A test of Ca II H & K photometry for isolating massive globular clusters below the metallicity floor

Context. The serendipitous discovery of the M31 globular cluster (GC) EXT8 has presented a significant challenge to current theories of GC formation. By finding other GCs similar to EXT8, it should become clear if and/or how EXT8 can fit into our current understanding of GC formation. Aims. We aim to test the potential of integrated-light narrowband Ca  II H & K photometry as a proxy for the metallicity of GCs to be able to provide effective candidate selection for massive GCs below the GC metallicity floor ([Fe/H] ≤ −2.5). Methods. We investigate the behaviour of two colours involving the CaHK filter employed by the Pristine survey, CaHK- u and CaHK- g , as a function of metallicity through CFHT MegaCam imaging of EXT8 and a wide set of M31 GCs covering the metallicity range −2.9 ≤ [Fe/H] ≤ +0.4. Additionally, we investigate if the CaHK colours are strongly influenced by horizontal branch morphology through available morphology measurements. Results. In both of the CaHK colours, EXT8 and two other potential GCs below the metallicity floor could be selected from other metal-poor GCs ([Fe/H] ≤ −1.5), with ( C a H K − g ) o showing the greater metallicity sensitivity. The RMS values of the linear fits to the metal-poor GCs for both colours show an uncertainty of 0.3 dex on metallicity estimations. Comparisons with u − g and g − z /F450W-F850L colours reinforce the notion that CaHK photometry can be used for effective candidate selection, as they reduce false positive selection rates by at least a factor of 2. We find no strong influence of the horizontal branch morphology on the CaHK colours that would interfere with candidate selection, although the assessment is limited by the quantity and quality of available data.