New insights into the star formation histories of candidate intermediate-age early-type galaxies from K′-band imaging of globular clusters

Abstract

We investigate age and metallicity distributions of bright globular clusters (GCs) in the candidate intermediate-age early-type galaxies NGC 3610, NGC 584 and NGC 3377 using a combination of new Gemini/NIRI K'-band imaging and existing optical V,I photometry from HST data. The V-I vs I-K' colour-colour diagram is found to break the age-metallicity degeneracy present in optical colours, as I-K' primarily measures a populations' metallicity and is relatively insensitive, unlike optical spectroscopy, to the effect of hot horizontal branch (HB) stars, known to be present in massive old GCs. We derive GCs' photometric age, Z and masses. In general, metal-poor ([Z/H]<-0.7dex) GCs are older than more metal-rich GCs. For the most massive GCs (M>6x10^5 M_sol) in NGC 3610 with available spectroscopic data, photometric ages are older by ~2 Gyr, and this difference is more pronounced for the metal-poor GCs. However, photometric and spectroscopic metallicities are in good agreement. We suggest that this indicates the presence of a hot HB in these massive clusters, which renders spectroscopic ages from Balmer line strengths to be underestimated. To support this suggestion we show that all Galactic GCs with M>6x10^5 M_sol feature hot HBs, except 47 Tuc. Using the relation between the most massive GC mass and the galaxy's SFR, we find that the galaxies' peak SFR was attained at the epoch of the formation of the oldest (metal-poor) GCs. Age and [Z/H] distributions of the metal-rich GCs are broad, indicating prolonged galaxy star formation histories. The peak value of the age and [Z/H] distributions of the GCs correlates with host galaxy integrated age and [Z/H], showing that GCs can indeed be used as relevant proxies of the star formation histories of galaxies.(Abridged)