Star Formation in SpARCS J0330: Bridging the Gap Between Protoclusters and Clusters 1Imperial College London, Blackett Laboratory, Prince Consort Road, London, SW7 2AZ, UK ● 2ASU School of Earth and Space Exploration, 781 Terrace Mall, Tempe, AZ 85287, USA 3MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA 4Departamento de Ciencias Físicas, Universidad Andres Bello, Fernandez Concha 700, Las Condes 7591538, Santiago, Región Metropolitana, Chile ● 5Department of Physics and Astronomy, University of California, Riverside, CA 92521 J. Cairns1, D. L. Clements1, A. Noble2, M. McDonald3, Y. Ding1, J. Nantais4, G. Wilson5 1. Background At z > 2, many studies have identified protoclusters - overdensities of galaxies spanning tens of Mpc in size and hosting some of the most vigorous star formation observed in the Universe to date. Conversely, galaxy clusters are typically observed at z < 1 and are massive, virialised and abundant in quiescent, elliptical galaxies. Current models of galaxy formation and evolution predict that protoclusters will evolve into z ~ 0 massive galaxy clusters (e.g. Cooke et al. 2015), and so there must be some rapid, environmentally-driven quenching of star formation at that transforms high-z, star-forming protoclusters into massive, quiescent galaxy clusters. However, the exact nature of this evolution remains poorly understood. In order to understand this evolution, we must search for overdensities of galaxies at the onset of cluster formation at which time we can observe the full web of activity: the assembly of a collapsed cluster core feeding on the star-forming filamentary material that resides in the protocluster web. z ∼1 4. SED Fitting 2. SpARCS J0330 We present observations of SpARCS J0330, one of the most massive, high-z galaxy clusters ever discovered (Lidman et al. 2012). SpARCS J0330 resides in the Chandra Deep Field South (CDFS) at a redshift of , with a dynamical mass of . We combine the photometry and spectroscopy compiled by Nantais et al. (2016) with our own observations, as well as ancillary data in the field, to build up Spectral Energy Distributions (SEDs) for the 40 spectroscopically confirmed cluster members, as well as potential cluster members, with up to 20-band photometry. We use the photometric fitting codes EAZY (Brammer et al. 2008) and CIGALE (Boquien et al. 2019) to estimate the properties of these sources and hence map out distant, star-forming galaxies over a region in the cluster. z = 1.626 ∼2.5 × 1014 M⊙ ∼15 Mpc 3. Sample Selection 1. Spec-z sample: Comprising the 40 spectroscopically confirmed cluster members. Available photometry includes u, g, r, i, z, y, J, H and K observations from IMACS, HAWK-I and HST, IRAC and MIPS photometry from Spitzer and PACS and SPIRE photometry from Herschel. 2. Photo-z sample: Sources with the same photometry as the spec-z sample but without a spectroscopically confirmed redshift. 3. Photo-z wide sample: Sources in the wider field from the Spitzer Data Fusion catalogue, including photometry from the HerMES, SWIRE, SERVS, VHS and VIDEO surveys, as well as CTIO MOSAIC2. 4. Field sample: We additionally assemble a field sample of sources from the Spitzer Data Fusion catalogue spread across the XMM-LSS field. We use the photometric fitting code EAZY to estimate photometric redshifts for the photo-z, photo-z wide and field samples, allowing us to select likely cluster candidates (see Figure 1). We then use CIGALE to estimate galaxy properties for all four samples. We use the sfhdelayedbq module to parameterise the star formation history according to: for with the option of an instantaneous recent variation in SFR. We include a Chabrier IMF with solar metallicity and a dust attenuation curve based on that of Calzetti et al. (2000). We use the nebular emission module and the Casey2012 dust module which models the FIR dust peak as a simple modified blackb