Ages for globular clusters in the outer galactic halo: The second-parameter clusters Palomar 3, Palomar 4, and Eridanus

We have used the Wide Field Planetary Camera 2 on the Hubble Space Telescope to obtain photometry of the outer halo globular clusters Palomar 3, Palomar 4, and Eridanus. These three are classic examples of the "second-parameter" anomaly because of their red horizontal-branch morphologies in combination with their low-to-intermediate metallicities. Our color-magnitude diagrams (CMDs) in V, V - I reach V_lim ≃ 27.0, clearly delineating the subgiant and turnoff regions and about 3 mag of the unevolved main sequences. The slopes and dereddened colors of the giant branches are consistent with published [Fe/H] estimates that rank the clusters (Pal 3, Eridanus, Pal 4) in order of increasing metallicity, with all three falling near or between the abundance values of the classic nearby halo clusters M3 and M5. Differential fits of their CMDs are made to each other and to M3 and M5 for relative age determinations. We find that the three outer halo cluster CMDs differ from the nearby clusters in a way that is consistent with their being younger by ∼ 1.5-2 Gyr, if we have correctly estimated the clusters' chemical abundance ratios. Conversely, the inferred age difference could be smaller (≲1 Gyr) if either [Fe/H] or [α/Fe] for the outer halo clusters is significantly lower than we have assumed. Possible age spreads of order 1 Gyr among both the nearby and outer halo clusters may also be present.