THE GLOBULAR-CLUSTER SYSTEM OF NGC-5481 AND FAINT BACKGROUND GALAXIES

Deep, subarcsecond BVR CCD imaging was obtained for one 10 arcmin(2) field centered on the elliptical galaxy NGC 5481 (E3). The radial velocity of NGC 5481 with respect to the rest frame of the Local Group is 2050kms(-1). Slight morphological perturbations reflect a weak tidal interaction with the Sc companion galaxy NGC 5480. The observed excess of objects with a strong concentration towards the centre of the galaxy is mainly due to globular clusters. A cluster of background galaxies is visible in the same field. The good imaging quality allows many of the background galaxies to be resolved. The largest of these galaxies with a size of 2 " have mean colours B-V = 1.6 and V-R = 1.2, so that the redshift of these galaxies is estimated to z > 0.25. A considerable number of the fainter and unresolved objects may be due to smaller galaxies belonging to the same cluster. According to colour, a contaminating background galaxy population is determined so that the remaining cleaned sample of compact objects with mean colours B-V = 0.8, V-R = 0.6 is assumed to be the globular cluster system of NGC 5481 with its brightest clusters appearing at R = 20.5 (B = 21.7). Although the colour selection removes many cluster galaxies, there remains a considerable uncertainty in the subtraction of the contaminating background counts. The differential number counts show a turnover at B = 25.2(+/-0.6), V = 24.3(+/-0.5) and R = 23.7(+/-0.4), with the large errors mainly reflecting the uncertainty of the background subtraction. The globular cluster luminosity function (GCLF) of Virgo ellipticals at the distance of NGC 5481 would give a turnover 0.5mag fainter, so that the globular clusters in NGC 5481 appear intrinsically brighter, but observational uncertainties are of the same order. Assuming a GCLF as in the Local Group, a distance modulus of m-M=32.2(+/-0.4) (or d=27 Mpc) can be formally determined. Combining this distance estimate with the corrected radial velocity of NGC 5481, the Hubble constant is formally determined to H-0 = 80 kms(-1) Mpc(-1). Counting the globular clusters until the turnover and assuming the faint end of the globular cluster luminosity function symmetric to the bright end, a total globular cluster population N-total = 180(+/-40) within the observed field is determined, giving a corrected specific frequency S = 2.5 (+1.5)(-0.9). The surface density of globular clusters gives a slope Delta logN/Delta logr = - 1.7, similar as observed for Virgo ellipticals. At larger galactocentric distances the surface density appears to have a steeper slope, which, if not caused by a wrong background subtraction in the small observed field, reflects the tidal interaction with NGC 5480.