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 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.

Based on observations made with the VLT at Paranal Observatory under programme 088.B-0288(A) and 093.B-066(A), and with the Subaru Telescope under programme S10A-039.