Chemical Abundances for the Outer Halo Cluster Pal 4 from Co-added
  High-Resolution Spectroscopy
  Context: Chemical element abundances for distant Galactic globular clusters
(GCs) hold important clues to the origin of the Milky Way halo and its
substructures. Aims: We study the chemical composition of red giant stars in
Pal 4 - one of the most remote GCs in the Milky Way - and compare our abundance
measurements to those for both low surface brightness dwarf galaxies, and GCs
in the inner and the outer halo. Methods: By co-adding high-resolution, low-S/N
Keck/HIRES spectra of 19 stars along the red giant branch, we estimate chemical
abundance ratios of 20 alpha-, iron peak-, and neutron-capture elements. Our
method gives total uncertainties on most element-to-iron ratios of typically
0.2 dex. Results: We measure [Fe/H] = -1.41 +- 0.04 (statistical) +- 0.17
(systematic) and an alpha-enhancement of [alpha/Fe] = +0.38 +- 0.11 dex, which
is consistent with the canonical value of +0.4 dex found for Galactic halo
field stars and most halo GCs at this metallicity. Although Pal 4 has higher
enhancements in the heavier elements with respect to the halo, the majority of
the element ratios are, within the measurement errors, consistent with those
for local halo field stars. We find, however, evidence for a lower [Mg/Ca]
ratio than in other halo clusters. Conclusions: Based on the available
evidence, we conclude that the material from which Pal 4 and the Galactic halo
formed experienced similar enrichment processes, despite the apparently younger
age of this cluster. Within the limitations of our methodology we find no
significant indication of an iron spread, as is typical of genuine GCs of the
Milky Way. However, abundance ratios for individual stars in Pal 4 and other
distant satellites are urgently needed to understand the relationship, if any,
between remote GCs and other halo substructures (i.e., luminous and ultra-faint
dwarf spheroidal galaxies).
