Are GRB hosts a distinct population of star forming galaxies? This question has been the core of many studies in the research field of GRB hosts (e.g., Le Floc’h et al. 2003; Christensen et al. 2004; Fruchter et al. 2006; Savaglio et al. 2009; Fynbo et al. 2009; Castro Cerón et al. 2010; Krühler et al. 2015; Greiner et al. 2015; Schulze et al. 2015; Perley et al. 2016a; Vergani et al. 2017). Understanding any systematic differences between GRB hosts and the average field galaxy population underpins attempts to use GRBs as probes of galaxy evolution, and provide important clues as to the conditions required to produce GRBs. Many works have indicated that GRB production is disfavoured in high-metallicity environments (e.g., Modjaz et al. 2008; Graham & Fruchter 2013; Vergani et al. 2015; Perley et al. 2016b), but it is less clear whether other factors are also relevant as some GRBs have been associated with metal-rich galaxies.

A powerful diagnostic is the mass-metallicity (MZ) relation, one of the most fundamental scaling relations addressing the star formation history of galaxies. This relation effectively allows us to investigate whether galaxies hosting GRBs have metallicities similar to those of general star forming galaxies at a given stellar mass and redshift. The MZ relation of GRB hosts had frequently been discussed in the literature and several studies had found the GRB hosts to fall well below the MZ relation of the general population, towards lower metallicities (Stanek et al. 2006; Kewley et al. 2007; Levesque et al. 2010; Mannucci et al. 2011; Japelj et al. 2016; Vergani et al. 2017). However, all these studies suffer from low statistics and are significantly affected by systematics. We studied the MZ relation of the largest assembled sample of GRB hosts to date, spanning a redshift range of z =0.3-3.4, minimizing the effects of systematics (see Fig. 1). This study showed that GRB hosts follow the MZ relation of the general star forming galaxies (Arabsalmani et al. 2018a). Our findings revealed that the average offset of the GRB hosts from the MZ relation is comparable with the intrinsic scatter of the relation, as well as with the error-bars of metallicity measurements. This motivates the use of absorption-line metallicity measurements of GRB hosts for studying the MZ relation of star forming galaxies at high redshifts