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Holmberg 15A (H15A), the brightest cluster galaxy of A85, has an exceptionally low central surface brightness even among local massive elliptical galaxies with distinct stellar cores, making it exceedingly challenging to obtain high-quality spectroscopy to detect a supermassive black hole (SMBH) at its center. Aided by the superb sensitivity and efficiency of KCWI at the Keck II Telescope, we have obtained spatially resolved stellar kinematics over a ∼100″ x 100″ contiguous field of H15A for this purpose. The velocity field exhibits a low-amplitude (∼20 km/s) rotation along a kinematic axis that is prominently misaligned from the photometric major axis, a strong indicator that H15A is triaxially shaped with unequal lengths for the three principal axes. Using 2500 observed kinematic constraints, we perform extensive calculations of stellar orbits with the triaxial Schwarzschild code, TriOS, and search over ~40,000 galaxy models to simultaneously determine the mass and intrinsic 3D shape parameters of H15A. We determine a ratio of p = 0.89 for the middle-to-long principal axes and q = 0.65 for the short-to-long principal axes. Our best estimate of the SMBH mass, M_BH = (2.16_{−0.18}^{+0.23})x10^{10} M⊙, makes H15A — along with NGC 4889 — the galaxy hosting the most massive SMBHs known in the local Universe. Both SMBHs lie significantly above the mean M_BH–σ scaling relation. Repeating the orbit modeling with the axisymmetrized version of TriOS produces worse fits to the KCWI kinematics and increases M_BH to (2.55 ± 0.20)x10^{10} M⊙, which is still significantly below the M_BH = (4.0 ± 0.8)x10^{10} M⊙ reported in a prior axisymmetric study of H15A.
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The MASSIVE Survey. XVII. A Triaxial Orbit-based Determination of the Black Hole Mass and Intrinsic Shape of Elliptical Galaxy NGC 2693
Abstract We present a stellar dynamical mass measurement of a newly detected supermassive black hole (SMBH) at the center of the fast-rotating, massive elliptical galaxy NGC 2693 as part of the MASSIVE survey. We combine high signal-to-noise ratio integral field spectroscopy (IFS) from the Gemini Multi-Object Spectrograph with wide-field data from the Mitchell Spectrograph at McDonald Observatory to extract and model stellar kinematics of NGC 2693 from the central ∼150 pc out to ∼2.5 effective radii. Observations from Hubble Space Telescope WFC3 are used to determine the stellar light distribution. We perform fully triaxial Schwarzschild orbit modeling using the latest TriOS code and a Bayesian search in 6D galaxy model parameter space to determine NGC 2693's SMBH mass ( M BH ), stellar mass-to-light ratio, dark matter content, and intrinsic shape. We find M BH = 1.7 ± 0.4 × 10 9 M ⊙ and a triaxial intrinsic shape with axis ratios p = b / a = 0.902 ± 0.009 and q = c / a = 0.721 − 0.010 + 0.011 , triaxiality parameter T = 0.39 ± 0.04. In comparison, the best-fit orbit model in the axisymmetric limit and (cylindrical) Jeans anisotropic model of NGC 2693 prefer M BH = 2.4 ± 0.6 × 10 9 M ⊙ and M BH = 2.9 ± 0.3 × 10 9 M ⊙ , respectively. Neither model can account for the non-axisymmetric stellar velocity features present in the IFS data.
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- PAR ID:
- 10352834
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 928
- Issue:
- 2
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- 178
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.3847/1538-4357/ac58fd
