Ages for Globular Clusters in the Outer Galactic Halo: The Second-Parameter Clusters Palomar 3, Palomar 4, and Eridanus

We have used the WFPC2 camera on the Hubble Space Telescope to obtainphotometry of the outer-halo globular clusters Palomar 3, Palomar 4, andEridanus. These three are classic examples of the "second parameter" anomalybecause of their red horizontal-branch morphologies in combination with theirlow-to-intermediate metallicities. Our color-magnitude diagrams in (V, V-I)reach $V_{lim} \simeq 27.0$, clearly delineating the subgiant and turnoffregions and about three magnitudes of the unevolved main sequences. The slopesand dereddened colors of the giant branches are consistent with published[Fe/H] estimates that rank the clusters (Pal 3, Eridanus, Pal 4) in order ofincreasing metallicity, with all three falling near or between the abundancevalues of the classic nearby halo clusters M3 and M5. Differential fits oftheir color-magnitude diagrams are made to each other and to M3 and M5 forrelative age determinations. We find that the three outer-halo cluster CMDsdiffer from the nearby clusters in a way that is consistent with their beingyounger by $\sim 1.5 - 2 $Gyr, if we have correctly estimated the clusters'chemical-abundance ratios. Conversely, the inferred age difference could besmaller ($\ltsim 1 $Gyr) if either [Fe/H] or [$\alpha$/Fe] for the outer-haloclusters is significantly lower than we have assumed. Possible age spreads oforder 1$ $Gyr among both the nearby and outer-halo clusters may also bepresent.Comment: 20 figures, 6 tables, 59 postscript page