Is Terzan 5 the remnant of a building block of the Galactic bulge? Evidence from APOGEE

TL;DR

This study uses APOGEE data to compare the chemical abundances of stars in Terzan 5 with bulge stars, finding significant differences that suggest Terzan 5 is not a primordial bulge building block.

Contribution

It provides the first detailed chemical abundance comparison between Terzan 5 and bulge stars using SDSS-IV APOGEE data, challenging previous hypotheses about its origin.

Findings

Terzan 5 stars differ significantly in Ca, Mn, C, O, Al, Si, Mg abundances from bulge stars.

Terzan 5 has lower [$ ext{α}$/Fe] and higher [Mn/Fe] than bulge stars.

Estimated progenitor mass of Terzan 5 is about 3×10^8 solar masses, too small to influence the bulge significantly.

Abstract

It has been proposed that the globular cluster-like system Terzan 5 is the surviving remnant of a primordial building block of the Milky Way bulge, mainly due to the age/metallicity spread and the distribution of its stars in the $\alpha$-Fe plane. We employ Sloan Digital Sky Survey (SDSS-IV) data from the Apache Point Observatory Galactic Evolution Experiment (APOGEE 2) to test this hypothesis. Adopting a random sampling technique, we contrast the abundances of 10 elements in Terzan 5 stars with those of their bulge field counterparts with comparable atmospheric parameters, finding that they differ at statistically significant levels. Abundances between the two groups differ by more than 1$\sigma$ in Ca, Mn, C, O, and Al, and more than 2$\sigma$ in Si and Mg. Terzan 5 stars have lower [$\alpha$/Fe] and higher [Mn/Fe] than their bulge counterparts. Given those differences, we conclude that Terzan 5 is not the remnant of a $major$ building block of the bulge. We also estimate the stellar mass of the Terzan 5 progenitor based on predictions by the Evolution and Assembly of GaLaxies and their Environments (EAGLE) suite of cosmological numerical simulations, concluding that it may have been as low as $\sim3\times10^8$ M$_\odot$ so that it was likely unable to significantly influence the mean chemistry of the bulge/inner disk, which is significantly more massive ($\sim10^{10}$ M$_\odot$). We briefly discuss existing scenarios for the nature of Terzan 5 and propose an observational test that may help elucidate its origin.