Euclid: A blue galaxy population and a brightest cluster galaxy in the making in a z ∼ 1.74 MaDCoWS2 galaxy cluster candidate

We present an example cluster follow-up study with Euclid. Our target, a z similar to 1.74 candidate cluster nicknamed the 'Puddle', was initially discovered by the Massive and Distant Clusters of WISE Survey 2 as a z(phot) similar to 1.65 candidate cluster. It was also detected independently as a z(phot) similar to 1.5 candidate with the two cluster-finding algorithms in Euclid Quick Release 1 (Q1). A Keck MOSFIRE spectrum shows the brightest nucleus is at z = 1.74 and is dominated by an active galactic nucleus. Our analysis focused on the galaxy population and the brightest cluster galaxy (BCG), and is based on Euclid and ancillary photometry. Compared to similar fields, we measured an overdensity of 110 +/- 14 galaxies with H-E similar to 22.25 in a 2 ' radius around the BCG. About 18 +/- 4% of the completeness-corrected galaxy population is red, which is consistent with some clusters at z > 1.5 but lower than others. Euclid imaging revealed that six or seven galaxies appear to be assembling to form the future BCG. Spectral energy distribution fitting suggests that the merging BCG has a stellar mass of 5.7 +/- 0.3 similar to 10(11) M-circle dot and that it experienced a short burst of star formation similar to 300 Myr ago. Its morphology and star-formation history suggest that the proto-BCG is a more evolved version of the merging core of SPT2349 56. These systems indicate that multiobject mergers might be a common BCG formation process. Assuming a similar density of mergers in the Euclid Wide Survey, we expect that Euclid will discover approximately 400 assembling BCGs by the end of its mission.