The outer halo globular cluster system of M31 – III. Relationship to the stellar halo

Abstract

We utilise the final catalogue from the Pan-Andromeda Archaeological Survey to investigate the links between the globular cluster system and field halo in M31 at projected radii R proj = 25 − 150 kpc. In this region the cluster radial density profile exhibits a power-law decline with index Γ = −2.37 ± 0.17, matching that for the stellar halo component with [Fe/H] < −1.1. Spatial density maps reveal a striking correspondence between the most luminous substructures in the metal-poor field halo and the positions of many globular clusters. By comparing the density of metal-poor halo stars local to each cluster with the azimuthal distribution at commensurate radius, we reject the possibility of no correlation between clusters and field overdensities at 99.95% significance. We use our stellar density measurements and previous kinematic data to demonstrate that ≈ 35 − 60% of clusters exhibit properties consistent with having been accreted into the outskirts of M31 at late times with their parent dwarfs. Conversely, at least ∼ 40% of remote clusters show no evidence for a link with halo substructure. The radial density profile for this subgroup is featureless and closely mirrors that observed for the apparently smooth component of the metal-poor stellar halo. We speculate that these clusters are associated with the smooth halo; if so, their properties appear consistent with a scenario where the smooth halo was built up at early times via the destruction of primitive satellites. In this picture the entire M31 globular cluster system outside R proj = 25 kpc comprises objects accumulated from external galaxies over a Hubble time of growth.

Estimating ages and metallicities of M31 star clusters from LAMOST DR6

“…The furthest cluster is located at projected radii R ∼ 200 kpc from the M31 centre. At the region of R = 25 ∼ 150 kpc, the cluster radial density profile exhibits a power-law decline (Mackey et al 2019).…”

Integrated spectroscopy of extragalactic Globular Clusters

“…Images of M31's outer halo (R proj > 25 kpc) from the Pan-Andromeda Archaeological Survey (PAndAS; McConnachie et al 2009) reveal a number of stellar streams, which are debris from dwarf satellite galaxies that are being accreted into M31. There are 92 GCs known in the outer halo of M31 (Mackey et al 2019), many of which were discovered in PAndAS and proceeding surveys. On average, the outer halo GC population is generally more metal-poor than the innermost GCs, as expected from the overall metallicity gradient in the field stars (Gilbert et al 2014;Ibata et al 2014).…”

“…The [Fe/H] and [Ca/Fe] ratios of several of these GCs place constraints on the masses of the dwarf galaxies that have created these streams (see Sakari et al 2015). For example, the high [Ca/Fe] abundance of the GC H10, at [Fe/H] ∼ −1.4 (Sakari et al 2015), suggests that it formed in a fairly massive dwarf galaxy; H10 might also be a member of the 3 SW Cloud, a bright feature to the southwest of M31 (Mackey et al 2019). The chemical abundances of H10 therefore suggest that the progenitor galaxy of the SW Cloud was more massive than the current stream indicates, in agreement with other papers (e.g., McMonigal et al 2016).…”

Detecting the dark matter halos with star clusters in M31/M33 with PFS, SDSS-V and LAMOST

“…More and more observational evidence shows the hierarchical assembly of galaxies through accretion of satellite dwarf galaxies (Searle & Zinn 1978). For M31, the Pan Andromeda Archeological Survey (PAndAS) (McConnachie et al 2009) have discovered large number of star streams and substructures in M31 halo, extended to around 150 kpc from the galaxy center, with deep observations of CFHT (g = 26.0 and i = 24.8 at signal-to-noise ratio of S/N = 5 for the point sources, please see Mackey et al 2019). Recent studies imply that the variations of star stream density could trace the dark matter sub-halos.…”

“…Their simulation could show that the amount of substructure increases from the visible dwarf galaxies to thousands of dark matter sub-halos that the LCDM model predicted. More recently, Mackey et al (2019) systematically investigated the relations of the outer halo star clusters and the stellar halos of the M31 at the projected radii 25-150 kpc, with 106 Z. Fan et al the observational data of PAndAS.…”