The edge of the M87 halo and the kinematics of the diffuse light in the Virgo cluster core

TL;DR

This study uses high-resolution spectroscopy to analyze the kinematics of planetary nebulae in the Virgo cluster core, revealing the structure and dynamics of M87's halo and intracluster light, and suggesting M87 and M86 are approaching each other.

Contribution

It provides detailed kinematic measurements of planetary nebulae in the Virgo cluster core, offering new insights into the velocity dispersion profile and orbital anisotropy of M87's halo.

Findings

M87's velocity dispersion decreases in the outer halo to ~78 km/s.

M87's stellar orbits are strongly radially anisotropic, increasing with radius.

M87 and M86 are likely falling towards each other, observed before their first close pass.

Abstract

We present high resolution FLAMES/VLT spectroscopy of intracluster planetary nebula (ICPN) candidates, targeting three new fields in the Virgo cluster core with surface brightness down to mu_B = 28.5. Based on the projected phase space information we separate the old and 12 newly-confirmed PNs into galaxy and intracluster components. The M87 PNs are confined to the extended stellar envelope of M87, within a projected radius of ~ 160 kpc, while the ICPNs are scattered across the whole surveyed region between M87 and M86. The velocity dispersions determined from the M87 PNs at projected radii of 60 kpc and 144 kpc show that the galaxy's velocity dispersion profile decreases in the outer halo, down to 78 +/- 25 km/s. A Jeans model for the M87 halo stars in the gravitational potential traced by the X-ray emission fits the observed velocity dispersion profile only if the stellar orbits are strongly radially anisotropic (beta ~= 0.4 at r ~= 10 kpc increasing to 0.8 at the outer edge), and if additionally the stellar halo is truncated at ~= 150 kpc average elliptical radius. From the spatial and velocity distribution of the ICPNs we infer that M87 and M86 are falling towards each other and that we may be observing them just before the first close pass. The inferred luminosity-specific PN numbers for the M87 halo and the ICL are in the range of values observed for old (> 10 Gyr) stellar populations (abridged).