The Fornax 3D project: Non-linear colour–metallicity relation of globular clusters^⋆

¹ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching bei München, Germany
e-mail: kfahrion@eso.org
² Department of Astrophysics, University of Vienna, Türkenschanzstrasse 17, 1180 Wien, Austria
³ Instituto de Astrofísica de Canarias, Calle Via Láctea s/n, 38200 La Laguna, Tenerife, Spain
⁴ Depto. Astrofísica, Universidad de La Laguna, Calle Astrofísico Francisco Sánchez s/n, 38206 La Laguna, Tenerife, Spain
⁵ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁶ Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
⁷ University of California Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, USA
⁸ Dipartimento di Fisica e Astronomia “G. Galilei”, Università di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
⁹ INAF–Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹⁰ INAF-Astronomical Observatory of Capodimonte, Via Moiariello 16, 80131 Napoli, Italy
¹¹ Department of Physics and Astronomy, Macquarie University, North Ryde, NSW 2109, Australia
¹² Armagh Observatory and Planetarium, College Hill, Armagh BT61 9DG, UK
¹³ Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
¹⁴ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281, 9000 Gent, Belgium
¹⁵ Sterrewacht Leiden, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands
¹⁶ Max-Planck-Institut für Extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching bei München, Germany
¹⁷ Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, PR China

Received: 7 February 2020
Accepted: 28 March 2020

Abstract

Globular cluster (GC) systems of massive galaxies often show a bimodal colour distribution. This has been interpreted as a metallicity bimodality, created by a two-stage galaxy formation where the red, metal-rich GCs were formed in the parent halo and the blue metal-poor GCs were accreted. This interpretation, however, crucially depends on the assumption that GCs are exclusively old stellar systems with a linear colour–metallicity relation (CZR). The shape of the CZR and range of GC ages are currently under debate because their study requires high quality spectra to derive reliable stellar population properties. We determined metallicities with full spectral fitting from a sample of 187 GCs with a high spectral signal-to-noise ratio in 23 galaxies of the Fornax cluster that were observed as part of the Fornax 3D project. The derived CZR from this sample is non-linear and can be described by a piecewise linear function with a break point at (g−z) ∼ 1.1 mag. The less massive galaxies in our sample (M_* <  10¹⁰ M_⊙) appear to have slightly younger GCs, but the shape of the CZR is insensitive to the GC ages. Although the least massive galaxies lack red, metal-rich GCs, a non-linear CZR is found irrespective of the galaxy mass, even in the most massive galaxies (M_* ≥ 10¹¹ M_⊙). Our CZR predicts narrow unimodal GC metallicity distributions for low mass and broad unimodal distributions for very massive galaxies, dominated by a metal-poor and metal-rich peak, respectively, and bimodal distributions for galaxies with intermediate masses (10¹⁰ ≤ M_* <  10¹¹ M_⊙) as a consequence of the relative fraction of red and blue GCs. The diverse metallicity distributions challenge the simple differentiation of GC populations solely based on their colour.