Impact of Resonance, Raman, and Thomson Scattering on Hydrogen Line Formation in Little Red Dots

Abstract

Little Red Dots (LRDs) are compact sources at $z>5$ discovered through JWST spectroscopy. Their spectra exhibit broad Balmer emission lines ($\gtrsim 1000\mathrm{km}{\mathrm{s}}^{-1}$), alongside absorption features and a pronounced Balmer break -- evidence for a dense, neutral hydrogen medium with the $n=2$ state. When interpreted as arising from AGN broad-line regions, inferred black hole masses from local scaling relations exceed expectations given their stellar masses, challenging models of early black hole-galaxy co-evolution. However, radiative transfer effects in dense media may also impact the formation of hydrogen emission lines. We model three scattering processes shaping hydrogen line profiles: resonance scattering by hydrogen in the $n=2$ state, Raman scattering of UV radiation by ground-state hydrogen, and Thomson scattering by free electrons. Using 3D Monte Carlo radiative transfer simulations with multi-branching resonance transitions, we examine their imprint on line shapes and ratios. Resonance scattering produces strong deviations from Case B flux ratios, clear differences between H$\alpha$ and H$\beta$, and encodes gas kinematics in line profiles but cannot broaden H$\beta$ due to conversion to Pa$\alpha$. While Raman scattering can yield broad wings, scattering of UV continuum is disfavored given the absence of strong FWHM variations across transitions. Raman scattering of higher Lyman-series emission can produce H$\alpha$/H$\beta$ wing width ratios of $\gtrsim 1.28$, agreeing with observations. Thomson scattering can reproduce the observed $\gtrsim 1000\mathrm{km}{\mathrm{s}}^{-1}$ wings under plausible conditions, e.g., $T_{\mathrm{e}}\sim 10^4\mathrm{K}$ and $N_{\mathrm{e}}\sim 10^{24}\mathrm{c}{\mathrm{m}}^{-2}$ -- and lead to black hole mass overestimates by factors $\gtrsim 10$. Our results provide a framework for interpreting hydrogen lines in LRDs and similar systems.

arXiv comment22 pages, 21 figures, accepted for publication in MNRAS

References in corpus (43)

• Relations Between Central Black Hole Mass and Total Galaxy Stellar Mass in the Local Universe
• Estimating Black Hole Masses in Active Galaxies Using the Halpha Emission Line
• The black hole mass - stellar velocity dispersion relation of narrow-line Seyfert 1 galaxies
• What the Optical Recombination Lines Can Tell Us About the Broad Line Regions of Active Galactic Nuclei
• JWST CEERS & JADES Active Galaxies at z = 4-7 Violate the Local $M_{\bullet }-M_{\star }$ Relation at $>3\sigma$: Implications for Low-Mass Black Holes and Seeding Models
• Simple Models of Metal-Line Absorption and Emission from Cool Gas Outflows
• SN 1994W: An Interacting Supernova or Two Interacting Shells?
• RUBIES: Evolved Stellar Populations with Extended Formation Histories at $z\sim 7-8$ in Candidate Massive Galaxies Identified with JWST/NIRSpec
• JADES -- The Rosetta Stone of JWST-discovered AGN: deciphering the intriguing nature of early AGN
• Electron scattering wings on lines in interacting supernovae
• Evidence for Line Broadening by Electron Scattering in the Broad Line Region of NGC 4395
• A Balmer-line Broad Absorption Line Quasar
• JADES: comprehensive census of broad-line AGN from Reionization to Cosmic Noon revealed by JWST
• Broad H$\alpha$ Wing Formation in the Planetary Nebula IC 4997
• Challenging the AGN scenario for JWST/NIRSpec LRD and non-LRD broad H$\alpha$ emitters in light of non-detection of NIRCam photometric variability and X-ray
• A black hole in a near-pristine galaxy 700 million years after the Big Bang
• Another piece to the puzzle: radio detection of a JWST detected AGN candidate
• Asymmetric Absorption Profiles of Lyα and Lyβ in Damped Lyman Alpha Systems
• Radiative Transfer in Lyα Nebulae: I. Modeling a Continuous or Clumpy Spherical Halo with a Central Source
• Electron optical depths and temperatures of symbiotic nebulae from Thomson scattering
• COSMOS-Web: The over-abundance and physical nature of "little red dots"--Implications for early galaxy and SMBH assembly
• Raman mapping of photodissociation regions
• Polarisation of the Broad H alpha Wing in Symbiotic Stars
• A "Black Hole Star" Reveals the Remarkable Gas-Enshrouded Hearts of the Little Red Dots
• Rayleigh and Raman scattering cross-sections and phase matrices of the ground-state hydrogen atom, and their astrophysical implications
• The case for super-Eddington accretion in JWST broad-line AGN during the first billion years
• On the Polarization of H-alpha Lines Scattered by Neutral Hydrogen in Active Galactic Nuclei
• Polarization Structures in the Thomson-Scattered Emission Lines in Active Galactic Nuclei
• An episodically variable stellar wind in the planetary nebula IC\,4997
• The Discovery of Raman Scattering in HII Regions
• BlackTHUNDER strikes twice: rest-frame Balmer-line absorption and high Eddington accretion rate in a Little Red Dot at $z=7.04$
• Broad Wings around H$\alpha$ and H$\beta$ in the Two S-Type Symbiotic Stars Z Andromedae and AG Draconis
• Irony at z=6.68: a bright AGN with forbidden Fe emission and multi-component Balmer absorption
• Line Formation of Raman-Scattered He II $\lambda$ 4851 in an Expanding Spherical H I Shell in Young Planetary Nebulae
• The Discovery of Little Red Dots in the Local Universe: Signatures of Cool Gas Envelopes
• The H$\alpha$ and H$\beta$ Raman-scattering line profiles of the symbiotic star AG Peg
• Raman-scattered O VI features in the symbiotic nova RR Telescopii
• Distribution and Kinematics of H I through Raman He II Spectroscopy of NGC 6302
• Ruling out dominant electron scattering in Little Red Dots' Rosetta Stone using multiple hydrogen lines
• JWST/NIRSpec Observations of High Ionization Emission Lines in Galaxies at High Redshift
• Space UV polarimeters
• A deep dive down the broad-line region: permitted OI, CaII and FeII emission in an AGN Little Red Dot at z=5.3
• CAPERS-LRD-z9: A Gas Enshrouded Little Red Dot Hosting a Broad-line AGN at z=9.288

Cited by in corpus (5)

• Little Red Dots As Late-stage Quasi-stars
• The warm outer layer of a Little Red Dot as the source of [Fe II] and collisional Balmer lines with scattering wings
• Spectral Uniformity of Little Red Dots: A Natural Outcome of Coevolving Seed Black Holes and Nascent Starbursts
• The Cliff: A Metal-Poor Little Red Dot Hosting an Overmassive Black Hole at $z=3.55$
• Massive black holes in the first billion years with JWST