Letter to the Editor

Supermassive black hole mass measurement in the spiral galaxy NGC 4736 using JWST/NIRSpec stellar kinematics

¹ International Centre for Interdisciplinary Science and Education 07 Science Avenue, Ghenh Rang, 55121 Quy Nhon, Vietnam
² Faculty of Physics – Engineering Physics, University of Science, Vietnam National University, Ho Chi Minh City, Vietnam
³ Vietnam National Space Center, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Vietnam
⁴ Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, VIC 3122, Australia
⁵ INAF-Osservatorio Astrofisico di Torino, Strada Osservatorio 20, I-10025 Pino Torinese, Italy
⁶ Sydney Institute for Astronomy, School of Physics A28, The University of Sydney, Sydney, NSW 2006, Australia
⁷ Center for Astrophysics – Harvard and Smithsonian, 60 Garden St., Cambridge, MA 02138, USA
⁸ Sternberg Astronomical Institute, M.V. Lomonosov Moscow State University, 13 Universitetsky prospect, 119992 Moscow, Russia
⁹ Sub-Department of Astrophysics, Department of Physics, University of Oxford, DWB, Keble Road, Oxford OX1 3RH, UK
¹⁰ Korea Astronomy and Space Science Institute, Daejeon 34055, Republic of Korea
¹¹ Department of Astronomy and Space Science, UST, 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea
¹² Instituto de Física Fundamental, CSIC, Calle Serrano 123, 28006 Madrid, Spain
¹³ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands

^⋆ Corresponding authors: nddieuphys@gmail.com

Received: 21 March 2025
Accepted: 12 May 2025

Abstract

We present accurate mass measurements of the central supermassive black hole (SMBH) in NGC 4736 (M 94). We used the “gold-standard” stellar absorption features (CO band heads) at ∼2.3 μm, as opposed to gas emission lines, to trace the dynamics in the nuclear region, easily resolving the SMBH’s sphere of influence. The analysis uses observations made with the integral field unit of the Near-Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope and a surface brightness profile derived from Hubble Space Telescope archival images. We used Jeans anisotropic models within a Bayesian framework, and comprehensive Markov chain Monte Carlo optimization, to determine the best-fit black hole mass, orbital anisotropy, mass-to-light ratio, and nucleus kinematical inclination. We obtained a SMBH mass M_BH = (1.60 ± 0.16)×10⁷ M_⊙ (1σ random error), which is consistent with the M_BH–σ and M_BH–M_⋆ relations. This is the first dynamical measurement of a M_BH in NGC 4736 based on the stellar kinematics observed with NIRSpec. We thus settle a longstanding inconsistency between estimates based on nuclear emission-line tracers and the M_BH–σ relation. Our analysis shows that NIRSpec can detect SMBHs with M_BH, min ≈ 5 × 10⁶ M_⊙ in galaxies within 5 Mpc and σ ≈ 100 km s⁻¹.