer line profiles are correlated with the spectral shape of the broad-band spectrum. Each row shows the median stacked spectrum of a sub-set split by their UV to optical continuum color, increasingly red from top to bottom. Shaded regions illustrate the variation within each stack based on bootstrap resamples. In the left column, we show the stacked broad-band spectrum of the bin (normalised to the continuum level at 5500 Å rest-frame) in color, while the median stack of our full sample is shown in grey. The middle and right (log scale) panels show the H$\alpha$, H$\beta$ and [O iii] profiles, in pink, blue and green, respectively. The profiles are normalised to the peak flux at the systemic velocity. The linear scale in the middle column highlights variations in the line-center, while the logarithmic scale in the right column highlights variations in the broad wings. The Balmer lines of redder sources are increasingly more dominated by broad, exponential emission. The central parts of the line-profile changes from narrow and compact to blue-shifted absorption with red-shifted emission (like P Cygni), to broad absorption with narrow central emission, from blue to red sources.
• Figure 3: The variation in faint emission-line strengths across the range of UV to optical colors. Stacked PRISM spectra (as in the left column in Fig. \ref{['fig:prism_sample']}) in our four sub-sets of UV to optical color, with blue corresponding to the bluest subset, and red to the reddest. Shaded regions illustrate the variation within the sub-sets based on bootstrap resampling. In the top panel, we highlight emission features in the rest-frame UV, while we show faint features in the optical regime in the bottom panel. Differences that correlate with the UV to optical redness are the weakness of N iv] emission and the strength of Fe ii features. Redder sources show lower EWs in the higher-order Balmer lines and are absent in He ii. [Ne iii] appears relatively strong across the sub-sets.
• Figure 4: The similarity and variety among emission-line spectra of broad H$\alpha$ line emitters. Objects are sorted by the UV to optical color such that blue sources are shown on top-left and the reddest are in the bottom-right. To highlight variations in equivalent width of the various lines, lines are normalised to their corresponding continuum flux density. We highlight sources with significant detection of Balmer absorption lines.
• Figure 5: Example model fits to the H$\beta$ and H$\alpha$ line-profiles of four broad-line H$\alpha$ emitters with H-grating spectra, sorted by optical to UV color. Continuum-subtracted H$\beta$ profiles are shown in the left column, while H$\alpha$ is shown in the right. Each panel shows the data (black) and the residual of the best-fit (brown shows the full model), with associated $\chi^2_{\rm r}$ below. For the H$\alpha$ residuals, green dash-dotted lines mark the locations of the [N ii] doublet. The purple line shows the narrow-subtracted model. Blue shows the intermediate emission component and pink the exponential component. Grey regions are excluded from the fits as they are possibly affected by [Fe ii] emission. While exponential wings are present in virtually all sources, the core of the line-profile displays most variation. The cores of the H$\beta$ lines show stronger absorption compared to the H$\alpha$ line, and narrow central emission that is likely associated with the host galaxy.