Observational Biases Masquerading as Cosmological Effects? A Cautionary Tale about Blue Tilts and Other Trends in Globular Cluster Systems

Abstract

The high spatial resolution of the Hubble Space Telescope (HST) has led to tremendous progress in many areas of astronomy. The ability of the HST to peer into the bright inner regions of galaxies and distinguish between globular clusters (GCs) and background objects has been particularly beneficial to the study of clusters. But the very virtue of the HST that has been an asset to such research can be its pitfall if the consequences of superior angular resolution are not considered in detail. Recent HST studies show a small, but consistent, color-magnitude correlation in the metal-poor halo GCs of nearby galaxies. This `blue-tilt' has been interpreted as a mass-metallicity relationship, implying self-enrichment in the higher mass GCs. We show that the `blue-tilt' is likely the consequence of a small, but measurable with HST, mass-size relationship in GCs. The combined effects of Poisson noise and surface brightness fluctuations can explain other apparent correlations of the `blue-tilt' with environment. Some HST based studies have similarly suggested that the mean metallicity of the blue, metal-poor, halo clusters increases with host galaxy mass, indicating that GC metallicity is linked to the size of the host galaxy halo. We show that this correlation is also likely due to the effects of GC size in high resolution HST images. We also point out that the presumed fundamental plane of GCs itself varies with galactocentric distance due to GC size trends and ultra compact dwarfs may simply reflect the tail of the globular cluster distribution.