THE TWO-DIMENSIONAL PROJECTED SPATIAL DISTRIBUTION OF GLOBULAR CLUSTERS. I. METHOD AND APPLICATION TO NGC 4261

R D'Abrusco,S Mineo,J Strader,T Fragos,B Luo,D.-W Kim,A King,P Bonfini,A Zezas,G Fabbiano

doi:10.1088/0004-637x/773/2/87

Abstract

We present a new method for the determination of the two-dimensional (2D) projected spatial distribution of globular clusters (GCs) in external galaxies. This method is based on the K-Nearest Neighbor density estimator of Dressler (1980), complemented by MonteCarlo simulations to establish the statistical significance of the results. We apply this method to NGC4261, a "test galaxy" where significant 2D anisotropy in the GC distribution has been reported. We confirm that the 2D distribution of GC is not azimuthally isotropic. Moreover, we demonstrate that the 2D distribution departures from the average GC radial distribution results in highly significant spiral-like or broken shell features. Overall, the same perturbations are found in "red" and "blue" GCs, but with some differences. In particular, we observe a central feature, roughly aligned with the minor axis of NGC4261, composed of red and most luminous GCs. Blue and fainter GCs are more frequent at large radial distances and follow the spiral-like features of the overall density structure. These results suggest a complex merging history for NGC4261.

Highlights

A considerable body of work has been accumulated in the past two decades on the properties of globular cluster (GC) populations in elliptical galaxies
The GC system in NGC 4261 shows that the geometry of the GC distribution observed in the Wang et al (2013) sample is not universal, even though the different conclusions based on the results provided by our method may depend on the rich sample of NGC 4261 GCs used, which, does not cover the whole GC system
We have developed a new approach to the study of the 2D distribution of GCs in elliptical galaxies

Summary

INTRODUCTION

A considerable body of work has been accumulated in the past two decades on the properties of globular cluster (GC) populations in elliptical galaxies (see review by Brodie & Strader 2006). Other authors have explained the formation of low-metallicity GCs as starburst remnants of old dwarf galaxies that could have entered the halos of spiral galaxies (Elmegreen et al 2012) While these works have explored multiple possible explanation of the peculiar observational traits of the GC populations, no agreement has yet been reached in the literature. While the spectral and photometric properties of GC populations, as well as their radial distributions, have been explored in depth, relatively little work has addressed their two-dimensional (2D) distributions (e.g., NGC 4471 in Rhode & Zepf 2001, NGC 1399 in Dirsch et al 2003; Bassino et al 2006, NGC 3379, NGC 4406 and NGC 4594 in Rhode & Zepf 2004, NGC 4636 in Dirsch et al 2005, multiple galaxies in Hargis & Rhode 2012, NGC 3585 and NGC 5812 in Lane et al 2013). We have excluded the undersampled regions outside the D25 isophote (de Vaucouleurs et al 1991)

THE ANALYSIS

Density and Residual Maps

Color versus Spatial Distribution of GCs

DISCUSSION

CONCLUSIONS