GLOBULAR-CLUSTERS IN COMA GALAXY NGC-4881

We report HST-WFPC2 observations of the bright E0 galaxy NGC 4881, loc ated about 18' north of the center of the Coma Cluster. Counts of glob ular clusters (GCs) were made both within the PC1 (f/28) frame and wit hin the three WFC (f/13) frames. Attention was focused mainly on the P C1 data, in which faint GCs could be distinguished from noise spikes b y the use of a PSF fitting test, Within the 34 '' x 34 '' field of the PC1 detector (minus a circle of 3 '' radius around the NGC 4881 nucle us), 88 GCs have been identified to a threshold of F555 W = 27.6 mag a pproximate to V. Noise spikes are only beginning to appear near that t hreshold, and they have been excluded. Extended sources, such as dista nt background galaxies, have also been identified and excluded, If cor rected for incompleteness in the faintest bins, the numbers of GCs per 0.4 mag bin in NGC 4881 are rising monotonically to our threshold and therefore imply that we probably have not yet passed the turnover of the globular cluster luminosity function (GCLF). Alternatively, instea d of applying completeness corrections, we can compare uncorrected NGC 4881 counts with a model GCLF transported to various assumed Coma Clu ster distances. The relative likelihoods of various distances can then be calculated using Poisson statistics to quantify the fit. For a mod el we use the GCLFs of the Milky Way and M31, We find the likelihood o f the GCLF fit to rise with increasing Coma distance until it crests a t approximate to 108 Mpc, and then levels off. Therefore, if the model fairly represents the GCs in NGC 4881, our estimate of the minimum Co ma distance is 108 Mpc with a formal statistical error of (- 11, + inf inity) Mpc. A comparison of three independent calibrations indicates t hat any zero-point error in the photometry should be < 5%, Three other possible sources of systematic error include the RR Lyrae distance sc ale for the Milky Way GCs, the distance modulus of M31, and any intrin sic difference between NGC 4881 and the model in the MV of the GCLF tu rnover. In all three cases, there are envisaged adjustments which, if applied, would have the effect of increasing our estimate of the minim um distance to Coma, A distance of 108 (- 11, + infinity) Mpc implies a Hubble Constant H-0 = 67 (+ 7, - [67]) km/s per Mpc, We cannot set a n upper limit on the distance, nor a lower limit on H-0, until observa tions of Coma galaxies reach deep enough to show the turnover of the G CLF, and we also caution that the results above are based on only one Coma galaxy, If the GCs detected on all four CCDs are fitted with the GCLF model and if counts are adjusted for incomplete areal coverage, t he GC system in NGC 4881 is found to have a specific frequency of only about 1.0, which is lower than in the majority of bright ellipticals but not the lowest known. The GC system in NGC 4881 is also found to b e only a little more spatially extended than the luminosity profile of the galaxy itself. Those findings are somewhat sensitive to any nonun iformity in the distribution of background objects in the WFC frames. Even so, the specific frequency is almost certainly less than 2.0. The low specific frequency in NGC 4881 may have implications concerning t he formation, migration, and fate of GCs in a dense environment like t hat of Coma, where very large numbers of GCs and dwarf galaxies have b een reported near the center of the cluster. There is no evidence that the GCs of NGC 4881 are abnormal: they have a mean color index (V - I ) = 0.97 mag with relatively small dispersion, and the implied metalli city, [Fe/H] similar to - 1.2, is within the range of GC metallicities found in the Milky Way, though a little more metal rich than the mean . Moreover, the NGC 4881 GCLF has the same slope as the bright side of the GCLF in the Milky Way. (C) 1995 American Astronomical Society.