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

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 phasespace. 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 NPN,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 × 10 9 L� ,V , a colour of (B − V) = 0.76 ± 0.05, and a stellar mass of M = 6.4 ± 2.3 × 10 9 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.