Letter to the Editor

Double-peaked Ca II traces a relativistic broad-line region disk in NGC 4593

¹ Institut für Astrophysik und Geophysik, Universität Göttingen, Friedrich-Hund Platz 1, 37077 Göttingen, Germany
² Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
³ Department of Physics, Faculty of Natural Sciences, University of Haifa, Haifa 3498838, Israel
⁴ Haifa Research Center for Theoretical Physics and Astrophysics, University of Haifa, Haifa 3498838, Israel
⁵ Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute (AIRUB), 44780 Bochum, Germany
⁶ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Bartycka 18, 00-716 Warszawa, Poland
⁷ Universidad Católica del Norte, Instituto de Astronomía, Avenida Angamos, 0610 Antofagasta, Chile
⁸ Department of Physics, Geology, and Engineering Technology, Northern Kentucky University, 1 Nunn Drive, Highland Heights, KY 41099, USA
⁹ School of Physics & Astronomy and the Wise Observatory, The Raymond and Beverly Sackler Faculty of Exact Sciences, Tel-Aviv University, Tel-Aviv 6997801, Israel
¹⁰ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

^⋆ Corresponding author: martin.ochmann@uni-goettingen.de

Received: 27 March 2025
Accepted: 16 April 2025

Abstract

Context. Double-peaked emission lines are observed in a small percentage of active galactic nuclei (AGN). These lines allow the determination of fundamental properties of the line-emitting region, known as the broad-line region (BLR).

Aims. We investigated the structure and kinematics of the BLR in the nearby Seyfert galaxy NGC 4593 through an analysis of the near-infrared (NIR) line blend of Ca IIλ8498, λ8542, λ8662, and O Iλ8446 observed in a 2019 VLT/MUSE spectrum.

Methods. We performed a detailed decomposition of the NIR Ca II triplet and O Iλ8446 blend, extracting clean profiles of Ca IIλ8498, λ8542, λ8662, and O Iλ8446. We then fitted Ca IIλ8662 with a relativistic elliptical line-emitting accretion disk model.

Results. The extracted line profiles are double-peaked with a full width at half maximum (FWHM) of ∼3700 km s⁻¹ and exhibit a redward asymmetry with a red-to-blue peak ratio of 4:3. The Ca II triplet lines have an intensity ratio of 1:1:1 and show no evidence of a central narrow or intermediate-width component. The profiles of Ca II and O I are remarkably similar, suggesting a common region of origin. Given the 1:1:1 ratio of the Ca II triplet, this region is likely a high-density emission zone, and the Ca IIλ8662 profile is well described by a mildly eccentric, low-inclination relativistic line-emitting disk with minimal internal turbulence. The profile represents one of the clearest kinematic signatures of a relativistic disk observed in BLR emission lines to date.

Conclusions. The double-peaked profiles of the NIR Ca II triplet and O Iλ8446 in NGC 4593 represent the first detection of double-peaked Ca II and O Iλ8446 in a nontransient AGN spectrum. The minimal intrinsic turbulence (the lowest value reported for an AGN emission line to date) and the absence of narrow or intermediate-width components in Ca IIλ8662 make it a powerful diagnostic tool of BLR structure and kinematics. Further investigations of the profiles of Ca II and O I in other AGN are recommended to better constrain BLR properties and the nature of the underlying accretion flow.