Session 79 - Star Clusters in Other Galaxies.
Display session, Friday, January 09
Exhibit Hall,

## [79.01] Duration of the Early Galactic Formation Epoch: HST Photometry for Red-Horizontal Branch Clusters in the Outer Halo

J. E. Hesser, P. B. Stetson, R. D. McClure, S. vandenBergh (NRC,HIA,DAO), M. Bolte (Lick), W. E. Harris (McMaster), D. A. VandenBerg (U.Victora), R. A. Bell (U.Md.), G. G. Fahlman, H. B. Richer (U.B.C.), H. E. Bond (STScI)

Last year we presented evidence from HST photometry of the low-metallicity cluster NGC 2419 (M_V = -9.5, R_\sun\sim90 kpc, [Fe/H] = -2.2) that globular cluster formation began at essentially the same time throughout a region of the Galactic halo now almost 200 kpc in diameter (Harris et al. 1997 AJ 114, 1030). We now turn to the time spread of halo formation, with the ultimate aim of addressing the relative roles of mergers over the first 4 or more Gyrs (Searle amp; Zinn 1978, ApJ, 225, 357; Lee, Demarque amp; Zinn 1994 ApJ, 423, 248) versus models favoring a rapid collapse (Eggen, Lynden-Bell amp; Sandage 1962, ApJ, 236, 748; Stetson, VandenBerg amp; Bolte 1996, PASP, 108, 560), or some combination of those and other processes. We provide the first reliable measurements from the giant branch through the main-sequence turnoffs of red-horizontal-branch clusters in the outer halo, which are frequently postulated to be younger than most other globular clusters. From WFPC2 F555W (V') and F814W (I') photometry for Pal 3 (M_V = -5.2, R_\sun\sim87 kpc), Pal 4 (M_V = -5.8, R_\sun\sim98 kpc), and Eridanus (M_V = -4.8, R_\sun\sim78 kpc), all with [Fe/H]\sim-1.5, we estimate their relative ages by making differential comparisons among them and with respect to inner-halo objects of, presumably, comparable chemical compositions. It seems likely at this stage of our analysis that (a) the three clusters are the same age to our measurement precision of \sim 1 Gyr, and, (b) the CMDs of all three outer halo clusters differ from those of M 3 and M 5 (our template clusters of similar metallicity), in the sense that the outer halo clusters are younger by \sim 3 Gyr, or they are \sim 0.5 dex more metal-rich than currently thought. Large uncertainties in chemical compositions (He, [\alpha/Fe], [CNO/Fe]) for outer halo and template clusters alike mask the true interpretation.