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

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 deg 2 . 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 m 5007 = 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 −8 N PN L −1 ,bol and α ICL = 2.72 × 10 −8 N PN L −1 ,bol , 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.

THE FORNAX DEEP SURVEY WITH VST. I. THE EXTENDED AND DIFFUSE STELLAR HALO OF NGC 1399 OUT TO 192 kpc

“…In this case, this interpretation is further supported by kinematic measurements. In M87, the kinematics of PNe, their α parameter, and the shape of the luminosity function allow us to separate the halo component from the ICL population (Longobardi et al 2015b). As stressed by Bender et al (2015a), deep photometry alone is not enough to recognize the structure of the stellar halo, and kinematics measurements are necessary when possible.…”

Rhapsody-G simulations – II. Baryonic growth and metal enrichment in massive galaxy clusters

“…Deep optical and near infrared observations revealed the existence of the so-called intracluster light (ICL): extended haloes of diffuse light that typically surround BCGs and that are considered to be generated by the stellar material stripped from satellite galaxies (e.g. Longobardi et al 2015;Burke et al 2015, for recent studies).…”

Extended halos and intracluster light using Planetary Nebulae as tracers in nearby clusters

“…With the narrow band imaging carried out with Suprime-Cam at Subaru, they detected 688 PN candidates in an area of nearly 0.5 sq-deg covering M87. The spectroscopic followup was performed with Flames at UT2 on the VLT; 287 spectra were obtained for single PNs (Longobardi et al 2015). This sample was analyzed co-jointly with 12 PNs from Doherty et al (2009) in the same area, giving a merged total sample of 301 PNs.…”

“…This sample was analyzed co-jointly with 12 PNs from Doherty et al (2009) in the same area, giving a merged total sample of 301 PNs. Longobardi et al (2015) built the LOSVD for the entire sample of the spectroscopically confirmed PNs. The LOSVD displays broad asymmetric wings that cannot be matched by a single Gaussian, see Figure 5 in Longobardi et al (2015).…”

“…Longobardi et al (2015) built the LOSVD for the entire sample of the spectroscopically confirmed PNs. The LOSVD displays broad asymmetric wings that cannot be matched by a single Gaussian, see Figure 5 in Longobardi et al (2015). The entire LOSVD was fitted by a narrow Gaussian centered at the systemic velocity of M87 and σ n ≃ 300 kms −1 and a broad Gaussian component with average velocity at v b ≃ 1000 kms −1 and…”