Extragalactic GCs in the near-infrared: genuinely old in E/S0's?

Extract

Globular cluster (GC) systems are powerful probes to study the evolutionary histories of galaxies, being tracers of major star fomation episodes (Brodie & Strader 2006). They are found around all major galaxies and are easy to see far beyond the local group. Age dating GCs therefore helps pinpoint epochs of major star forming events. Spectroscopic age dating though (Strader et al. 2005) is extremely time consuming and can only access the few brightest clusters. An alternative is to combine near-infrared (NIR) and optical photometry, and therefore have a better chance in lifting the age metallicity degeneracy than with optical colours alone. This approach relies in testing GC colours against simple stellar population (SSP) models. The first studies following this technique showed the possible existence of a high percentage of intermediate age (2-3 Gyrs) GCs in early-type galaxies known to contain old stellar populations from integrated light studies. Two strong cases can be listed: NGC 4365 (Puzia et al. 2002, Larsen et al. 2005) and NGC 5846 (Hempel et al. 2003). In the present study we combine NIR deep photometry obtained with the WHT/LIRIS instrument and archival HST/ACS optical images to determine g(F475W), z(F840LP) and K(2.2m) magnitudes and colours of GCs in 14 early-type galaxies.