The build-up of the cD halo of M 87: evidence for accretion in the last Gyr^⋆

¹ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse, 85741 Garching, Germany
e-mail: alongobardi@mpe.mpg.de; gerhard@mpe.mpg.de
² European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
e-mail: marnabol@eso.org
³ Department of Physics and Astronomy, Case Western University, 10900 Euclid Avenue, Cleveland, OH 44106, USA
e-mail: mihos@case.edu

Received: 8 April 2015
Accepted: 29 May 2015

Abstract

Aims. We present kinematic and photometric evidence for an accretion event in the halo of the cD galaxy M 87 in the last Gyr.

Methods. Using velocities for ~300 planetary nebulas (PNs) in the M 87 halo, we identify a chevron-like substructure in the PN phase-space. We implement a probabilistic Gaussian mixture model to identify PNs that belong to the chevron. From analysis of deep V-band images of M 87, we find that the region with the highest density of chevron PNs is a crown-shaped substructure in the light.

Results. We assign a total of N_PN,sub = 54 to the substructure, which extends over ~50 kpc along the major axis where we also observe radial variations of the ellipticity profile and a colour gradient. The substructure has highest surface brightness in a 20 kpc × 60 kpc region around 70 kpc in radius. In this region, the substructure causes an increase in surface brightness by ≳60%. The accretion event is consistent with a progenitor galaxy with a V-band luminosity of L = 2.8±1.0×10⁹ L_{⊙ ,V}, a colour of (B − V) = 0.76±0.05, and a stellar mass of M = 6.4±2.3×10⁹ M_⊙.

Conclusions. The accretion of this progenitor galaxy has caused an important modification of the outer halo of M 87 in the last Gyr. This result provides strong evidence that the galaxy’s cD halo is growing through the accretion of smaller galaxies as predicted by hierarchical galaxy evolution models.