Nitrogen abundances and multiple stellar populations in the globular clusters of the Fornax dSph

Abstract

We usemeasurements of nitrogen abundances in red giants to search formultiple stellar populations in the four most metal-poor globular clusters (GCs) in the Fornax dwarf spheroidal galaxy (Fornax 1, 2, 3, and 5). New imaging in the F343N filter, obtained with the Wide Field Camera 3 on the Hubble Space Telescope, is combined with archival F555W and F814W observations to determine the strength of the NH band near 3370 A˚. After accounting for observational errors, the spread in the F343N-F555W colors of red giants in the Fornax GCs is similar to that in M15 and corresponds to an abundance range of Δ[N/Fe] ∼ 2 dex, as observed also in several Galactic GCs. The spread in F555W-F814W is, instead, fully accounted for by observational errors. The stars with the reddest F343N-F555W colors (indicative of N-enhanced composition) have more centrally concentrated radial distributions in all four clusters, although the difference is not highly statistically significant within any individual cluster. From double-Gaussian fits to the color distributions, we find roughly equal numbers of "N-normal" and "N-enhanced" stars (formally ∼40% N-normal stars in Fornax 1, 3, and 5 and ∼60% in Fornax 2). We conclude that GC formation, in particular, regarding the processes responsible for the origin of multiple stellar populations, appears to have operated similarly in the Milky Way and in the Fornax dSph. Combined with the high ratio of metal-poor GCs to field stars in the Fornax dSph, this places an important constraint on scenarios for the origin of multiple stellar populations in GCs.