Cluster Ages to Reconstruct the Milky Way Assembly (CARMA). VI. The age-metallicity relation of in situ clusters and the age of the Universe
Year: 2026
Type: article
Abstract: Within the homogeneous framework for the derivation of globular cluster ages defined by the CARMA collaboration, we focus on the clusters born in situ in the Milky Way (MW). As they are the oldest among the old globular clusters (GCs), their absolute age provides crucial constraints about the age of the Universe. Moreover, the shape of their age-metallicity relation (AMR) is expected to show the imprints of past merger events, in case these were able to trigger cluster 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 NGC 6642 and Pal 1. While the former is likely a metal-rich and low-orbital-energy member of the Low-energy-Kraken-Heracles (LKH) merger event, and thus puts the 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 episodic, merger-driven GC formation, we use a Gaussian mixture model to describe the observed age distribution. This is statistically best-fit by four components, which we associate with significant events of the Milky Way 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 NGC 6791, and the resulting AMR was fit to compute the look-back time for the beginning of cluster formation in the Milky Way. The fit provides t_ GC,i =13.59^ +0.33 _ -0.32 Gyr for the initial time of cluster formation, and t_ U =13.79± 0.34 Gyr for the age of the Universe. This value translates to H_ =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. This result is in good agreement with probes from the early Universe, though still consistent with local probes within the uncertainties.
Source: Astronomy and Astrophysics
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $1,865
Funders: National Aeronautics and Space Administration, Space Telescope Science Institute, European Commission, Ministero dell’Istruzione, dell’Università e della Ricerca, Ministerio de Ciencia e Innovación, Atlantic Research Center for Information and Communication Technologies, European Regional Development Fund, Agencia Estatal de Investigación
Awards: G1089506725, G1382750735, G1794310344, G1849103467, G1855990336, G195395952, G2309174360, G2313451655, G252646220, G2660406951, G3518538256, G3694284866, G4059861418, G410571670, G430777504, G433322167, G4518818167, G451917667, G4553926751, G4651445947, G5195188938, G5421974903, G5539308778, G5592398465, G5611110931, G5805502524, G58316358, G5845768719, G5910366635, G5967599077, G6273510985, G6285680282, G6797201117, G6950864665, G6998206882, G745419941, G99296377
OpenAlex ID: W7221780920