A JWST View of the Overmassive Black Hole in NGC 4486B

Behzad Tahmasebzadeh, Matthew A. Taylor, Monica Valluri, Haruka Yoshino, Eugene Vasiliev, Michael J. Drinkwater, Solveig Thompson, Kristen Dage, Patrick Côté, Laura Ferrarese, Tatsuya Akiba, Vivienne Baldassare, Misty C. Bentz, John P. Blakeslee, Holger Baumgardt, Youkyung Ko, Chengze Liu, Ann-Marie Madigan, Eric W. Peng, Joel Roediger, Kaixiang Wang, Tyrone E. Woods

First listed 2025-05-20 | Last updated 2025-08-01

Abstract

We present a new stellar dynamical measurement of the supermassive black hole (SMBH) in the compact elliptical galaxy NGC 4486B, based on integral field spectroscopy with JWST/NIRSpec. The two-dimensional kinematic maps reveal a resolved double nucleus and a velocity dispersion peak offset from the photometric center. Utilizing two independent methods-Schwarzschild orbit-superposition and Jeans Anisotropic Modeling-we tightly constrain the black hole mass by fitting the full line-of-sight velocity distribution. Our axisymmetric Schwarzschild models yield a best-fit black hole mass of $M_{BH} = 3.6^{+0.7}_{-0.7} \times 10^8 \, M_{\odot}$, slightly lower but significantly more precise than previous estimates. However, since our models do not account for the non-equilibrium nature of the double nucleus, this value may represent a lower limit. Across all tested dynamical models, the inferred $M_{BH}/M_*$ ratio ranges from ~ 4-13%, providing robust evidence for an overmassive SMBH in NGC 4486B. Combined with the galaxy's location deep within the Virgo Cluster, our results support the interpretation that NGC 4486B is the tidally stripped remnant core of a formerly massive galaxy. As the JWST/NIRSpec field of view is insufficient to constrain the dark matter halo, we incorporate archival ground-based long-slit kinematics extending to 5 arcsec. While this provides some leverage on the dark matter content, the constraints remain relatively weak. We place only an upper limit on the dark matter fraction, with $M_{DM}/M_{*} < 0.5$ within 1 kpc-well beyond the effective radius. The inferred black hole mass remains unchanged with or without a dark matter halo.

Short digest

Using JWST/NIRSpec IFU stellar kinematics plus archival long-slit data, the authors model the compact elliptical NGC 4486B and resolve a double nucleus with a velocity-dispersion peak offset from the photometric center. Axisymmetric Schwarzschild modeling of the full LOSVD yields M_BH = 3.6 ± 0.7 × 10^8 M_sun and a host ratio M_BH/M_* ≈ 4–13%, firmly placing the black hole in the overmassive regime. Dark matter within 1 kpc is weakly constrained (M_DM/M_* < 0.5), and the inferred M_BH is unchanged with or without a halo. The system’s location deep in Virgo supports a stripped-core origin, with the caveat that non-equilibrium double-nucleus dynamics are not modeled, so the mass may be a lower limit.

Key figures to inspect

• Figure 1: Compare the HST F850LP image to the NIRSpec IFU S/N map to see the resolved P1/P2 double nucleus and how the IFU coverage samples it; inspect the central vs outer pPXF fits to gauge LOSVD quality where dispersion is highest.
• Figure 2: Examine GH moment maps (V, σ, h3, h4) to locate the off-center σ peak and assess how the best-fit Schwarzschild model reproduces asymmetries; contrast the baseline, peak-masked, and peak-shifted cases to test robustness.
• Figure 3: Trace V and σ along the kinematic major axis, comparing JWST points to CFHT/SIS data from Kormendy et al. (1997); verify the amplitude and location of the dispersion peak and how the axisymmetric model tracks both datasets.
• Figure 4: Inspect marginalized likelihoods for M_BH, M/L, DM circular velocity, and inclination; note the tight M_BH constraints and how data treatments (masking/shifting the peak) affect parameter posteriors and confidence intervals.