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

Journal article : Letter

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.

Authors: Nguyen, DD; Ngo, HN; Le, TQT; Graham, AW; Soria, R

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: EDP Sciences
• Journal: Astronomy & Astrophysics
• Volume: 698
• Article number: L9
• Publication date: 2025-06-03
• Acceptance date: 2025-05-12
• DOI: 10.1051/0004-6361/202554672
• EISSN: 1432-0746
• ISSN: 0004-6361
• Language: English
• Keywords: galaxies: individual: NGC 4736; galaxies: kinematics and dynamics; quasars: supermassive black holes
• Subtype: Letter