Velocity profiles of galaxies with claimed black holes - III. Observations and models for M87

Abstract

We report on high-S/N subarcsec resolution spectra of M87, obtained with the 4.2-m William Herschel Telescope in the spectral regions around the blue G-band and the IR Ca II triplet. From the spectra we determine the line strengths, the mean and dispersion of the best-fitting Gaussian velocity profiles (i.e. the line-of-sight velocity distributions) and the Gauss–Hermite moments h₃,...h₆ that measure deviations from a Gaussian. We find that the main results derived from the two spectral regions agree, in contradiction to recent measurements by Jarvis & Melnick. The observed line strengths have a central minimum in both spectral regions and are consistent with the central luminosity ‘spike’ of M87 being completely non-thermal. The coefficients h₃,...h₆ are close to zero at all radii. The velocity dispersion rises from ~ 270 km s^–1 at ~ 15 arcsec to ~ 305 km s^–1 at ~ 5 arcsec, and then to ~ 400 km s^–1 at 0.5 arcsec. We model the observed velocity dispersions by solving the Jeans equation for hydrostatic equilibrium. Radial anisotropy (β ≈ 0.5) is required in the outer parts to fit the observed velocity dispersion gradient. Near the centre, the data can still be fitted equally well with radially anisotropic models without a central black hole as they can be with less anisotropic models with a central black hole of mass M_BH ≲ 5 × 10⁹ M_⊙. However, the radially anisotropic Jeans models without a central black hole need not necessarily correspond to a positive and stable distribution function. We study the central velocity profile of isotropic dynamical models with a central black hole. The wings of the velocity profile are more extended than those of a Gaussian. This is due to the stars that orbit close to the hole at high velocities. The wings contribute significantly to the normalization and the dispersion of the velocity profile. A Gaussian fit to the velocity profile is insensitive to the wings, and thus underestimates both the line strength γ and the velocity dispersion σ. In the analysis of real data, this effect is even more pronounced, since low-frequency information is lost due to continuum subtraction. If M87 has a 5 × 10⁹ M_⊙ black hole, we show that for our observational setup, the central line strength will be underestimated by ~ 2 per cent and the central velocity dispersion by ~ 8 per cent. Our blue data show two puzzling features, seen also in the data of other authors: the central line strength is too small to be accounted for solely by the dilution from non-thermal light, and the velocity dispersion in the centre is ~ 30 km s^–1 smaller than that at R ≈ 0.5 arcsec. The presence of a central black hole can provide a qualitative explanation for both features. In addition to the stellar kinematics, we also determine the ionized gas kinematics from the data, by analysing the H_γ emission line. The central velocity dispersion of the ionized gas is very high at ~ 516 km s^–1, and drops steeply to ~ 125 km s^–1 at R = 2 arcsec. Interpretation of the ionized gas kinematics in terms of a naive isotropic hydrostatic equilibrium model implies the presence of a central black hole of mass M_BH ≈ 3 × 10⁹ M_⊙.