Astronomy and Astrophysics – Astrophysics
Scientific paper
May 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995a%26a...297..660m&link_type=abstract
Astronomy and Astrophysics, v.297, p.660
Astronomy and Astrophysics
Astrophysics
6
Galaxies: Clusters Of, Galaxies: Ngc 5481, Galaxies: Star Clusters, Distance Scale
Scientific paper
Deep, subarcsecond BVR CCD imaging was obtained for one 10arcmin^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 2050km/s. 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=27Mpc) 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_=80km/s/Mpc. 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.
Madejsky Rainer
Rabolli Monica
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