A wide-field photometric study of the globular cluster system of NGC 4636^*

¹ Universidad de Concepción, Departamento de Física, Casilla 160-C, Concepción, Chile e-mail: bdirsch@cepheid.cfm.udec.cl
² Radioastronomisches Institut der Universität Bonn, Auf dem Hügel 71, 53123 Bonn, Germany

Received: 24 November 2003
Accepted: 3 November 2004

Abstract

Previous smaller-scale studies of the globular cluster system of NGC 4636, an elliptical galaxy in the southern part of the Virgo cluster, have revealed an unusually rich globular cluster system. We re-investigate the cluster system of NGC 4636 with wide-field Washington photometry. The globular cluster luminosity function can be followed roughly 1 mag beyond the turn-over magnitude found at for the blue cluster sub-population. This corresponds to a distance modulus of , 0.4 mag larger than the distance determined from surface brightness fluctuations. The high specific frequency is confirmed, yet the exact value remains uncertain because of the uncertain distance: it varies between and . The globular cluster system has a clearly bimodal color distribution. The color peak positions show no radial dependence and are in good agreement with the values found for other galaxies studied in the same filter system. However, a luminosity dependence is found: brighter clusters with an “intermediate” color exist. The clusters exhibit a shallow radial distribution within 7´, represented by a power-law with an exponent of -1.4. Within the same radial interval, the galaxy light has a distinctly steeper profile. Because of the difference in the cluster and light distribution the specific frequency increases considerably with radius. At 7´ and 9´ the density profiles of the red and blue clusters, respectively, change strongly: the power-law indices decrease to around -5 and become similar to the galaxy profile. This steep profile indicates that we reach the outer rim of the cluster system at approximately 11´. This interpretation is supported by the fact that in particular the density distribution of the blue cluster population can be well fit by the projection of a truncated power-law model with a core. This feature is seen for the first time in a globular cluster system. While the radial distribution of the cluster and field populations are rather different, this is not true for the ellipticity of the system: the elongation as well as the position angle of the cluster system agree well with the galaxy light. We compare the radial distribution of globular clusters with the light profiles for a sample of elliptical galaxies. The difference observed in NGC 4636 is typical of an elliptical galaxy of this luminosity. The intrinsic specific frequency of the blue population is considerably larger than that of the red one.