The Peculiar Velocity of Messier~87 from Microarcsecond Geodetic VLBI Astrometry

TL;DR

Using 28 years of VLBI measurements, we precisely determined Messier 87's proper motion and peculiar velocity, revealing its significant tangential motion relative to the cosmic microwave background.

Contribution

First direct measurement of M87's full 3D velocity vector using long-term high-precision astrometry, combining statistical methods with VLBI data.

Findings

M87's proper motion magnitude is 10.19 μas/yr with 0.64 μas/yr uncertainty.

The galaxy's peculiar velocity is approximately 1037 km/s relative to the CMB rest frame.

M87's motion aligns with the Virgo filament's flow towards the Great Attractor.

Abstract

Our knowledge of the space velocity of Messier 87, which is the dominant galaxy in the Virgo cluster, has been limited to the radial velocity component. Using a cadence of precision position measurements with the global geodetic very long baseline interferometry (VLBI) system over 28 years, we determined the proper motion vector of the radio-emitting core by a robust statistical method involving 1-norm optimization and bootstrapping. The proper motion vector is directed at a position angle $189.2\degr \pm 3.5\degr$ in the equatorial International Celestial Reference Frame, and its magnitude is $10.19$ $\mu$as yr$^{-1}$ with an uncertainty of $0.64$ $\mu$as yr$^{-1}$. The projected velocity of the AGN in the tangential sky plane is ($787\pm50$)~km~s$^{-1}$. The peculiar velocity of Messier 87 with respect to the preferred rest frame of the cosmic microwave background field is approximately 1037 km s$^{-1}$ (assuming a distance of 16.1 Mpc) with an angle of 65$^\circ$ to the current line of sight, which implies a tangential relative motion of M87 and the Galaxy. The peculiar velocity of M87 is directionally concordant with the reconstructed and $\Lambda$CDM-simulated motion of the Virgo filament towards the Great Attractor, but the Milky Way moves slower by 470 ~km~s$^{-1}$ in that direction.