2017A&A...603A.104H

Context. The galaxy M 49 (NGC 4472) is the brightest early-type galaxy in the Virgo Cluster. It is located in subcluster B and has an unusually blue, metal-poor outer halo. Planetary nebulae (PNe) are excellent tracers of diffuse galaxy and intragroup light (IGL).
Aims. We aim to present a photometric survey of PNe in the galaxy's extended halo to characterise its PN population, as well as the surrounding IGL of the subcluster B.
Methods. PNe were identified based on their bright [OIII]5007Å emission and absence of a broad-band continuum through automated detection techniques.
Results. We identify 738 PNe out to a radius of ∼155kpc from M 49's centre from which we define a complete sample of 624 PNe within a limiting magnitude of m_5007,lim=28.8. Comparing the PN number density to the broad-band stellar surface brightness profile, we find a variation of the PN-specific frequency (α-parameter) with radius. The outer halo beyond 60kpc has a 3.2 times higher α-parameter compared to the main galaxy halo (α_2.5,inner^M49=(3.20±0.43)x10^–9PN/L_☉,bol), which is likely due to contribution from the surrounding blue IGL. We use the planetary nebulae luminosity function (PNLF) as an indicator of distance and stellar population. Its slope, which correlates empirically with galaxy type, varies within the inner halo. In the eastern quadrant of M 49, the PNLF slope is shallower, indicating an additional localised, bright PN population following an accretion event, likely that of the dwarf irregular galaxy VCC1249. We also determined a distance modulus of µ_PNLF=31.29^+0.07_–0.08 for M 49, corresponding to a physical distance of 18.1±0.6Mpc, which agrees with a recent surface-brightness fluctuations distance.
Conclusions. The PN populations in the outer halo of M 49 are consistent with the presence of a main Sersic galaxy halo with a slight (B-V) colour gradient of 10^–4mag/arcsec surrounded by IGL with a very blue colour of (B-V)=0.25 and a constant surface brightness µ_V=28.0mag/arcsec².