Terzan 5: the remnant of a pristine fragment of the Galactic Bulge?

Terzan 5 is a stellar system commonly catalogued as a Globular Cluster
(GC), located in the inner Bulge of our Galaxy. Two distinct
sub-populations have been recently discovered in this system (Ferraro
et al. 2009, Nature, 462, 483). They define two well separated red
clumps in the (K, J-K) color-magnitude diagram (CMD) and show very
different iron content: ΔFe ~0.5 (such a large difference in iron
abundance has been found only in another GC-like stellar system, omega
Centauri, in the Halo of the Galaxy). Moreover, the abundance of light
elements measured in both sub-populations has been found not to follow
the typical anti-correlations observed in genuine GCs and the overall
chemical patterns of the two populations appear strikingly similar to
those of the Bulge stars (Origlia et al. 2011). These observational
results demonstrate that Terzan 5 is not a genuine GC, but a stellar
system that experienced complex star formation and chemical enrichment
histories. The strong chemical link with the Bulge, together with the
location in the inner region of it, suggest that Terzan 5 (at odds
with omega Centauri) is not the nucleus of an accreted dwarf galaxy,
but possibly the relic of one of the pristine fragments that
contributed to form the Bulge itself.