Kinematics of the outer halo of M 87 as mapped by planetary nebulae^⋆,⋆⋆

¹ Kavli Institute for Astronomy and Astrophysics, Peking University, 5 Yiheyuan Road, 100871 Beijing, PR China
e-mail: alongobardi@pku.edu.cn
² Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse, 85741 Garching, Germany
e-mail: gerhard@mpe.mpg.de, cpulsoni@mpe.mpg.de
³ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany

Received: 29 January 2018
Accepted: 23 September 2018

Abstract

Aims. We present a kinematic study of a sample of 298 planetary nebulas (PNs) in the outer halo of the central Virgo galaxy M 87 (NGC 4486). The line-of-sight velocities of these PNs are used to identify subcomponents, to measure the angular momentum content of the main M 87 halo, and to constrain the orbital distribution of the stars at these large radii.

Methods. We use Gaussian mixture modelling to statistically separate distinct velocity components and identify the M 87 smooth halo component, its unrelaxed substructures, and the intra-cluster (IC) PNs. We compute probability weighted velocity and velocity dispersion maps for the smooth halo, and its specific angular momentum profile (λ_R) and velocity dispersion profile.

Results. The classification of the PNs into smooth halo and ICPNs is supported by their different PN luminosity functions. Based on a Kolmogorov–Smirnov (K–S) test, we conclude that the ICPN line-of-sight velocity distribution (LOSVD) is consistent with the LOSVD of the galaxies in Virgo subcluster A. The surface density profile of the ICPNS at 100 kpc radii has a shallow logarithmic slope, −α_ICL ≃ −0.8, dominating the light at the largest radii. Previous B − V colour and resolved star metallicity data indicate masses for the ICPN progenitor galaxies of a few ×10⁸ M_⊙. The angular momentum-related λ_R profile for the smooth halo remains below 0.1, in the slow rotator regime, out to 135 kpc average ellipse radius (170 kpc major axis distance). Combining the PN velocity dispersion measurements for the M 87 halo with literature data in the central 15 kpc, we obtain a complete velocity dispersion profile out to R_avg = 135 kpc. The σ_halo profile decreases from the central 400 km s⁻¹ to about 270 km s⁻¹ at 2–10 kpc, then rises again to ≃300 ± 50 km s⁻¹ at 50–70 kpc, to finally decrease sharply to σ_halo ∼ 100 km s⁻¹ at R_avg = 135 kpc. The steeply decreasing outer σ_halo profile and the surface density profile of the smooth halo can be reconciled with the circular velocity curve inferred from assuming hydrostatic equilibrium for the hot X-ray gas. Because this rises to ν_c,X ∼ km s⁻¹ at 200 kpc, the orbit distribution of the smooth M 87 halo is required to change strongly from approximately isotropic within R_avg ∼ 60 kpc to very radially anisotropic at the largest distances probed.

Conclusions. The extended LOSVD of the PNs in the M 87 halo allows the identification of several subcomponents: the ICPNs, the “crown” accretion event, and the smooth M 87 halo. In galaxies like M 87, the presence of these subcomponents needs to be taken into account to avoid systematic biases in estimating the total enclosed mass. The dynamical structure inferred from the velocity dispersion profile indicates that the smooth halo of M 87 steepens beyond R_avg = 60 kpc and becomes strongly radially anisotropic, and that the velocity dispersion profile is consistent with the X-ray circular velocity curve at these radii without non-thermal pressure effects.