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

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; rhanusch@eso.org

Received: 30 January 2015
Accepted: 11 May 2015

Abstract

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 Å [OIII] emission line, the presence of the second λ4959 Å [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, I_ICL ∝ R^γ 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 α_halo = 1.06 × 10^-8N_PN L_⊙,bol^-1 and α_ICL = 2.72 × 10^-8N_PN 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.