The chemical compositions of accreted and in situ galactic globular clusters according to SDSS/APOGEE

Abstract

Studies of the kinematics and chemical compositions of Galactic globular clusters (GCs) enable the reconstruction of the history of star formation, chemical evolution, and mass assembly of the Galaxy. Using the latest data release (DR16) of the SDSS/APOGEE survey, we identify 3,090 stars associated with 46 GCs. Using a previously defined kinematic association, we break the sample down into eight separate groups and examine how the kinematics-based classification maps into chemical composition space, considering only α (mostly Si and Mg) elements and Fe. Our results show that: (i) The loci of both in situ and accreted subgroups in chemical space match those of their field counterparts; (ii) GCs from different individual accreted subgroups occupy the same locus in chemical space. This could either mean that they share a similar origin or that they are associated with distinct satellites which underwent similar chemical enrichment histories; (iii) The chemical compositions of the GCs associated with the low orbital energy subgroup defined by Massari and collaborators is broadly consistent with an in situ origin. However, at the low metallicity end, the distinction between accreted and in situ populations is blurred; (iv) Regarding the status of GCs whose origin is ambiguous, we conclude the following: the position in Si-Fe plane suggests an in situ origin for Liller 1 and a likely accreted origin for NGC 5904 and NGC 6388. The case of NGC 288 is unclear, as its orbital properties suggest an accretion origin, its chemical composition suggests it may have formed in situ.

Absolute -band magnitudes and mass-to-light ratios of Galactic globular clusters

“…Ferraro et al 1999;Pritzl et al 2005). Figure 3 in Horta et al (2020), for example, shows that clusters with [Fe/H]< −0.7 have more or less constant [Si/Fe] of about [Si/Fe]= +0.25, followed by a downturn in the [Si/Fe] values similar to what is seen for field stars. The two most metal-rich clusters in their sample (Liller 1 and Pal 10) have [Si/Fe] of 0.01 ± 0.05 and 0.0 ± 0.10, respectively, i.e., they are compatible with a solar abundance ratio.…”

Kraken reveals itself – the merger history of the Milky Way reconstructed with the E-MOSAICS simulations

“…In practice, this means that we exclude E3 from our analysis (also because of its low mass), and consider versions of the low-energy group both including and excluding NGC 6441. Finally, Horta et al (2020b) argue that NGC 6121, which Massari et al (2019) associate with the low-energy group, has an in-situ origin. Based on the age and metallicity of NGC 6121 (τ = 12.2 ± 0.5 Gyr and [Fe/H] = −1.14), we consider it too metal poor to have formed in situ and follow the choice of Massari et al (2019) to associate it with the low-energy group, even if we acknowledge that this is an edge case.…”

Low-α metal-rich stars with sausage kinematics in the LAMOST survey: Are they from the Gaia-Sausage-Enceladus galaxy?

“…Therefore, we favor for the suggestion that no obvious difference between the low-α and high-α groups is found in this diagram. The energy of MRSK stars is above the low boundary of the GSE galaxy at E tot /10 5 ∼ −1.9 km 2 s −2 , although its metal poor component lies at E tot /10 5 ∼ −1.5 km 2 s −2 according to Horta et al [46]. In addition, the energy of the GSE galaxy is significantly lower than the Sgr galaxy at E tot /10 5 ∼ −1.0 km 2 s −2 according to Naidu et al [14] (see their Figure 23).…”