Hubble Space Telescope imaging of the extremely metal-poor globular cluster EXT8 in Messier 31

Abstract

We recently found the globular cluster (GC) EXT8 in M 31 to have an extremely low metallicity of [Fe/H] = −2.91 ± 0.04 using high-resolution spectroscopy. Here we present a colour–magnitude diagram (CMD) for EXT8, obtained with the Wide Field Camera 3 on board the Hubble Space Telescope. Compared with the CMDs of metal-poor Galactic GCs, we find that the upper red giant branch (RGB) of EXT8 is ∼0.03 mag bluer in MF606W − MF814W and slightly steeper, as expected from the low spectroscopic metallicity. The observed colour spread on the upper RGB is consistent with being caused entirely by the measurement uncertainties, and we place an upper limit of σF606W − F814W ≈ 0.015 mag on any intrinsic colour spread. The corresponding metallicity spread can be up to σ[Fe/H] ∼ 0.2 dex or > 0.7 dex, depending on the isochrone library adopted. The horizontal branch is located mostly on the blue side of the instability strip and has a tail extending to at least MF606W = +3, as in the Galactic GC M 15. We identify two candidate RR Lyrae variables and several ultraviolet-luminous post-horizontal-branch and/or post-asymptotic-giant-branch star candidates, including one very bright (MF300X ≈ −3.2) source near the centre of EXT8. The surface brightness of EXT8 out to a radius of 25″ is well fitted by a Wilson-type profile with an ellipticity of ϵ = 0.20, a semi-major axis core radius of 0.″25, and a central surface brightness of μF606W, 0 = 15.2 mag arcsec−2, with no evidence of extra-tidal structure. Overall, EXT8 has properties consistent with it being a ‘normal’, but very metal-poor, GC, and its combination of relatively high mass and very low metallicity thus remains challenging to explain in the context of GC formation theories operating within the hierarchical galaxy assembly paradigm.

The chemical composition of globular clusters in the Local Group

“…By far the most metal-poor GC in our sample is M31 EXT8. The spectroscopic observations of this cluster were previously discussed in Larsen et al (2020) and its CMD was discussed in Larsen et al (2021).…”

Metallicities of Outer Halo M31 Globular Clusters from Integrated Light Calcium-II Triplet Spectroscopy

“…Larsen et al (2020) quote [Fe/H] = − 2.91 ± 0.04, which would make EXT 8 the lowest metallicity GC ever found. From resolved HST photometry,Larsen et al (2021a) find that EXT 8 is consistent with being as or more metal-poor than the Milky Way GC M15 (whichSakari et al 2013 find to have [Fe/H] = − 2.3, from high-resolution IL spectroscopy). The metallicities for EXT 8 from the other papers range from [Fe/H] = − 1.8(Wang et al 2021) to [Fe/H] = − 2.3(Chen et al 2016).…”

“…The GCs in M31 are close enough to be partially resolved for photometry (e.g., Mackey et al 2013) and low-resolution spectroscopy (e.g., Reitzel et al 2004), but the brightest red giant branch (RGB) stars are too faint for chemical abundance studies at high spectral resolution, at least with current telescopes and instrumentation. Instead, GCs can be studied with Integrated Light (IL) spectroscopy, which yields radial velocities, metallicities, and ages (at lower resolution; e.g., Caldwell et al 2009;Schiavon et al 2012Schiavon et al , 2013 as well as detailed chemical abundances (at higher resolution; e.g., Colucci et al 2009Colucci et al , 2014Sakari et al 2013, 2015, 2016, Larsen et al 2018, 2021a ).…”

“…The GCs that are not associated with substructure appear to be from older accretion events (Mackey et al 2019b), while the others are likely from more recent accretion events (McConnachie et al 2018). Several of the outer halo GCs have been spectroscopically studied, providing metallicities or detailed chemical abundances (Colucci et al 2009(Colucci et al , 2014Sakari et al 2015Sakari et al , 2021Sakari & Wallerstein 2016;Larsen et al 2018Larsen et al , 2021a, but many more remain to be studied.…”

Chemical composition and ages of four globular clusters in M31 from the analysis of their integrated-light spectra

“…For several objects, integrated-light spectroscopic studies were performed in order to determine their chemical composition (Colucci et al 2014 [hereafter: C14], Sakari et al 2016 [hereafter: S16], Sharina et al 2018, Larsen et al 2018) and age (C14). To date, the deepest colour-magnitude diagrams (CMDs) for clusters in M31 reach confidently the level of the horizontal branch (Federici et al 2012, Larsen et al 2021.…”