A Subaru/Suprime-Cam wide-field survey of globular cluster populations around M87 - II. Colour and spatial distribution

Abstract

We have performed a wide-field imaging survey of the globular cluster (GC) populations around M87 with Suprime-Cam on the 8.2-m Subaru Telescope. A 2° x 0.°5 field extending from the centre of M87 out to a radius of ∼0.5 Mpc was observed through the BVI filters. By investigating the GC colour distribution as a function of distance from M87 and NGC 4552, another luminous Virgo elliptical in our survey field, it is found that clear bimodality [(V - I) peak ∼ 1-0 and 1.2] is seen only in the inner regions (≤10 kpc) of the host galaxies and that it becomes weaker with radius due to the decreasing contribution of the red GC (V - I > 1.1) subpopulation. It is also found (both around M87 and around NGC 4552) that while the spatial distribution of the red GCs is as centrally concentrated as the host galaxy halo light distribution, the distribution of the blue GCs (V - I ≤ 1.1) tends to be more extended. However, the blue GC distribution around M87 is not as extended as the Virgo cluster mass density profile. Based on these facts, we argue that the contribution of GCs associated with the Virgo cluster [e.g. intergalactic GCs (i-GCs)] is not significant around M87 and is not the primary reason for the high S N value of M87. Instead, most of the blue GCs around luminous ellipticals, as well as the red GCs, are presumed to be associated with the host galaxy. We model the radial profile of GC surface densities out to ∼0.5 Mpc from M87 by a superposition of the GC populations associated with M87 and with NGC 4552. It is found that there are some regions where the GC surface densities are larger than that which is expected from this model, suggesting the existence of an additional i-GC population independent of the luminous ellipticals. By comparing the estimated i-GC surface density with the luminosity density of the intracluster stellar population inferred from the intergalactic planetary nebulae in the Virgo cluster, we obtain a crude estimate of i-GC specific frequency S N = 2.9 +4.2 -1.5 . If this S N value represents the stellar population tidally stripped by a massive central galaxy from other (less-luminous) galaxies, the contribution of tidally captured GCs in the GC population of M87 would need to be low to be consistent with the high S N value of M87.