Cluster Ages to Reconstruct the Milky Way Assembly (CARMA). VI. The age-metallicity relation of in-situ clusters and the age of the Universe

Abstract

Within the homogeneous framework for the derivation of GC ages defined by the CARMA collaboration, we focus on the clusters born in-situ in the MW. As they are the oldest among the old GCs, their absolute age provides constraints about the age of the Universe. Moreover, the shape of their AMR is expected to show the imprints of past merger events, in case these were able to trigger GC formation in the Galaxy. We derive new homogeneous ages for 18 GCs dynamically and/or chemically classified as in-situ, and build the MW AMR from a total sample of 34 GCs. We find that all the selected GCs follow a well defined AMR, with the exception of NGC6642 and Pal1. While the former is likely a member of the LKH merger event, thus putting chronological evidence for the existence of this progenitor on even more robust ground, for the latter we propose an alternative scenario according to which it formed in-situ, but in a metal-depleted region of the Galaxy, possibly due to the interaction with an unspecified merger event. To find evidence of merger-driven GC formation, we use a GMM for the observed age distribution. This is statistically best-fit by 4 components, which we associate with significant events of the MW merger history. Cluster ages were finally calibrated to a more accurate absolute age scale based on asteroseismic measurements of the mass of red giant stars in NGC6791, and the resulting AMR was fit to compute the look-back time for the beginning of cluster formation in the MW. The fit yields t_GC,i=13.59+0.33-0.32 Gyr for initial cluster formation, and tU=13.79+/-0.34 Gyr for the age of the Universe. This value translates to H0=68.72+3.95-3.49(stat.)+2.65-2.32(sys.) km s-1 Mpc-1, where the systematic error comes from a realistic estimate of the zero point uncertainty on the GC absolute ages derived from stellar models.