Globular cluster system and Milky Way properties revisited

Abstract

Aims. Updated data of the 153 Galactic globular clusters are used to readdress fundamental parameters of the Milky Way, such as the distance of the Sun to the Galactic centre, the bulge and halo structural parameters, and cluster destruction rates. Methods. We build a reduced sample that has been decontaminated of all the clusters younger than 10Gyr and of those with retrograde orbits and/or evidence of relation to dwarf galaxies. The reduced sample contains 116 globular clusters that are tested for whether they were formed in the primordial collapse. Results. The 33 metal-rich globular clusters ([Fe/H] ≥ −0.75) of the reduced sample basically extend to the Solar circle and are distributed over a region with the projected axial-ratios typical of an oblate spheroidal, Δx : Δy : Δz ≈ 1.0 : 0.9 : 0.4. Those outside this region appear to be related to accretion. The 81 metal-poor globular clusters span a nearly spherical region of axial-ratios ≈1.0 : 1.0 : 0.8 extending from the central parts to the outer halo, although several clusters in the external region still require detailed studies to unravel their origin as accretion or collapse. A new estimate of the Sun’s distance to the Galactic centre, based on the symmetries of the spatial distribution of 116 globular clusters, is provided with a considerably smaller uncertainty than in previous determinations using globular clusters, RO = 7.2 ± 0.3 kpc. The metal-rich and metal-poor radial-density distributions flatten for RGC ≤ 2 kpc and are represented well over the full Galactocentric distance range both by a power-law with a core-like term and Sérsic’s law; at large distances they fall off as ∼R−3.9. Conclusions. Both metallicity components appear to have a common origin that is different from that of the dark matter halo. Structural similarities between the metal-rich and metal-poor radial distributions and the stellar halo are consistent with a scenario where part of the reduced sample was formed in the primordial collapse and part was accreted in an early period of merging. This applies to the bulge as well, suggesting an early merger affecting the central parts of the Galaxy. The present decontamination procedure is not sensitive to all accretions (especially prograde) during the first Gyr, since the observed radial density profiles still preserve traces of the earliest merger(s). We estimate that the present globular cluster population corresponds to ≤ 2.3 = ± 6% of the original one. The fact that the volume-density radial distributions of the metal-rich and metal-poor globular clusters of the reduced sample follow both a core-like power-law, and Sersicfs law indicates that we are dealing with spheroidal subsystems at all scales.