Multiple stellar populations in the globular cluster ω Centauri as tracers of a merger event

Abstract

The discovery of the Sagittarius dwarf galaxy 1 , which is being tidally disrupted by and merging with the Milky Way, supports the view that the halo of the Galaxy has been built up at least partially by the accretion of similar dwarf systems. The Sagittarius dwarf contains several distinct populations of stars 2,3 , and includes M54 as its nucleus, which is the second most massive globular cluster associated with the Milky Way. The most massive globular cluster is ω Centauri, and here we report that ω Centauri also has several distinct stellar populations, as traced by red-giant-branch stars. The most metal-rich red-giant-branch stars are about 2 Gyr younger than the dominant metal-poor component, indicating that ω Centauri was enriched over this timescale. The presence of more than one epoch of star formation in a globular cluster is quite surprising, and suggests that ω Centauri was once part of a more massive system that merged with the Milky Way, as the Sagittarius dwarf galaxy is in the process of doing now. Mergers probably were much more frequent in the early history of the Galaxy and ω Centauri appears to be a relict of this era.As part of our investigation of the luminosity-metallicity relation of the RR Lyrae stars in the globular cluster ω Cen, we have obtained 2K BV CCD frames with the CTIO 0.9-m b 0.0 0.4 0.8

The HST large programme on ω Centauri – I. Multiple stellar populations at the bottom of the main sequence probed in NIR–Optical

“…More-recent papers have shown that the MS is even more complex than we had previously imagined and that both the blue and the red MS host stellar subpopulations (e.g., Bellini et al 2010). The complexity of multiple stellar populations in this cluster is further demonstrated by the multiple sub-giant branches (SGBs) and red-giant branches (RGBs) and by the presence of a double white-dwarf cooling sequence (Lee et al 1999;Pancino et al 2000;Sollima et al 2005;Villanova et al 2007;Bellini et al 2010;Milone et al 2017).…”

Flash Mixing on the White Dwarf Cooling Curve: Understanding Hot Horizontal Branch Anomalies in NGC 2808

“…However, this possibility raises an important question : why are EHB gaps not apparent in the optical CMDs of other globular clusters with extended blue HB tails ? For example, the optical CMDs of u Cen reported by Kaluzny et al (1997) and Lee et al (1999) do not seem to show an EHB gap, despite the substantial population of EHB stars. Quite possibly, any EHB gap in u Cen has been blurred by the metallicity distribution in the cluster.…”

Radial distribution of the multiple stellar populations in $\mathsf{\omega}$ Centauri

“…Finally, the red side of the main sequence is contaminated by the anomalous metal-rich population (hereafter MS-a), which is clearly connected with RGB-a. Even if these stars include only ∼5% of the total cluster members (Lee et al 1999;Pancino et al 2000;Sollima et al 2005a;Villanova et al 2007), they are an additional source of pollution for rMS stars -against which we now take specific precautions.…”

“…Its stars cover a wide range in metallicity (Cannon & Stobie 1973;Norris & Bessell 1975, 1977Freeman & Rodgers 1975;Bessell & Norris 1976;Butler et al 1978;Norris & Da Costa 1995;Suntzeff & Kraft 1996;Norris et al 1996), with a primary component at [Fe/H] ∼ −1.7 to −1.8, and a long tail extending up to [Fe/H] ∼ −0.6, containing three or four secondary peaks (see Johnson et al 2009, for a recent update). It has been shown, both with ground-based photometry (Lee et al 1999;Pancino et al 2000;Rey et al 2004;Sollima et al 2005a;Villanova et al 2007) and Hubble Space Telescope (HST) photometry (Anderson 1997;Bedin et al 2004;Ferraro et al 2004), that ω Cen hosts different stellar populations, most of them clearly visible in most of their evolutionary phases.…”