Little Red Dots at an Inflection Point: Ubiquitous "V-Shaped" Turnover Consistently Occurs at the Balmer Limit

TL;DR

This study investigates the spectral features of 'Little Red Dots' at high redshift, revealing that their characteristic 'V-shaped' spectral inflections are most likely caused by stellar populations near the Balmer limit, rather than AGN continuum effects.

Contribution

It demonstrates that the 'V shape' in Little Red Dots' spectra is best explained by stellar populations around the Balmer limit, challenging previous models based on AGN continuum dominance.

Findings

Most unresolved red Hα emitters show a strong spectral inflection at the Balmer limit.

Models with a stellar component near the Balmer limit can reproduce the observed spectral shapes.

The spectral inflection suggests a dominant component with T~10^4 K hydrogen in Little Red Dots.

Abstract

Among the most puzzling early discoveries of JWST are "Little Red Dots" -- compact red sources that host broad Balmer emission lines and, in many cases, exhibit a "V shaped" change in slope in the rest-optical. The physical properties of Little Red Dots currently have order-of-magnitude uncertainties, because models to explain the continuum of these sources differ immensely. Here, we leverage the complete selection of red sources in the RUBIES program, supplemented with public PRISM spectra, to study the origin of this "V shape". By fitting a broken power law with a flexible inflection point, we find that a large fraction (20/44, nearly all spatially unresolved) of extremely red H$\alpha$ emitters at $2<z<6$ exhibit a strong change in slope, and that all strong inflections appear associated with the Balmer limit ($0.3645$ $\mu$m). Using a simple model of a reddened AGN with an unobscured scattered light component, we demonstrate that the observed "V shape" in Little Red Dots is unlikely to occur at any specific wavelength if the entire continuum is dominated by light from a power law AGN continuum. In contrast, models with an intrinsic feature at the Balmer limit, such as those that are dominated by evolved stellar populations in the rest-UV-to-optical, can produce the observed spectral shapes, provided that a reddened component picks up sufficiently redward of the break. While no model can comfortably explain the full Little Red Dot spectral energy distribution, the common inflection location suggests that it is most likely a single component that consistently dominates the rest-UV-to-optical in Little Red Dots, and that this component is associated with $T\sim10^4$ K hydrogen due to the clear preference for a break at H$_\infty$.