Distance to the Coma Cluster and a Value for HO Inferred from Globular Clusters in IC 4051

May 1997 • 1997AJ....113.1483B

Authors • Baum, William A. • Hammergren, Mark • Thomsen, Bjarne • Groth, Edward J. • Faber, Sandra M. • Grillmair, Carl J. • Ajhar, Edward A.

Abstract • We report HST-WFPC2 observations of the giant E2 galaxy IC 4051 located 14' east of the center of the Coma Cluster. Eight full-orbit exposures were obtained with the F606W filter, and two with the F814W filter. Counts of globular clusters (GCs) in 0.2-mag wide bins were made within the PC1 (f/28) frame, where faint GCs can be distinguished from noise spikes and background objects by PSF fitting. The detection probability was above 86% for F606W<27.6 mag, and it was 40% at F606W=28.1 mag (bin center), which we chose as a working threshold. The latter is equivalent to V≈28.4 mag, which is beyond the peak (turnover) of the globular cluster luminosity function (GCLF). After very small k and reddening corrections, we found the mean rest-frame color index of the IC 4051 GCs to be (V- 1)₀=1.06 mag, which implies [Fe/H]=-0.74 and is about average for the GC systems of giant elliptical galaxies. The intrinsic dispersion of color indices is σ_v-I=0.16 mag. The specific frequency of the GC system in IC 4051 is ∼10, which is an order of magnitude larger than we found in the Coma E0 galaxy NGC 4881. Using a maximum likelihood method, we have compared the observed GCLF of IC 4051 with model GCLFs transported to various strawman Coma Cluster distances, with photometric errors and incompleteness taken into account. For a model based on the GC systems of the Milky Way and M31, the inferred bolometric modulus of IC 4051 is 35.15±0.16 mag. For a model based on the GC system of M87, the apparent V modulus difference (IC 4051 minus M87) is 4.15±0.07 mag, and the inferred bolometric modulus of IC 4051 is 35.07±0.17 mag. These moduli rest on RR Lyrae and Cepheid zero points that may be subject to future adjustments. The difference between the IC 4051 modulus based on the spiral-galaxy GCLF model (Milky Way and M31) and that based on the elliptical-galaxy GCLF model (M87) is only 0.08±0.23 mag, which is not statistically significant. Adopting the mean IC 4051 bolometric modulus, 35.11±0.12 mag, and assuming q₀=0.5, we obtain a luminosity distance of 105±6 Mpc (random error). For a cosmological redshift of km/s, the implied Hubble Constant H₀ =68±6 km/s per Mpc.

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Carl Grillmair

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