Purveyors of fine halos: Re-assessing globular cluster contributions to the Milky Way halo buildup with SDSS-IV

Abstract

There is ample evidence in the Milky Way for globular cluster (GC) disruption. Hence one may expect that also part of the Galactic halo field star population may once have formed in GCs. We seek to quantify the fraction of halo stars donated by GCs by searching for stars that bear the unique chemical fingerprints typical for a subset of GC stars often dubbed as "second-generation stars". These are stars showing light element abundance anomalies such as a pronounced CN-band strength accompanied by weak CH-bands. Based on this indicator, past studies have placed the fraction of halo stars with a GC origin between a few to up to 50%. Using low-resolution spectra from the most recent data release (DR14) of the latest extension of the Sloan Digital Sky Survey (SDSS-IV), we were able to identify 118 metal-poor (−1.8 ≤ [Fe/H] ≤ −1.3) CN-strong stars in a sample of 4470 halo giant stars out to ∼50 kpc. This increases the number of known halo stars with GC-like light-element abundances by a factor of two and results in an observed fraction of these stars of 2.6±0.2%. Using an updated formalism to account for the fraction of stars lost early on in the GCs' evolution we thus estimate the fraction of the Galactic halo that stems from disrupted clusters to be very low, at 11±1%. This number would represent the case that stars lost from GCs were entirely from the first generation and is thus merely an upper limit. Our conclusions are sensitive to our assumptions of the mass lost early on from the first generation formed in the GCs, the ratio of first-to-second generation stars, and other GC parameters. We carefully test the influence of varying these parameters on the final result and find that, under realistic scenarios, the above fraction depends on the main assumptions at less than 10 percentage points. We further recover a flat trend in this fraction with Galactocentric radius, with a marginal indication of a rise beyond 30 kpc that could reflect the ex-situ origin of the outer halo as is also seen in other stellar tracers.

The mass fraction of halo stars contributed by the disruption of globular clusters in the E-MOSAICS simulations

“…1) estimated in this work are consistent with the observational estimates (e.g. Martell et al 2016;Koch et al 2019), although perhaps a bit low. In contrast, assuming that chemically-distinct stars can also form in lowmass stellar clusters (more massive than 5 × 10 3 M at birth), we find that the mass fraction of chemically-distinct stars in the halo exhibit better agreement with the observational results (bottom left panel in Fig.…”

“…Stars lying farther away than this radius and within 50 kpc, R 1/2, * < r < 50 kpc 2 , are considered to belong to the halo and we determine its mass from the halo field stellar population. According to this definition, we measure stellar halo masses of a median ∼ 3.4 × 10 9 M among our suite of galaxies, which decreases by a factor ∼ 6 when restricted to a metallicity range typically used in chemical-tagging studies Koch et al 2019). These masses are lower limits, as the EAGLE model is known to underpredict the peak of the ratio of stellar mass to halo mass of central galaxies (Crain et al 2015).…”

“…The horizontal dotted lines and the shaded regions indicate the median and the 25th-75th percentiles, respectively, of the arrow bases (top) and arrow heads (bottom), for each population over our galaxy sample. The red line and downward arrow shown in the second and fourth rows mark the derived upper limit of the mass fraction of halo stars contributed by GCs of ∼ 11 per cent from Koch et al (2019), and the observationally-inferred fraction of chemically-distinct stars in the Galactic halo (2.6 ± 0.2 per cent, Koch et al 2019), respectively, whereas the blue box and downward arrow shown in the second row correspond to the revised observational upper limit of 2-5 per cent calculated in this work (see Sect. 4).…”

“…These latter studies find that ∼1.4-2.6 per cent of halo field stars exhibit light-element abundance patterns resembling those of GCs. This reina.campos@uni-heidelberg.de suggests that, if GCs are the unique formation sites of chemically distinct stellar populations 1 , an upper limit of ∼ 11-47 per cent of halo stars have originated in GCs, with the exact number depending on the details of the GC formation and evolution model, as well as the fraction of enriched-to-unenriched stars considered (Carretta 2016;Martell et al 2016;Koch et al 2019).…”

A high-resolution spectroscopic study of two new Na- and Al-rich field giants–likely globular cluster escapees in the Galactic halo

“…Following Martell et al (2011), the original contribution from GCs amounts to 13% of halo stars, in order to account for the 2% being chemically taggable as second-generation stars. From the DR14 release of SDSS-IV, Koch et al (2019) have further refined the observed fraction of such objects to be 2.6%, bringing the original contribution from GCs to ∼11%.…”