Ages and metallicities of globular clusters in M81 using GTC/OSIRIS spectra

Abstract

ABSTRACT We here present the results of an analysis of the optical spectroscopy of 42 globular cluster (GC) candidates in the nearby spiral galaxy M81 (3.61 Mpc). The spectra were obtained using the long-slit and multi-object spectroscopic modes of the OSIRIS instrument at the 10.4-m Gran Telescopio Canarias at a spectral resolution of ∼1000. We used the classical H β versus [MgFe]′ index diagram to separate genuine old GCs from clusters younger than 3 Gyr. Of the 30 spectra with continuum signal-to-noise ratio >10, we confirm 17 objects to be classical GCs (age >10 Gyr, −1.4 < [Fe/H] < −0.4), with the remaining 13 being intermediate-age clusters (1–7.5 Gyr). We combined age and metallicity data of other nearby spiral galaxies (≲18 Mpc) obtained using similar methodology like the one we have used here to understand the origin of GCs in spiral galaxies in the cosmological context. We find that the metal-poor ([Fe/H] < −1) GCs continued to form up to 6 Gyr after the first GCs were formed, with all younger systems (age <8 Gyr) being metal rich.