The most metal-poor Galactic globular cluster: the first spectroscopic observations of ESO280-SC06

Abstract

We present the first spectroscopic observations of the very metal-poor Milky Way globular cluster ESO280-SC06. Using spectra acquired with the 2dF/AAOmega spectrograph on the Anglo-Australian Telescope, we have identified 13 members of the cluster, and estimate from their infrared calcium triplet lines that the cluster has a metallicity of [Fe/H] = −2.48 +0.06 −0.11 . This would make it the most metal-poor globular cluster known in the Milky Way. This result was verified with comparisons to three other metal-poor globular clusters that had been observed and analyzed in the same manner. We also present new photometry of the cluster from EFOSC2 and SkyMapper and confirm that the cluster is located 22.9 ± 2.1 kpc from the Sun and 15.2 ± 2.1 kpc from the Galactic centre, and has a radial velocity of 92.5 +2.4 −1.6 km s −1 . These new data finds the cluster to have a radius about half that previously estimated, and we find that the cluster has a dynamical mass of the cluster of (12 ± 2) × 10 3 M . Unfortunately, we lack reliable proper motions to fully characterize its orbit about the Galaxy. Intriguingly, the photometry suggests that the cluster lacks a well-populated horizontal branch, something that has not been observed in a cluster so ancient or metal-poor.

Birds of a Feather? Magellan/IMACS Spectroscopy of the Ultra-faint Satellites Grus II, Tucana IV, and Tucana V*

“…For GruII, we are not able to detect spectroscopically any evidence for dark matter, although the measurements certainly do not rule out a substantial dark matter content either. The mean metallicity of GruII is similar to or just below those of the most metal-poor known globular clusters (e.g., Sobeck et al 2011;Simpson 2018), providing some support for the idea that it is a dwarf. Combining that information with its size, we conclude that GruII is very likely a galaxy, although more direct evidence would still be desirable.…”

The tidal remnant of an unusually metal-poor globular cluster

“…For giant-branch stars of a given luminosity (that is, absolute magnitude), a smaller equivalent width of the Ca ii triplet means that the star has a lower metallicity. The stars are colour-coded by the metallicity of their parent globular cluster 17 4,23,24 ). This empirical metallicity floor extends not only to all other Local Group galaxies 6,25 , but even further, spanning roughly 6 dex in galaxy stellar mass and a wide variety of morphologies and assembly histories [5][6][7] .…”

SkyMapper Southern Survey: Second data release (DR2)

“…DR1 has also helped with discovering extremely metal-poor stars in the Tucana II Dwarf Galaxy (Chiti et al 2018), with studies of the most metal-poor Galactic globular cluster (Simpson 2018), and with characterising the lowest-mass ultra metal-poor star known (Schlaufman, Thompson, & Casey 2018). Many other scientific endeavours in the Southern skies are underway using DR1 data.…”