Whiting 1: the youngest globular cluster associated with the Sagittarius dwarf spheroidal galaxy

Abstract

Context. Recently, Carraro ( 2005) drew attention to the remarkable star cluster Whiting 1 by showing that it lies about 40 kpc from the Sun and is therefore unquestionably a member of the Galactic halo (b = -60.6 deg). Its Color Magnitude Diagram (CMD) indicated that Whiting 1 is very young (similar to 5 Gyr) for a globular cluster. It is very likely that Whiting 1 originated in a dwarf galaxy that has since been disrupted by the Milky Way. Aims. The main goals of this investigation were to constrain better the age, metallicity, and distance of Whiting 1 and to assess whether it belongs to a stellar stream from the Sagittarius dwarf spheroidal galaxy (Sgr dSph). Methods. Deep CCD photometry in the BVI pass-bands obtained with the VLT is used to improve the quality of the CMD and to determine the cluster's luminosity function and surface density profile. High-resolution spectrograms obtained with Magellan are used to measure the cluster's radial velocity and to place limits on its possible metallicity. The measurements of distance and radial velocity are used to test the cluster's membership in the stellar streams from the Sgr dSph. Results. From our CMD of Whiting 1, we derive new estimates for the cluster's age (6.5(-0.5)(+1.0) Gyr), metallicity (Z = 0.004 +/- 0.001, [Fe/H] = -0.65), and distance (29.4(-2.0)(+1.8) kpc). From echelle spectrograms of three stars, we obtain -130.6 +/- 1.8 km s(-1) for the cluster's radial velocity and show from measurements of two infra-red CaII lines that the [Fe/H] of the cluster probably lies in the range -1.1 to -0.4. Both the luminosity function and the surface density profile suggest that the cluster has undergone tidal stripping by the Milky Way. We demonstrate that the position of Whiting 1 on the sky, its distance from the Sun, and its radial velocity are identical to within the errors of both the theoretical predictions of the trailing stream of stars from the Sgr dSph galaxy and the previous observations of the M giant stars that delineate the streams. Conclusions. With the addition of Whiting 1, there is now strong evidence that 6 globular clusters formed within the Sgr dSph. Whiting 1 is particularly interesting because it is the youngest and among the most metal rich. The relatively young age of Whiting 1 demonstrates that this dwarf galaxy was able to form star clusters for a period of at least 6 Gyr, and the age and metallicity of Whiting 1 are consistent with the age-metallicity relationship in the main body of the Sgr dSph. The presence now of Whiting 1 in the Galactic halo provides additional support for the view that the young halo clusters originated in dwarf galaxies that have been accreted by the Milky Way.