The Space Distribution of Globular Clusters in M87

Abstract

view Abstract Citations (52) References (18) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Space Distribution of Globular Clusters in M87 Harris, William E. ; Smith, Malcolm G. Abstract We report the results of a new program to determine the radial distribution of the globular clusters around the giant elliptical galaxy M87. From a IlIal plate taken with the CTIO 4 meter telescope, all faint (B > 20) stellar objects around M87 were counted out to a radius r = 23' and down to a limiting magnitude Biim 23.8. The projected number density of these objects falls off smoothly outward, from a(FS) 50 objects arcmin -2 at r = 1' down to a 6 arcmin -2 for r > 20', after which the counts appear to level off. Over the entire surveyed region we counted 14,000 faint stellar objects, which after subtraction of the adopted "background" count level of a(bkgd) = (6.0 + 1.0) arcmin -2, yields a total of 4000 + 1500 objects that are presumably M87 globular clusters. The true total of clusters in M87 may be much higher than this for several reasons having to do with possible incompleteness factors. The observed distribution of the M87 clusters is shown to be accurately described by a single power-law function a01(r) r -1.6 over most of the observed range of r. The slope of the a01 curve appears to turn down noticeably past r > 17'; our data therefore indicate that the cluster system extends detectably at least this far (corresponding to 100 kpc for a distance scale of H0 = 55 km s ' Mpc -1). Within the errors, the shape of the a01(r) curve also matches the surface- brightness distribution Iv(r) from the M87 halo itself for r > 4', demonstrating that the globular clusters are indeed "representative" halo objects in this respect and confirming the earlier results of Baum (1955) and Racine (l968a). From the cluster counts alone we are unable to identify confidently any "tidal radius" or outer boundary to the M87 halo because of the uncertainties in the level of background counts across the plate. Compared with the globular-cluster system in the Galaxy, the distribution for the M87 clusters appears to be significantly flatter (a01 r -2.5 for the Galaxy). It is suggested that this may have resulted from the M87 cluster system forming relatively earlier during the collapse phase of its protogalactic cloud, before the gas density distribution became as centrally condensed as it did in the Galaxy. Finally, the projected distribution of the clusters around M87 appears to be noticeably elliptical, in agreement with the halo isophotes. Significantly more clusters are seen along the major axis of the isophotes than along the minor axis. Thus within our ability to distinguish, we conclude that the globular clusters mimic the spatial structure of the halo in all important respects. Subject headings: clusters: globular - galaxies: individual - galaxies: structure Publication: The Astrophysical Journal Pub Date: August 1976 DOI: 10.1086/154573 Bibcode: 1976ApJ...207.1036H full text sources ADS | data products NED (1)