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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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Keck Integral-field Spectroscopy of M87 Reveals an Intrinsically Triaxial Galaxy and a Revised Black Hole Mass
Abstract The three-dimensional intrinsic shape of a galaxy and the mass of the central supermassive black hole provide key insight into the galaxy’s growth history over cosmic time. Standard assumptions of a spherical or axisymmetric shape can be simplistic and can bias the black hole mass inferred from the motions of stars within a galaxy. Here, we present spatially resolved stellar kinematics of M87 over a two-dimensional 250″ × 300″ contiguous field covering a radial range of 50 pc–12 kpc from integral-field spectroscopic observations at the Keck II Telescope. From about 5 kpc and outward, we detect a prominent 25 km s −1 rotational pattern, in which the kinematic axis (connecting the maximal receding and approaching velocities) is 40° misaligned with the photometric major axis of M87. The rotational amplitude and misalignment angle both decrease in the inner 5 kpc. Such misaligned and twisted velocity fields are a hallmark of triaxiality, indicating that M87 is not an axisymmetrically shaped galaxy. Triaxial Schwarzschild orbit modeling with more than 4000 observational constraints enabled us to determine simultaneously the shape and mass parameters. The models incorporate a radially declining profile for the stellar mass-to-light ratio suggested by stellar population studies. We find that M87 is strongly triaxial, with ratios of p = 0.845 for the middle-to-long principal axes and q = 0.722 for the short-to-long principal axes, and determine the black hole mass to be ( 5.37 − 0.25 + 0.37 ± 0.22 ) × 10 9 M ⊙ , where the second error indicates the systematic uncertainty associated with the distance to M87.
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- PAR ID:
- 10412070
- Date Published:
- Journal Name:
- The Astrophysical Journal Letters
- Volume:
- 945
- Issue:
- 2
- ISSN:
- 2041-8205
- Page Range / eLocation ID:
- L35
- 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/10.3847/2041-8213/acbbcf
