The outer regions of the giant Virgo galaxy M?87 Kinematic separation of stellar halo and intracluster light

摘要

Aims. We present a spectroscopic study of a sample of 287 planetary nebulas (PNs) around the brightest cluster galaxy (BCG) M?87 in Virgo A, of which 211 are located between 40 kpc and 150 kpc from the galaxy centre. With these data we can distinguish the stellar halo from the co-spatial intracluster light (ICL) and study both components separately. Methods. We obtained PN velocities with a high resolution FLAMES/VLT survey targeting eight fields in a total area of ~0.4 deg2. We identified PNs from their narrow and symmetric redshifted λ5007 ? [OIII] emission line, the presence of the second λ4959 ? [OIII] emission line, and the absence of significant continuum. We implement a robust technique to measure the halo velocity dispersion from the projected phase-space to identify PNs associated with the M?87 halo and ICL. Using photometric magnitudes, we construct PN luminosity functions (PNLFs), which are complete down to m5007 = 28.8. Results. The velocity distribution of the spectroscopically confirmed PNs is bimodal, containing a narrow component centred on the systemic velocity of the BCG and an off-centred broader component, which we identify as halo and ICL, respectively. We find that 243 PNs are part of the velocity distribution of the M?87 halo, while the remaining subsample of 44 PNs are intracluster PNs (ICPNs). Halo and ICPNs have different spatial distributions: the number density of halo PNs follow the galaxy’s surface brightness profile, whereas the ICPNs are characterised by a shallower power-law profile, IICL ∝ Rγ with γ in the range [?0.34, ?0.04 ]. No evidence is found for an asymmetry in the halo and ICPN density distributions when the NW and SE fields are studied separately. A study of the composite PN number density profile confirms the superposition of different PN populations associated with the M?87 halo and the ICL, characterised by different PN specific numbers α. We derive αhalo = 1.06 × 10-8NPN L⊙,bol-1 and αICL = 2.72 × 10-8NPN L⊙,bol-1, respectively. The M?87 halo PNLF has fewer bright PNs and a steeper slope towards faint magnitudes than the ICPNLF, and both are steeper than the standard PNLF for the M?31 bulge. Moreover, the ICPNLF has a dip at ~1?1.5 mag fainter than the bright cut-off, reminiscent of the PNLFs of systems with extended star formation history, such as M?33 or the Magellanic clouds. Conclusions. The BCG halo of M?87 and the Virgo ICL are dynamically distinct components with different density profiles and velocity distributions. Moreover, the different α-parameter values and PNLF shapes of the halo and ICL indicate distinct parent stellar populations, consistent with the existence of a gradient towards bluer colours at large radii. These results reflect the hierarchical build-up of the Virgo cluster.