The M31 globular cluster system: ugriz and K-band photometry and structural parameters

We present an updated catalogue of M31 globular clusters (GCs) based on images from the Wide Field Camera (WFCAM) on the United Kingdom Infrared Telescope and from the Sloan Digital Sky Survey (SDSS). Our catalogue includes new, self-consistent ugriz and K-band photometry of these clusters. We discuss the difficulty of obtaining accurate photometry of clusters projected against M31 due to small-scale background structure in the galaxy. We consider the effect of this on the accuracy of our photometry and provide realistic photometric error estimates. We investigate possible contamination in the current M31 GC catalogues using the excellent spatial resolution of these WFCAM images combined with the SDSS multicolour photometry. We identify a large population of clusters with very blue colours. Most of these have recently been proposed by other works as young clusters. We distinguish between these, and old clusters, in the final classifications. Our final catalogue includes 416 old clusters, 156 young clusters and 373 candidate clusters. We also investigate the structure of M31's old GCs using previously published King model fits to these WFCAM images. We demonstrate that the structure and colours of M31's old GC system are similar to those of the Milky Way. One GC (B383) is found to be significantly brighter in previous observations than observed here. We investigate all of the previous photometry of this GC and suggest that this variability appears to be genuine and short lived. We propose that the large increase in its luminosity may have been due to a classical nova in the GC at the time of the previous observations in 1989